Underground A/C Alternate Vertical Version

The standard underground tube arrangement that we describe in the main web-pages is a fine system. It is very economical in cost, potentially only a few hundred dollars for the materials. Because it is normally an air-based system, there is only one heat exchange involved (soil directly to house air through the PVC tube wall), in contrast to the TWO heat exchanges necessary in any water-filled system (first, house air to water inside a heat exchange radiator in the house and second, a water to soil heat exchange through the walls of the tubes underground).

In Thermodynamics, a temperature differential is required to drive any heat exchange, at least several degrees Fahrenheit difference. With our air-based system, the house air can be just those few degrees hotter than the soil to still be able to drive the needed heat exchange. With the tubes being water-filled, the total necessary difference between the house air temp and the soil temp generally has to be several degrees F greater, to drive both of the two heat exchanges. Heat exchanges to and from water work pretty well, so the total net effect of this is not as bad as it might first sound, but it still generally has to be at least a couple degrees greater difference.

This, and the rather nice economical cost, construction, have been the three primary reasons why we have always recommended the air-filled tubes several (3 or 4 at least) feet deep. However, there is one disadvantage of that air-filled-tube approach, due primarily to the tubes only being a few feet deep.

Please re-read our discussion on our main page regarding the effect of different depths of soil.

Specifically, in a really hot climate where there are many long hot sunny days in the Summer, the soil GRADUALLY warms up over time, but only for shallow locations. An inch or two deep heats up within minutes! Two feet deep heats up in around six days. At five feet deep, that takes around 40 days (which explains a lot of why we see value in recommending 3 or 4 feet deep tubes for most climates.) At 12 feet deep, that heat up requires around 4 months, so the temp down there does not change very much at all. But the ultimate is at 16 feet deep, where the six month time lag means that the soil at that depth is only still getting warmer during the winter and conversely! Other than how inconvenient it would be to try to dig trenches that deep, and keeping them from collapsing on you, deeper is always better!

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So the central reason for the vertical-water-filled is to get the heat exchange areas down really deep, as in a well.

You might ask why not use a vertical-air-filled system. That would be GREAT, but the area of each vertical well pipe is limited, and so to match the SURFACE AREA of the 9 horizontal tubes which are each 100 feet long, you would need to bore around 36 separate boreholes for 25 foot deep tubes. Worse, the DIAMETER of the air tubes would need to be near a foot in diameter, while this water-filled version can fit into a standard 4" well (for heat flow reasons). If you like to do that, it is a great way to go! Personally, boring just one or two boreholes for wells or these things is sufficient to get me to try to find some football game on TV!

WATER has a density which is around a thousand times that of air, and then it has a Heat Capacity of around four times it as well, so, without having to dig 36 wells, it is pretty easy to get PLENTY of cool water constantly coming up from the (four or six or eight, as starters) heat exchangers that you install in boreholes of maybe 25 to 50 feet deep. (The DEPTH might be dependent on what SORT of deep soil there is. If you are at a desert, where loose sand is piled a hundred feet deep before there is anything more solid, then water, either natural or any that you might add, would probably instantly seep away and NOT be available for your other tube which wants to suck the water back up to the surface!) IF you do not know what kind of material is down there, I generally recommend boring ONE borehole and setting up a small version of the NON-SEALED system, just to make sure there IS water down there and to confirm what temperature it is at.

As a lot of the heat from the house air is ALSO dumped into the soil, and now right close to where you want the heat exchange to occur, the soil still tends to warm up a little. Most types of soils take a while, some hours, before that heat can be naturally conducted or convected away from the area of the tubes. So, for climates where the soil is initially rather warm (where air conditioning is also needed most greatly), either the (horizontal air) system needs to be really over-designed or it will act as though it runs out of cooling after a severe load of air conditioning for many hours. Once the horizontal tube system has a few hours to have time to replenish its cooling, by ridding itself of all that excess heat you dumped into it, it works fine again!

Obviously, the deeper the tubes are buried, the better this is, because more and cooler soil is down there, and also that there is also a greater chance that the tubes might be at or near the level of the water table, where heat could be carried away very quickly by water flowing through the soil down there. Trying to make really deep trenches is quite hard and more time consuming and expensive, and even potentially dangerous regarding cave-ins. If there are issues with the water table, there can also be problems with wall collapses of the trenches. We felt that around 4 foot deep trenches is as deep as it is generally safe and economically realistic to go with that standard horizontal air tube system. But, as noted above, that limits the long term performance IF the climate is such that severe and long-term air conditioning is to be expected. In more Moderate climates, it works excellently!

A Vertical Approach, Water-Filled

For such climates where the soil near the surface might become too warm too quickly, we now have another approach. Instead of the tubes running horizontally just a few feet below the surface, we are recommending boring water-well-type holes straight downward! Rather than boring one really deep (500 foot) hole, we think it more economical to instead bore maybe 10 holes each 50 feet deep, or 20 holes each 25 feet deep, or even 30 holes each 17 feet deep. These numbers are for a really severe climate, and a less harsh location might do fine with fewer. That is another reason for making ONE initially so you can see what performance you actually get from it. You may then see that you need FOUR more boreholes or whatever other number seems able to provide you all the cooling you may want. You can ALWAYS make some more boreholes in the future if you see need! A central basic idea is to still maintain around the same 500 square feet of tube surface area in contact with the soil, as about 500 feet length of the PVC tubing by its one foot circumference (of the 4 inch diameter). This then provides the same short-term heat-transfer performance as presented before, which is greatly dependent on that area of contact with the soil.

Moisture in the house air would now condense out at the radiator inside the house (exactly as your conventional air conditioner has always done before due to the A-coil) and need to be drained away. From that point on, the system pipes and tubes are always forever filled with water, in order to get best heat exchange!

We think that these vertical systems should NOT be used as an air-based system and ONLY be water-filled versions, unless a solution to the condensation issue is found. Water would condense and accumulate in the bottom of each borehole, with no realistic way to remove it. Also, hiring equipment that can make 12" diameter boreholes is a LOT more expensive than for the standard 4" diameter wellpipe borehole.

The structure of each vertical tube is pretty simple. EITHER a standard 4-inch pipe well housing OR a SCHEDULE-40 PVC (NOT thinwall!) pipe is used. An End Cap CAN BE installed on the bottom, and sealed properly so there is no leak! That End Cap MIGHT be really important but it might even be unnecessary, depending on your soil and climate. You need to ask a local Engineer about that matter. The pipe then would get lowered down and sections added, very much as is done for conventional water wells, again, always making sure that every joint is water-tight. These tubes will HOLD water inside them, but will never be able to exchange water with the soil, that is important!

However, IF you know that the water table is fairly close to the surface, you might be able to skip the End Cap, and let your tube draw water directly from the natural soil (with a mesh screen to keep debris out) If an End Cap is used, where the system naturally stays fuall of water, a much smaller water pump, resembling that in an automobile engine, can be used. If you do NOT use an End Cap, then the system will naturally tend to drain out when the pump is not running. Ask the Engineer to explain!

Note that you MUST therefore make everything absolutely watertight. You do NOT want your trapped water to ever seep out and empty the system, and you do not want nearby groundwater to be seeping into the system.

IF you use the End Cap where your tube is permanently full of water, then you need to SEAL the top of the whole thing TIGHT. You WANT a situation where when RETURN WATER is going back DOWN one pipe, that it automatically PULLS water UP the other pipe in the process! Once that pipe housing is all completed, a similar length of EITHER 1.25-inch PVC pipe OR 1.25-inch diameter black poly irrigation piping is lowered down inside, BUT with its end sawed off at a 45 degree angle. This piping goes all the way down to the bottom, so it rests on the inside of the end cap at the bottom. The angle cut allows water to easily flow in or out of it.

You now have two different places of connection for external pipes to the house, at the top of the 4-inch housing pipe and the top of the 1.25-inch inner pipe.

Here is a drawing:

Water System Connections The blue lines are cooler water from the bottom of the wells which are drawn up an INNER pipe by the effect of a circulating pump (in the house) and then are joined to a (blue) manifold to be able to send larger amounts of cool water to the house. As an example, in the Chicago area, the deep soil IS at around 52°F or 11 °C, and cooler water naturally settles down to the bottom of the vertical tank. This makes the water going to the house to be rather frigid at around that temperature, to then be sent through standard car radiators in various rooms. The cold water passing through the radiator enables a fan to chill down large amounts of house air (such as by a 12 volt standard car radiator fan or by a 110 volt standard blower or fan.)

This drawing should show a larger diameter horizonal blue line, to carry the water coming up from many vertical wells. That (blue, cooler) water gets sent through one or more car radiators inside the house, which can be inserted into the air-handler's air ducts. Therefore, house air being moved by the air-handler has to pass THROUGH the car radiator and be cooled by it (due to the cooler water inside that radiator). More expensive and more sophisticated heat exchangers can be used instead of car radiators, but these really seem ideal for this use. They have LARGE pipe connections to be able to have a lot of cool water pass through them, they do NOT have to withstand any tremendous water pressure, and they are normally intended to have a LOT of air passing through them!

The water which has passed through the car radiator has been warmed up by the hotter house air that has given up heat to this water. This water is drawn with red arrows here and (and which we have shown as being much larger diameter which is not really necessary) referred to as return (hot) in the drawing. Hot is a relative term! That water is probably only warmed by 3°F in passing through the car radiator, so the water may only be minimally warmer than it was when you pulled it up and through the pipes and radiators.

This system is meant to RECIRCULATE the water pretty often. That slightly warmed water is sent down into the deep vertical (larger) tubes to have a chance to be re-cooled by the surrounding deep soil. You probably noted that we designed this to draw water from the well at the BOTTOM where the coolest water would accumulate, and we feed the return water in at the TOP where it will have several minutes inside the larger vertical chamber to cool down. These things maximize the performance of this system.

One other detail that is important. BOTH the horizontal lines (blue and red) need to be well INSULATED rather than laying on hot soil in direct hot sunlight. No sense in wasting the benefit achieved by this system!

There are yet other benefits from how this system is designed. One is that by having everything SEALED, no water ever has to be LIFTED by a powerful pump, but instead simply has to be CIRCULATED by a far less powerful pump. The other reason is a little more scientific! It is because of something called thermosiphon. Say that all the water in both pipes has been raised to 72°F temp, while the soil outside the pipe is at 64°F. The water in the OUTER (called annular) pipe will get cooled by the contact with the cooler tube surface. That water in the OUTER pipe will become slightly more dense due to that cooling. Because it is then heavier, the outer water tends to very slowly NATURALLY flow DOWNWARD which forces the inner water to be pushed upward. Now, where did that water just come from? The very bottom of the whole thing, right? The place where the water would be coolest of all! The water that would provide the best cooling for the house! And it tends to flow even without any pump (although we still recommend using a "hydronic circulating pump" to get the water efficiently back and forth between the house and these vertical tubes.)

We recommend one other portion to this system. Since there will be a number of separate wells, which should be 6 feet apart, they could certainly be bored in a straight line! A single trench should then be dug first, and all those wells drilled down from the bottom of that trench. This would allow the pipe connections from the house to all the wells to be several feet deep, and with good (blue) insulation above them, so that the cooling effect is not lost on the way to the building.

If you use 1.25-inch PVC pipe for the inner pipes, the connections can actually be a little simpler. At the top of the 4-inch pipe, you could attach a Tee first, and then a 4-inch to 1.25-inch reducer coupling or bushing on top. The side connection from the tee would then also have a bushing reducer, and the bushing in the top would be modified (drilled out a little) so that the 1.25-inch PVC pipe could continue completely through that bushing, instead of running into a stop that is normally there. That would allow a pretty secure and water tight connection everywhere. If you use the black poly pipe, making it securely watertight is much harder.

Only 1.25-inch pipe is then required to allow sufficient flow of the water (unless the house is a mansion!) A conventional hydronic circulating pump can circulate at least 10 gallons per minute through that size piping. That is about 75 pounds of water per minute, or 4500 pounds of water per hour. If the radiator(s) in the house hydronic system can capture say 7°F of heat into the water (from the house air), that would be 4500 * 7 or about 32,000 Btu/hr, fine for most houses. Hydronic pumps can circulate far more water than that, so there should be a lot of spare capability.

From this previous paragraph, you can see that if you know the circulation rate of the hydronic pump, and you put (digital) thermometers for the water before and after the heat exchangers, you would always be able to know exactly how much cooling you were getting at any moment!

As noted, before filling in that trench with the connecting pipes, we recommend that you place some blue slab insulation across on top of them, to provide maybe R-10 or R-20 separation from any hot ground above the connecting pipes. The yard would have no evidence whatever that the air conditioning system is under it! (As with the other versions).

Any of the water-filled versions is essentially like any other hydronic heating system or any swimming pool plumbing system. Some bleach or other chlorine should be put into the water from time to time. There is NO significant pressure anywhere in the system. This means that a simple standpipe could be provided to pour some bleach in from time to time. A cap should be left on that standpipe, both to avoid water evaporation and to avoid any smell of the bleach ever getting into the house.

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E-mail to: cj@mb-soft.com

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