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Actually, the temperatures shown here are nearly always very close to the average of the summer and winter average temperatures. (In Chicago, the average December temperature is 28ºF and the average June temperature is 72ºF, which averages to 50ºF, close to what this map shows.)

Heat-pumps are much more efficient than normal air conditioners, but they still have compressors that use a lot of expensive electricity. Because they use that compressor, they have the capability of sort of "multiplying" the heat/cool from the ground to even provide complete winter heating (for some mild climates), where our system, without a compressor, does not try to accomplish that. But we don't have to pay for electricity for that compressor, either!

Because any substantial volume of earth contains enormous amounts of heat/cool storage but the many soils are so poor at heat conduction, the AREA of the interface (tube wall) between the soil and the tube contents nearly always is the greatest limiting effect on short-term system performance. Ground-source heat pumps generally have very small diameter copper tubing, so the circumference and the outside surface area in direct contact with the soil is very limited. This tends to make ground-source heat pumps to often act like they have depleted the energy source very quickly. Fortunately, they have their compressor that just has to work harder, but can still continue to work during fairly severe depletion. And, in any case, energy from nearby soil replenishes the energy source in just a few hours.

Our approach is to use much larger diameter tubing underground, which greatly increases the contact area between the soil and the tubing surface. This total "area of contact" is one of the most important characteristics to design for in this system! Again, in more technical terms, heat exchange is proportional to the surface area of the heat exchanger. It determines the amount of heat (or coolness) that can be given to the house air that is flowing through the tubes, in other words, the Btu/hr rating of the system (or short-term performance). Even normal ground-source heat pumps would benefit from such a larger surface contact area, but their compressor allows designers to calculate very minimal systems, to keep their digging (and copper tubing) costs at a minimum.

Further, our approach is to divide the underground air passageway into several "parallel" paths, separated from each other by around six horizontal feet. This configuration takes into account the very slow soil heat conductance and yet allows using a FAR larger mass of soil to participate in this system, which determines the long-term performance of the system.

This drawing suggests a very compact arrangement. For example, if a 50 by 50 foot area of yard was involved (about 1/16 acre), nine parallel tubes (six feet apart, or 48 feet across all of them), and each 50 feet in functioning length as shown, a total of around 450 feet of functional tube length would dissipate house heat into the soil. You would probably need around double that amount of the sewer pipe, since they are "bundled" together on their way to and from the building, as the small cross sectional drawing shows. Our drawing does not show the three (or more) identical tubes above the top of the drawing This arrangement would represent a way of installing around 500 lineal feet of heat exchange surfaces in that fairly compact area (counting the bundles). Such a configuration makes sure that one or another of the tubes is within three feet of well over a million pounds of cool soil. (Simple engineering calculations show that that much soil represents over 10,000,000 Btus of cooling available!)

In case you are curious, we chose bundling all the tubes together on the way to the house as the simplest and least expensive way to join the tubes and also make sure they are all watertight. Other, better ways exist, where the bundles are replaced by large ducts, and especially tapered ones, to make sure that the same amount of air goes through each tube. All such ways that we know of are very expensive to do! So even though the bundling approach has some disadvantages, it is reliably sealable to be water-tight/air-tight, fairly simple and easy to install, and fairly inexpensive. For the technical ones out there, yes, we intentionally over-designed the generic system to account for the different amounts of air flowing through the different length tube air paths!

In case you're still skeptical, the "heat exchanger" arrangement we are describing has a coefficient U that is around 8 Btu/hr/square foot/ºF difference. If you look at ONE of our nine tubes, its circumference is around one foot and it is 50 feet long, so its area is around 50 square feet. If the house air begins at 90ºF and the deep soil temperature is 53ºF, there is 37ºF difference. Multiplying these (8 * 50 * 37) gives an effective rating of 14,800 Btu/hr. That's ONE of our tubes, and there are nine of them! This suggests that the total system is capable of about 135,000 Btu/hr cooling, around four times as much as the normal house requires! Well, it could (almost) actually do that for a few minutes, but there are a number of factors that would get it down below 100,000 Btu/hr of cooling in under an hour and within a few hours to the 36,000 Btu/hr that we designed our "standard configuration" for.

Many larger houses and hot climates should probably have specific engineering done to determine that that is enough, but this system's million pounds of cool soil would often be sufficient for many houses and climates.

Basically, we are using modern engineering concepts to maximize the effectiveness of a natural phenomenon!

There are many variables involved, but many installations should be able to use 4" plastic (thinwall) PVC drain/sewer pipe, available at any local 'home' store. (Technical Comments). At around $3.50 per ten-foot length, the 900 feet of pipe mentioned above (90 lengths) and a bunch of elbows would only cost around $350. A trencher (sort of like a roto-tiller) can be rented for around $220 per day. The sections are light and easy to handle, and available almost everywhere. Standard PVC Cement glues the sections together. A do-it-yourselfer could do this amazingly inexpensively and quickly and easily!

Materials List
90 pieces 4" PVC THINWALL sewer (ASTM D-3034)	~$316
18 pieces compatible PVC sewer elbows (ASTM D-3034)	~$36
a lot of PVC cement	~$15
Rental of DitchWitch #1820 Trencher	~$220

The drawing above certainly does not show the only possible arrangement. But it shows a compact arrangement with the three important considerations.

1. There is a lot of total pipe surface area (which defines the short term performance of the system). In this case, there is over 500 square feet of exterior pipe surface in contact with the soil. Even though PVC pipe does not have the thermal conductivity of aluminum or copper, it turns out that that fact is of minimal importance (because the soil is so much worse!) And PVC sewer pipe is, could we say, "dirt" cheap???
2. Pipes are distributed so that a lot of mass of soil is within about three feet of some tube (which defines the long term performance of the system), (in this case, around 2.1 million pounds) and
3. The AREA of the air path always totals an area comparable to the area of the main house ducts (which allows proper airflow through the whole system and house). In this example, each 4" tube has around 13 square inches of cross-sectional area, so 9 of them total about 120 square inches, fairly similar to an 8" by 16" house trunk duct of 128 square inches. If the house has larger main ducting, then more tubes should be "bundled" together, like 3x4 (12 tubes or 156 total sq.in.) or 4x4 (16 tubes or 210 total sq.in.) We could describe this in a different way. A 4" diameter tube can realistically carry around 60 to 80 cfm (cubic feet per minute) of air through it. Nine of them in "parallel" can therefore carry around 700 cfm, 12 can carry around 1,000 cfm, and 16 can carry around 1,300 cfm of total airflow. Conventional A/C units generally have greater airflow rates than this, but it is actually because their performance is poorer at slower airflow rates, and the house does not actually need that great an airflow. In fact, that same central A/C/furnace uses far less airflow during heating in the winter! The total airflow rate is mostly related to the comfort level inside the house, regarding how well the cooled (or heated) air is distributed to all rooms and to corners of those rooms.

   This is mentioned here because we have been told by a lot of people that they intended to "customize" or "improve" our system! One man said he intended to run ONE extremely long tube (around a mile long underground, as I recall) instead of our 9 or 12 or 16. He certainly would have cooled the air down, but would have caused himself two new problems! (1) No matter how powerful a blower he would use, that one tube would only be able to supply 60 or 80 cfm of that cooled air to his house! That is enough for a small bedroom, but no more! (2) Whatever blower he would have tried to use, that blower would have to be extremely powerful, probably several horsepower, because it would have to be constantly be pushing that entire mile of air inside the tube! He likely would spend more for all the electricity needed than he would save on A/C bills!

   There have been (many) other people who have said they intended to only install four tubes (mostly to save on the cost of digging the trenches) and they figured that was an "improvement". Note again that such an installation would likely only provide around 300 cfm of cooled air to the house. That is certainly enough to nicely cool two or three rooms, but not really enough airflow to give the comfort level that we spoiled Americans expect for a whole house!

If you do a reasonable job of providing these three considerations, your system is bound to work! But please don't "improve" it unless you really know what you are doing!

(Alternate Configurations).
(More Technical Comments).

• (Vertical Water-filled Configuration).
  This version has the tubes vertical rather than horizontal, so they are generally much deeper and down where hot climates could less degrade their performance. So it might be a version that would be more suitable for really hot climates. However, the cost of essentially drilling a bunch of shallow water wells figures to be more expensive than the cost of making all those trenches.

• (Configuration for a whole community).
  This version is a larger example of the water-filled version, suitable for providing A/C for all of the houses on a particular city block. This system clearly needs to have cooperation among neighbors and with the local government and street department!

Virtually all of the heat that replenishes this system (or ground-source or water-source heat pumps, close cousins) originally began as solar energy that heated the surrounding fields, often months before. Virtually no actual "geothermal" heat is involved. However, there is no commonly accepted name for this process and we suspect that it will generally be thought of as geothermal heating/cooling. Oh, well!

This article is presented separately from the Solar System presentation because this feature could be added to most houses, either while they are being built or to be retrofitted later to existing houses. This sub-system can be fairly inexpensive and can be configured to completely eliminate ALL conventional summer air conditioning usage (and those huge electric bills!) In the winter, the same system significantly reduces home heating cost by reducing the necessary heat load. Yet another benefit is that recent Indoor Air Purity concerns can be addressed in a way that is not costly or wasteful. Finally, you cannot get much more environmentally friendly than this system!

Now, you probably already see the obvious logic of this system. Some fairly large-scale (expensive) systems have been occasionally installed for at least 30 years, and the relatively similar ground source heat pump system has been marketed for twenty years, so the concept is well proven. We are just presenting a low-cost version of it for residential use.

You are probably curious if the designer of this system uses it! Actually, his house has an unusual environment, and he uses a variation of this system which required much more design engineering but works even better! Here is a description of it.

This might lead you to believe that no planning or engineering is actually necessary! Well, technically, you would be right! If you chose to bury a mile-long culvert, it would certainly work excellently! However, most people couldn't afford to do that! They would want the shortest, most compact arrangement possible, both to fit under their yard and to minimize the digging expense.

The example described above will work extremely well for most medium-sized homes in moderate climates. So, no additional engineering is necessary for such applications! But, where soil is extremely dry, or the climate is extremely hot, or the house is especially large, prudent design might involved extra engineering. There is even often an alternative here, too! OK! You make 4' deep trenches and place the PVC tubing in the bottom of them. Instead of immediately filling in the entire trenches, for such applications, consider filling in ONE foot deep of fill in the trenches! THEN, get (cheap) 100 foot-long coils of black polyethylene 1/2" water line from the store, and drill LOTS of small holes in it! Make water connections to this set of water pipes, which are now one foot above the much bigger air tubes below. THEN fill in the trenches and re-plant your grass.

IF the system should ever seem to lose effectiveness in the future, it would generally be because the soil had gotten too dry down there, and the coolness was not able to flow through the soil to the tube. By briefly running some water through those water lines, you can quickly saturate the soil around the air tubes, TREMENDOUSLY increasing the system's performance (often by over a factor of EIGHT!) This "accessory" to this system is really inexpensive to add, and if your climate or soil dryness or house size is even moderately unusual, it might be a good idea to add this feature, even if it turns out that you never need it.

Comparison to Conventional Air Conditioning

Let's say you have a moderate sized house in a moderate climate and you want to have 36,000 Btu/hr of cooling. Again, there are straightforward engineering conversion formulas that indicate that this is equivalent to about 10.5 Kilowatts of actual cooling effect. Our thoughtful government has provided us with a SEER (Seasonal Energy Efficiency Ratio) or COP (Coefficient of performance) for each air conditioning system sold. Let's say for a moment, that yours happens to have a SEER of 10 (about the same as a COP of 3.0). You would just divide the cooling load (36,000) by the SEER (10) to know how much electricity will actually be used (about 3.6 kW) of electricity. If you knew the COP instead, you would divide the cooling effect (10.5 kW) by the COP (3.0), or again, about 3.6 kW of electricity.

Most actual central air conditioners have a lower SEER than 10. Government studies have established that home central air conditioners average using 1.49 kW of electricity for the compressor and another 0.14 kW for fan motors, for each "ton" (12,000 Btu) of rating. Our example 36,000 Btu/hr system (3 tons) would therefore probably use about 4.9 kW of electricity (which calculates to a SEER of about 7.3).

Still with us? Now say you have a REALLY hot day, and the compressor ran virtually continuously for all 24 hours (not particularly good for the compressor!). You would have used up 4.9 * 24 or around 118 kWh of electricity that day. Look at your latest electric bill and it will tell you what each kWh costs you. Recently, electric rates have been going wild in many parts of the country. In an area where such rates are still relatively stable (Chicago), the rate is still only around 10 cents per kWh. One hundred eighteen kWh would therefore cost 118 * 0.10 or about $12. That single very hot day would have cost $12 in air conditioning. This is for a very moderate sized house and air conditioning system and for very reasonable priced electricity. We have shown you this logic here so you could figure these things out for YOUR system using electricity available to YOU.

You can probably now better understand the very large electric bills you receive during the summer. Twelve dollars for a hot day could easily account for $200 per month. Far more, for larger homes and for where electricity is more expensive than ten cents per kilowatt-hour.

The system described here does not involve any compressor! The only electricity necessary is for a fan or blower to push the house air through the underground tunnels! In many cases, this can be accomplished with a fan or blower that only uses 200 watts (0.2 kW) or less of electricity. In terms of SEER, the effective rating would be (36,000/200) around 180! That's nearly TWENTY TIMES as energy efficient as the very BEST heat pump or air conditioning system!

On that example very hot day described above, let's look at the figures. Instead of continuously using up 4.9 kW of electricity, this system only needs 0.2 kW. In 24 hours, that's 4.8 kWh. At the electricity cost mentioned above, that's $0.48 of electricity instead of $12.00! A full month of such heavy cooling would cost around $10 instead of over $200!

There are actually even some possibilities of eliminating the cost of running the fan, too, eliminating even that last $10 of monthly operating expense! However, it is such a minimal remaining expense, it is probably not worth even trying to do that!

This basically states the point of this system (regarding air conditioning). If you have a moderate sized house in a moderate climate and you have moderate electricity costs, you are STILL likely to sometimes save $200 in a single month! In a whole summer, this "eliminating" of the cost of air conditioning the house or building, might easily save you $1,000. Basically, whatever your total electric bills were last summer, inflated for the recent price hikes, THAT'S what you would save! And, that savings would be EVERY year, for the rest of your life! What a deal!

In areas where electricity costs have drastically risen in recent months, they are not likely to ever fall back to where they were. Using the logic presented above, you should be able to figure out approximately how much air conditioning will cost you. Or just check the electric bills from a previous year and multiply by how much the cost of electricity has multiplied. We are guessing that there are many millions of homeowners who are going to find it no longer possible to regularly air-condition their homes, just because of this tremendous increase in the cost of electricity. Well, that's a main reason we are presenting and offering this page! Each homeowner who would install this simple and obvious system, will virtually certainly save more than $1000 every summer (depending on climate, of course), forever! If a new monthly TOTAL cooling cost becomes only $10, even if it some day doubles, that's only $20!

And the coolest (pun intended) part of this is that all of the comfort in the house is PRECISELY identical to that when using conventional air conditioning. The temperature and humidity levels will be identical. Of course, you would not have a noisy condenser unit running outside your house, so if you like that sound, you're outa luck! All this, in a system is about as "natural" as you can get! And bonus $$$ savings in the winter!

If you live in ANY house, and YOU pay the electric bills, you must now see the exquisite logic of this system. It even has a bunch of additional bonus characteristics. For example, there is virtually NO chance of ever being charged for any repairs to the system, because there is virtually nothing that could ever break or fail or leak!

It seems to us that such homeowners will even be doing good things for society and the environment, as well as pocketing an extra $1,000+ each year. Consider California and its amazing energy woes, particularly electricity. Even during the winter, their electric companies and power grid have great difficulties in keeping up with electrical demand. Consider if a million California homeowners decide to save $1,000 each, every summer, with this type of system. They're smiling! LOTS of happy people in California! But consider this! If a million California homeowners are NOT taking 5 kW each from the power grid during the summer, that's a reduction of load on the power grid of FIVE THOUSAND MEGAWATTS! It is unclear if that would "solve" the lack of planning of California's electricity needs, but it would have to help!

(I know! I know! There are far too many exclamation marks in the previous paragraphs! But several of these concepts are pretty amazing! Huh?!)

Winter Make-Up Air

Until around 1985, all houses built had significant infiltration (leakage), and many thousands of cubic feet of heated house air would therefore leak out each hour and be wasted. This added considerably to the heating load of the house or building, but it actually naturally assured indoor air purity because any air seldom actually remained in the house more than an hour or two.

After some energy shortages in the late 1970s and early 1980s, houses were built to be extremely tight, because of this very situation. Air infiltration was nearly eliminated. This significantly improved the energy efficiency of the house, but it caused the air in the house to not have any way of ever leaving. Where the smell of a cigar would have left in an hour or two in an older house, it is now trapped for days or even weeks in the new, tight house.

This situation eventually caused great concerns regarding Indoor Air Purity. (It is amazing that no one saw that coming!) Not because any more pollutants were being created in the house, but because the ones created in the house could never leave.

By the middle 1990s, Building Codes were starting to add new rules, where motorized house pressurization or similar schemes were required in new construction. In a darkly humorous sense, they are defeating the entire advantage accomplished by making the houses so very tight only a decade earlier! Some States' new (bad) rules require motorized exhausters, which is a truly bad idea because it causes the house to be at negative atmospheric pressure. That could cause fumes from a car in an attached garage to be sucked into the house, or smoke from a fireplace drawn out into the room as air is sucked down the chimney, or paint fumes from a workshop in the basement be pulled upstairs. Others of the new rules, slightly more thought through, require a similar motorized device to forcefully inject fresh outdoor air into the house. In principle, this is a far better idea, since it slightly pressurizes the house. If any leakage would happen associated with a garage, it would be (warmed) house air being forced out into the garage. Regarding a fireplace, warmed house air would be forced up the chimney. Wasteful of heat, but not a source of Indoor Air Pollution.

By the way, this pressurization approach has existed in most large commercial stores for decades. Did you ever notice how air whooshes (outward) past you as you enter the door of such a store? That's actually unnatural, because the "chimney effect" tends to always suck cold outdoor air INWARD near the ground and it leaks out through cracks high in a building. They pressurize such buildings for several reasons, but that is one of them. They don't want their incoming customers to have to feel an incoming blast of cold outdoor air!

Amazingly enough, some of the new laws realized that a motorized pressurizer couldn't actually push any new (clean) air into the house unless old (stale or polluted) air was able to get out, so they ALSO required PERMANENT openings in the house where this air could leave. Some new windows are built with these permanent openings in the frames! In principle, those windows act like they always remain slightly open! It's sort of hilarious! The older laws mandate super insulation and all that (costing the owner $$$) to make the house more energy efficient. These new laws require expensive motorized blowers and special windows and such which are designed to completely defeat the purpose of the original added expense, only at additional expense! Only in America!

Our Approach

Since some amount of outdoor air must then be brought into the house (by the motorized pressurizer), if the outdoor temperature is below zero, this adds greatly to the heating load for the house, and therefore to the cost of supplying heat for the house (unless you have the Solar Heating System!) In principle, you are bringing in extremely cold outside air and then having to add quite a bit of heat and humidity to it.

We're finally getting to our (winter) improvement! Assuming you have a large yard, imagine digging a trench about four feet deep all the way across the yard, and maybe then even zigzagging around the yard. In the bottom of this trench, place a large diameter pipe, and then fill the trench back in. At the one end of this pipe, have it pass through the basement wall and open into the basement or some other low part of the house. At the opposite end, (for primarily heating systems) have it elbow upwards so it sticks up out of the ground. (Make it decorative somehow!) (For primarily cooling systems, see below, and the descriptions earlier.)

The motorized pressurizer would be set up to draw its air through this long underground tube. When the air was first drawn into the tube at the outer end, that air might be below zero. But the ground several feet deep is much warmer. In the Chicago area, for example, it always stays around 52ºF. As the air was drawn through the long tube, as long as the tube was designed and dimensioned properly, it would pick up heat from the surrounding pipe and soil. By the time the air arrived at the house, it would have been (naturally!) heated from the original 0ºF to around 52ºF. Only a minor amount of heating would then be necessary to raise it to the 70ºF for the house, around 1/4 of that needed otherwise!

Summer Air Conditioning

Any existing furnace/air conditioning system already has a 'Summer Fan' switch on the wall thermostat. This switch just turns on the blower, without activating the furnace or air-conditioner, and could easily be used to control the airflow through the underground tubes. Alternately, the 'Air Conditioning' switch position on the thermostat could be used. This would then allow the wall thermostat to automatically turn on and off the blower, blowing the house air through the underground tubes as necessary to maintain the desired temperature set on the thermostat! Absolutely automatic! Absolutely identical in usage to traditional air conditioning!

Summer De-Humidification

Interestingly enough, this system usually actually lets you control a comfort parameter that normal air conditioning does not, the Indoor Relative Humidity! Massive government tests have determined that the ideal summer indoor conditions are 76ºF dry-bulb temperature (normal temperature) and 40% Indoor Relative Humidity (IRH) (See any ASHRAE Handbook or any psychrometric chart, as below). Store bought air conditioners were designed to accomplish approximately the right IRH as a function of the temperature, so you don't actually have any control.

This system actually does (if you wish it!) Much of the foregoing discussion has mentioned air returning to the house at 52ºF in our examples. This system can actually be used in that way, but that is usually not the ideal situation. If the air passing through the tube actually gets down to 52ºF, then the great majority of its moisture content would condense out on the walls of the underground tubes. That's a desirable goal, but in this case, we're doing TOO good a job! Once that air comes out into the house and becomes warmed to the 76ºF room temperature, it may have too low an IRH for best comfort.

You can use a standard Psychrometric Chart, as below, if you want to plan the humidity. Say you are starting with air that is 85ºF and 60% indoor relative humidity (the lower end of the red line in our example). The Psychrometric Chart immediately shows that there is 0.0155 pound of water vapor in every pound of the air. When it is cooled, it is first a reversible adiabatic process, which keeps the Enthalpy, the energy content of the mixture, constant (the red line in our example to the left and upward, along a line of constant Enthalpy) and the actual amount of water vapor does not change, so the movement is to the left on that Chart. As air is cooled, it can hold less water vapor, so the RELATIVE humidity rises. You can see the red line cross the higher relative humidity percentages in the chart. In this case, you can see that by the time the air is cooled to about 74ºF, the relative humidity has risen to 100%. Up to now, no de-humidification has occurred. But once the relative humidity has gotten to 100%, further cooling requires that water condense out, in this case, on the walls of our underground tubes. On the Psychrometric Chart, continued cooling follows the 100% humidity line downward and to the left (as the green line in our example). If the air is cooled all the way to available 53ºF as in our (Chicago) example, the Chart shows that we would now have 0.009 pounds of water per pound of air. This means that about 0.009 pound of water was removed (during the green line portion). (If the house was 1500 square feet, with 8 foot ceilings, and therefore contained 12,000 cubic feet of air, or around 930 pounds of air, then 930 * 0.009 or about 8 pounds of water was removed, more than a gallon.) When that air becomes heated to the desired room temperature of 76ºF, we go isentropically down to the right on the Chart, to find a resulting IRH of about 20%. That is too low for best comfort, and NOT inside the Comfort Zone of the ASHRAE Comfort Chart (a special version of the Psychrometric Chart). Therefore, conventional A/C units (and our recommendation) only cool the air down to around 60ºF, as we have shown in our example green line. Then, when that air warms up to the desired 76ºF of the house air (as along our blue line), the resulting Relative Humidity is the 40% that the ASHRAE Comfort Chart recommends as most comfortable (in summer).

We (and all conventional A/C systems) have reduced both the air temperature and the Relative Humidity to the desired levels.

So, how would we control the IRH? By controlling how long a time that individual air molecules would be inside the tube system! There are elegant engineering ways of calculating what cfm of blower airflow would provide final airflow temperature, but it is generally easier and more accurate to just measure the (web-bulb) temperature as the air re-enters the house. Or the Psychrometric Chart could be used to determine it. If it is higher or lower, just adjust the air flow cfm through the tube system so that the air returns at that saturation temperature. For locations where the ground temperature is higher than that 60ºF, the house humidity level may be higher than desired, and, under some circumstances, a de-humidifier may be necessary in the house.

It might seem that changing the air flow through the tubes, and therefore the temperature that the air returns to the house, would affect the overall system Btu/hr capability. In general, it doesn't. A LOT of air coming back at 60ºF has as much cooling effect as a lesser quantity of 52ºF air. The actual coolness being delivered is proportional to the PRODUCT of cfm and temperature differential. So, with 80ºF air entering the system, either 1000 cfm at 52ºF or 1400 cfm at 60ºF, would provide the same Btu/hr of cooling. Only the IRH would be different.

Depending on the climate of the house, it might be desirable to arrange this system for primarily or exclusively A/C operation, with little or no concern about winter benefits. In such a situation, there can be additional benefits from looping the tube around so that both ends of it come through the basement walls. Ducts (with dampers) would connect the existing furnace (or air handler) ducts to this path. At whatever point the wall thermostat would call for cooling, that existing blower would turn on and appropriate dampers would move so house air is blown through the underground tubes. This recirculating method has certain advantages, like better control of house humidity, better usage of the available cooling effects (higher net efficiency), and air filtration advantages. A pure recirculating system would have the potential of the super tight house Indoor Air Purity concerns. Probably the ideal solution for a primarily cooling installation would be a primarily recirculating system with a small intake provision for bringing in a little make-up air (for pressurizing the house).

With the underground tubing, the effect of all this is much more prominent, and the moisture that condenses out of the air is collected and removed, very much like a normal air conditioner does.

The basement floor has plenty of "interface area" so short-term performance can be great. If you live in a climate where air conditioning is only needed for a few hours at a time, you could probably get most of the benefits of air-conditioning from just recirculating your upstairs house air through the basement, using the existing house blower/air handler.

If there is NOT thermal insulation under the basement floor, then fairly simple engineering shows the short-term benefit you can get from this cool basement floor. Say the house is 25 feet by 40 feet, so the basement floor is 1,000 square feet. If the ground underneath it is at 52ºF and the house air is at 80ºF, then the "cooling effect" is seen to be (1,000) * 8 * (80-52) = 224,000 Btu/hr! That would be PLENTY to cool your house, and that's why recirculating the upstairs air through the basement can quickly cool the house. Just sending that air through the basement does not actually send all of that air right along the basement floor, so the "basement effect" winds up to be far less than 224,000 Btu/hr, but can definitely be the 36,000 Btu/hr of cooling that your house actually needs. For the record, if you do NOT cause the house air to blow along the basement floor, and leave it to natural convection, instead of 8 in the formula above, ASHRAE says that it is 2.347; also, the basement air is probably nearer to 65ºF instead of 80ºF, so the calculation gives around 104,000 Btu/hr of cooling, again, explaining why basements stay cool! (Keep in mind that this approach does NOT remove moisture from the house air, so there is no de-humidifying effect and a separate de-humidifier would be needed).

However, if your air conditioning needs are for more than a few hours at a time, this approach will soon lose its effect. Gradually, the soil underneath the basement floor will warm up. Since it is a finite volume of soil (basically the size of the house), once it has all warmed up, the cooling effect would be greatly reduced, until an extended period of non-use occurred so the soil could again cool back down. Anyone who has tried to cool their house in this way has noticed the reduction in cooling effect over time.

Any Engineer can solve this Integral Kelvin equation to determine this effect. (By the way, this is the scariest of the equations involved! And the Technical information Packet described below includes a table of solutions for this equation for all practical situations.) If a common, fairly dry, Midwestern soil is under the house, and if this example house needed continuous cooling of 36,000 Btu/hr, the solution shows that the soil a foot below the basement floor would have risen in temperature by 24ºF after just one week! By then, the floor would have been at 76ºF and there would have been no cooling effect at all. Even after just a couple days, the cooling ability would have dropped to about half, because the soil down there would have heated up to around 64ºF. With a more moist common Midwest soil down there, the effect is only half as bad, with a decent cooling effect existing beyond a full week.

A simplistic engineering approach could also be used to roughly estimate long-term performance. In VERY approximate terms, the soil temperature one foot down would probably reflect the overall effect on TWO feet deep of soil down there. One thousand square feet, two feet deep is 2,000 cubic feet or about 200,000 pounds of soil. The specific heat of dry soil is around 0.3, so the heat capacity of this mass of soil is around 0.3 * 200,000 or 60,000 Btu/ºF. If 60,000 Btus are put into that ground, it would rise an average of 1ºF. Since we are talking about putting 36,000 Btu/hr down into that soil, that's 864,000 Btu/day. This implies that the soil would rise in temperature by around 14ºF in a 24 hour period of operation, relatively in line with the solution of the more precise Integral Kelvin equation.

These comments are included to emphasize the need for calculating the long-term performance. Even though a basement floor starts out with incredible short-term cooling capability, in just a few days of use, that cooling effect gets depleted. Larger volumes of soil need to be involved when extended periods of cooling as necessary. The network of underground tubes accomplishes this.

We had mentioned using nine parallel 4" tubes for the total of about 120 square inches of (total cross-sectional) area for the airflow. Or, for larger houses that need greater airflow rates, 12 parallel 4" tubes for about 150 sq.in., or 16 parallel 4" tubes for around 200 square inches of air-flow (duct) area. We have discovered that some people have read this page and decided to "improve" on it by making one very long air path of a single 4" pipe 500 or more feet long. Well, that WOULD actually work, but it would have a disadvantage. In the same way that firemen use a 3" diameter hose instead of a 1/2" garden hose (to carry far more water for putting out the fire), a single 4" air path would greatly limit the amount of air that could be cooled. True, it would be cooled really well, but there would be very little air flow through the tube, the effect being a lot like the garden hose trying to supply enough water to put out a big fire. The larger area is very important!

Now, if you have 6" pipe available to you locally, roughly five parallel tubes would be necessary to match the air flow through the nine 4" pipes. In that case, the air flow would be fine, but the total tube surface area would be slightly less (making the short-term system performance a little less, and the amount of soil within three feet of a tube would only be 5/9 as much, substantially reducing the long-term performance. So, before you go changing any major aspect of this system, make sure you understand all the consequences of that change!

Four inch PVC drain pipe should be available everywhere. Six inch is nearly as widely available, but usually a lot more expensive.

Some people might fabricate a round or rectangular duct of some sort, and have the branch runs come off of it. A second similar possibility is to just have one large 15" diameter PVC Sewer Main duct come through the basement wall and have all nine separate 4" PVC pipes elbow branch off of it, but that requires more design engineering and probably more expensive large tubes.

We think it is most economical to simply continue all the tubes, in a bundle (3x3 for nine, 3x4 for twelve, or 4x4 for sixteen tubes) back to the building. This should minimize any chance of future leaks at any joints or connections. Better still, once it is all done, IF you discover that you accidentally made one joint badly, where one tube has a leak, that tube could be plugged up (at both ends) to eliminate it from the system!

Specifically, there are apparently furnace wall thermostats which are Bluetooth compatible. We understand that one use of that capability is to have a standard PC (computer) check and record the temperature regularly, as well as recording the minutes of use of the furnace and the conventional air conditioning system. Well, if TWO such thermostats were used, one inside the house as normal, and the other down inside one of the tubes, then the PC could record and track some really useful data. In the hours that NO airflow was going through the tubes, then the temperature readings would record how quickly the underground tube system was recovering from previous usage. In other words, making a record to be able to predict how quickly the system would be able to regain cooling ability after a very hot afternoon! This would essentially be the LONG-TERM performance guide. WHILE the air was going through the tubes, and knowing the current temperatures of the house and the tubes, the gradual increase in the air temperature inside the tubes would be an indication of SHORT-TERM performance. This might be a valuable addition to this type of installation. Therefore, if there are people out there with good knowledge of Bluetooth technology, and/or access to those Bluetooth-compatible wall thermostats, we encourage experimenting! And whether results are useful or not, please let us know so that we can inform others of what seems to work and what does not.

Ever since we first started offering this system for free in late 2000, we have also offered a Technical Packet, too. We have always asked people to make a Donation to ANY Charity or Church of their choice, anonymously, of $250, for a collection of equations, formulas, charts, (pre-calculated solutions of that Integral Kelvin equation!), and a lot of additional guidance regarding designing of the tube sizes, materials, lengths, and a bunch of general suggestions. If you happen to be or to know a Thermodynamics Engineer, he could probably do all of this for you and you wouldn't have to pay anyone anything! The planning involves the size of the house, the climate it is in, its estimated heating/cooling load, the number of members of the family, the size and shape of the yard available, the type of soil, etc., to determine the diameters, configurations, patterns, depths, etc. of the components of this system. Many variables are sometimes involved, including mountains, lakes, forests, and other local conditions.

For many climates, the necessary yard area that would have to be dug up for this network of tubes might only be 50 feet square! If you followed our "basement" discussion, this area of tubes would involve a volume of soil of about 56 feet square (about 3,000 square feet) and about double the vertical depth of soil (because the basement "hole" is not there). In other words, even this moderate area of yard could be used to supply around SIX TIMES the long-term cooling effect of the basement floor example. (3,000 * 4 or 12,000 cubic feet of soil instead of 2,000). In hot climates, a larger area of yard would obviously be necessary.

We don't want to LOSE money as a result of providing this offer! It isn't fair when many people ask for a lot of free engineering as well, seeming to have the attitude that we are somehow RESPONSIBLE to try to "sell" this system! IT IS FREE, with all the necessary information provided to you in this and linked pages. And even if you have doubts and insist on duplicating the calculations and logic for yourself, the Technical Packet even enables you to do that. It is fairly time consuming to do all of the necessary calculations and Engineering, especially when thousands of people expect this to all be done for them. We have no problem with a MODERATE number of questions, and we do our best to find the time to answer all of them, within reason. (But some people consider it reasonable to ask fifty to eighty very specific technical questions!) We encourage you to find a local Civil Engineer who could equally do the math. We created this Engineering presentation because we have already done a LOT of research and collected all the useful stuff in that collection of information. Prior to our deciding to provide this for free after watching the California energy crisis late in 2000, we had fully Engineered (and used) this system as a sub-system meant as part of of the NorthWarm Version 1 100% solar house heating system. We are hopeful that the $250 required Anonymous Donation for the Technical Packet (which is generally not even actually necessary) would not cause a hardship on anyone for that information.

By the way, we really LIKE the fact that many thousands of people, world-wide, have indicated that they planned to install this system. But just as much, we like the fact that, so far, in mid-2007, over 4,900 people have requested the Technical Packet. Assuming that they each fulfilled their word of Anonymously Donating $250 (US) to some Charity or Church, that means that those people have contributed over a million total dollars, all anonymously, to many different worthy causes! We REALLY like that! Can you imagine a Soup Kitchen that finds an envelope of $250 in their mailbox one day? Does it get any better than that?

Doing This Without Our Help

Water Condensation, Fungus, Etc.

You know how house rain gutters slightly slope, so they drain? Just an inch or two in a ten-foot length? That MUST be done for these underground tubes. NO corrugated tubing should be used, because it would trap such condensed moisture in lots of little puddles. The tubing should have a smooth interior. Since it is relatively hard to confirm that a minimal slope has no low spots (far harder than for gutters), it is generally a good idea to provide a somewhat greater slope than normally used in house gutters. The slope should probably go downhill along the direction of the airflow, so the moving air would tend to push the water along. This could terminate in a central condensate collection point in the tube system (possibly with an automatic sump pump), or it could continue all the way back into the house, where the water would be collected and then sent down a sewer drain. These considerations would eliminate any danger of a puddling or bacteria problem in the tube system.

There is actually an alternative available if you are not interested in the de-humidifying ability of this system. You could install a much stronger blower! The air would be cooled by passing through the underground tubes, so you would get the cooling part of the air conditioning. But if the air passes through the tubes so fast that it can never get cooled below its dew-point, then no condensation could occur, and you would eliminate the de-humidifying, which means no moisture would be removed from the house air and so no condensation droplets could ever form inside the tubes. We think this is foolish, in giving up half of the benefits of air-conditioning (the reduction of humidity), and we strongly encourage you to just make sure of adequate slope in all the tubes.

We have noticed that a lot of people are very concerned about even the possibility of accidentally not providing good drainage of the (hidden) underground tubes, and thereby having some sort of health consequences. We are confident that proper sloping for good drainage can absolutely eliminate this concern, however we have figured out a variant of this system for people who are really worried about this. The variant does not ever send the house air underground, and the condensation of humidity which occurs is exactly the same as with all other central air conditioners. This variant involves TWO heat exchange processes, which causes it to be slightly less efficient, but it certainly cannot have any underground puddles causing things to grow in them! You need to generally understand the system described above, in order to follow the differences described in water-filled variant.

NOTE: Before you go digging, make absolutely sure that no easements are across the property. You DEFINITELY do not want to dig into high voltage electrical cables or gas mains or water mains!

But, let's say you don't get our help and you happen to not bury enough pipe. Well, even in that case, you come out fine, because the system would do much of the air conditioning (depending on how well you planned the piping) and will greatly reduce your air conditioning bills anyway. So, even if you somewhat mess up by not doing any preliminary engineering, you still win!

Hot Climate Applications

The trenches should be as deep as can be managed (without danger to workers regarding collapses!), because in very extended hot weather, the top several feet of soil can get heated by a number of degrees. There is a VERY simple way to know how serious a problem this might be, regarding trench depth. On an August afternoon, after a long stretch of hot days, turn on a house water faucet of the Cold water, but from a MUNICIPAL supplied water source. That water gets distributed to all the houses by a pattern of water main distribution pipes, generally around 3 feet deep. So, after you let that Cold water run for about a minute, accurately measure its temperature. Whatever that temperature is, is probably very close to the temp of the three-foot-deep soil at that moment.

If it is 70ºF or below, the air-based system described above should be able to provide decent cooling (but with a larger underground tube pattern, maybe double in size). But if that water comes out at around 70ºF or above, then the benefits might be limited. But there are still some possibilities!

If you can dig a lot deeper, you can reduce the effects of the gradual summer heating of the soil. It is well known that at a depth of around 12 feet there is only one or two degrees of annual temperature change, and that it is actually delayed by several months to essentially completely eliminate the effect. At 8-foot depth, the delay is around two months and there are only a few degrees of seasonal difference at that depth. Shallower means that the seasonal differences are greater, and that the heat gets down there in weeks rather than months. We initially suggested the three- to four-foot depth for the tubes, for applications in MODERATE climates, and because fairly simple and inexpensive ditch-digging trenchers can be rented by the hour or day to do that. For deeper trenches, more involved planning and more expensive equipment needs to be involved.

Both of our main suggestions then involve the EXACT same buried tubes, but now filling them with water (as more fully described in our separate page on the water-filled version, linked a few paragraphs above). The simplest is to then use it IN COMBINATION with an existing central A/C. Normally, existing hot house air might be at 90ºF when it gets drawn into return air ducts to be blown through the A/C coil again, and the A/C then has to do a lot of work cooling that air down. Think now of installing a standard car radiator (or a more sophisticated and expensive equivalent) in that return duct, just before the A/C coil. The relatively cool (70ºF???) water from the underground tubes is run through the radiator with a small pump. As the house air passes through that radiator, it gets cooled! On mildly hot days, that cooling might be enough that the actual A/C does not even need to turn on! But on extremely hot days, the house air may not be cooled sufficiently by the radiator, and so the conventional A/C might need to turn on. This is still good! The conventional A/C would have a far easier job, as the air getting to it is already greatly cooled. AND it would probably have to run for far less time. Both effects would greatly reduce the electricity bills due to A/C.

The other main suggestion is more complex to install. It is also a water-filled system, but instead of just small tubes being underground, there would be either LARGE pipes or even a large tank buried as deep as possible! We will use here an example of a 1200-gallon concrete septic tank, placed in as deep an excavation as possible (here suggested, 10 feet deep). That tank would be carefully sealed shut (lid and body, and also around the connection holes) so there were never any leaks.

Here is the reasoning for that approach. Twenty-four hours every day, that tank would conduct heat away, including at night. The primary extreme heating of the house is during the afternoon, when maximum A/C is needed. The 1200 gallons of water in the tank should then generally be able to cool back down to around the 70ºF by every morning. That much water weighs around 9,000 pounds, so if the A/C heat removal from the house during the hot hours of that day were, say 90,000 Btu, that tank water would still only get up to around 80ºF and still be able to provide some cooling effect to even hotter house air! (During that night, it would cool back down!) We note that this only represents around 30,000 Btu/hr of full cooling for three hours, but we see it as better than nothing! Also, if THREE such tanks were buried in different parts of a yard, then they would have available as much as 270,000 Btu of total cooling, which might be enough for most houses!

Note that these water-filled systems have the water permanently trapped inside, which eliminates any possibility of the mold and fungus that could occur if an air-filled system did not have good drainage slopes. But the water-filled systems are slightly less efficient, because they require TWO heat-exchanges (tube wall to water, then water/radiator to house air) where the air-filled systems only require ONE heat-exchange (buried tube wall directly to house air). Also, before sealing it in, the trapped water should be given some pool chemicals or the equivalent to reduce future growths or contaminants.

Depending on the local cost for usage of a backhoe, or if you happen to love to shovel(!?), the installed cost of this system may be less than a conventional central air conditioning system. Depending on the climate and house size, the necessary pipes could cost around $500. Depending on how much a backhoe or trencher costs in your area, that might also be around $500. This suggests that some installations could realistically be put in for well under $2,000. If you can put in LOTS of narrower tubes, a Ditch Witch Model 1820 trencher can make 4 foot deep trenches for around $220 rental per day. And you essentially eliminate ALL those huge summer air conditioning electric bills! And you get significant energy savings in winter, too! Forever!

In nearly all cases, the existing furnace blower or air handler, and wall thermostat could be used, so there's virtually nothing necessary except for the tubing and the trenching. And there's nothing bizarre about operating the system, either, since the normal wall thermostat would be used exactly as before.

We believe this to be a feature that nearly all houses could benefit from. Considering recent large price hikes for electricity and natural gas, we felt it appropriate to present it as a separate system, where it has always been considered a relatively minor part of the full Solar Heating System.

Even if you have already paid for an existing central air conditioner, this intake arrangement could quickly pay for itself in combined heating and cooling savings. Just do the cost calculations suggested above to find out what YOU might save. And, even if you happen to be in a climate, like Miami, where you might feel it too involved and costly to bury all the necessary piping for an entire system, any size system that you would install would greatly reduce your air conditioning electricity costs.

Since we're basically telling you how to generally do this, for free, we feel it's fair to ask a single favor in return. If you happen to live in or near California, and you install this system, please call ANY local newspaper, radio or TV reporter to look at what you did. We don't really care if we get any credit in the matter, but it's important to get the word out to all California homeowners that they each have a way to greatly reduce their summer electric bills. And, if enough of them actually do that, collectively we might help avert a big summer problem of blackouts out there. In the process of this, you might even get yourself on TV, if that's important to you! It might seem surprising, but WE don't want any publicity from this effort at trying to help California deal with a big problem. We just believe that we have a grass-roots solution for it, and that people should help one another.

However, there are some known examples of SIMILAR installations:

It may be of interest to you that I built a 700 m² (6,500 square feet, extremely large) house near Sydney with six 250mm (10 inch) cool tubes. With the help of the University of Athens SUMMER programme I modelled my site and calculated that I would get 82% of my cooling in lines only 30 metres (100 feet) long. I buried them in a radial spread 3.5 metres (10 feet) deep in clay which has a year round 18.2ºC (65ºF) subsoil temperature. I have abundant cooling, virtually no refractory period and can keep the house at any temperature from 24ºC (75ºF) upwards effortlessly all year round. You will appreciate that with a climate similar to Houston Texas, air conditioning a huge house for an overall cost of $15.60 a year attracts many visitors.

I'd add to your summary that with high dehumidification, molds etc are a real problem but with a slope away from the house to a drainage sump I then use slow release chlorine tablets which knock this stone dead. When working gives a very faint ozone smell in the house which is quite pleasant. Not sure what the equivalent brand is in the US but marketed by Amatil for use in pools. A handful once a season does the job.

Thanks once again for your excellent summary.

Jim McKnight

(Note that he sloped the tubes AWAY from the house and therefore had condensation water accumulate at the outer ends of his star pattern of tubes, where he then has to sump-pump it out, meaning there are ALWAYS puddles out there. Our approach is very different, where we want the tubes to always slope slightly downward, as with rain gutters, and so the moving air tends to push along any droplets of condensation all the way through to a collection drain just inside the house.)

(His house is five times the size of most American houses, so he used larger diameter tubes and longer. He also buried them deeper, definitely a good idea, but really deep digging like that can be rather expensive, and our description here was to provide a reasonably economical installation. Our thoughts are that if an owner liked the benefits but would like a little more cooling, he could always later bury some additional tubes in a different part of the yard.)

The recent editors seem to have adopted a few concepts from our presentation but also still rely on some incorrect conclusions of the early 1980s. The main point regarding that web-page is that it has now been altered to see some value in the concept, although they still give bad guidance regarding the diameter of the tubes, and their web page now even admits that they have limited information! One great problem with nearly all of the installations of the 1980s was that they were single-pass, open-loop systems. Such systems were doomed to perform poorly, just as a single-pass, open-loop central furnace would do! Any well designed furnace or air-conditioning system gets much of its benefits from recirculating the house air over and over, therefore only having to slightly change its temperature, rather than forever having to heat or cool every cubic foot of outdoor air!

It is frightening that the government web-page referred to aluminum tubes as acceptable! ANY Engineer knows how rapidly aluminum corrodes in contact with damp soil due to electrolysis! NO reputable Engineer would place ANYTHING that was aluminum in direct contact with damp soil! (This is an example of weaknesses in the government's web-page!)

But they seem to have added comments (possibly from our site) regarding different types and moisture contents of soils and some comments about condensation/growth issues, which didn't use to be in that web-page.

This presentation is a GIFT! We didn't try to SELL you anything, or even try to FORCE you to make any decision! This information is LOOOOOONG because we tried to provide sufficient information and logic so that you might get a basic understanding of the concepts behind it, and that we attempted to include a reasonable amount of information regarding actually installing it.

We get a lot of notes that complain about how long this is. Sounds somewhat like Gift Horse to us! But a large number of people INSIST that we provide the names and addresses and phones of people who have installed such systems! We feel that is incredibly offensive! How dare people insist on bothering people (WITHOUT then being expected to pay any money for any product!) just because they happen to not understand the value of something? Hey, go ask for the pedigree papers on that Gift Horse!

Other people seem amazed that these web-pages do not include testimonials of people who installed the system. Again, if we were trying to SELL such a system, that would make complete sense, in trying to convince you to want to give money to us! But since that is not the case, we really see no reason at all to feel any need to provide such testimonials. Do you really believe all those TV ads where people claimed to have lost 80 pounds in three weeks while eating everything they like?

If you choose to find no value in these pages, fine! You are free to do that. If you are not sure, you are free to hire some local Civil Engineer or Thermodynamicist to examine these pages to tell you whether or not there is any value here. But we have pretty much gotten tired of responding to Gift Horse e-mails, and probably will never again respond to such notes!

• For NEW houses, where maximum performance is desired, so that an ENTIRE large house can be TOTALLY heated, in most climates, with ONLY solar energy! Solar1

  Performance and storage will be extremely great. The VERY large collector area and storage space make it certain that NO back-up heat will EVER be necessary for heating the house.

• For existing houses, where room for a two-car-garage-sized building is available nearby in the yard. Solar2

  Performance and storage will be great, but the necessarily smaller collector area and storage space make it possible that back-up heat will sometimes be necessary for heating the house.

• For existing houses, in very rural areas, where an array of tracking collectors and a high-temperature boiler system would be of advantage. Solar3

  This liquid-based version has potentially higher installation costs than either of the air-based systems above. It would certainly involve much more maintenance time and cost. It also has all the advantages and disadvantages of a water-based system.

• For existing or new houses. This system is quite different from all of the above. Solar4

  It has very good performance but has intrinsically less performance ability than any of the other three Versions. It is a low-tech approach to solar heating.

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How does the Sun create so much energy?
Inventions Which Might Help Deal With Coming Energy Catastrophes
An Invention to Efficiently Make Electricity from Solar
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A Method of Storing Summer Heat to (Nearly) Entirely Heat a House all Winter
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The Physics of Wood as a Heating Fuel
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A Possible way to greatly reduce Aerodynamic Drag of Airplanes

Solar Heating