Underground A/C Alternate Compact but Technical Version

This presentation is also intended as an overall LESSON regarding the design issues that need to be considered in order to get any of these systems to operate at best efficiency.

Unfortunately, the bulk of readers of the underground air conditioning presentations I composed only seem to read the first few paragraphs and then conclude that they do not need to read further or think more extensively. After all, the concept is REALLY obvious, isn't it?

Well, it appears that many thousands of people have done that during the past ten years that these presentations have been provided. And the odds are that most of those installations probably DO give some indication of doing some cooling! But as a Physicist, I know for a fact that most of those installations will likely seem to work great for FIVE MINUTES or even for an hour, but then they stop providing further coolness! And the owners certainly do NOT know why!

THIS presentation is about a different variant, but the basic Engineering involved is similar. I figured that maybe if people SEE all the Engineering that I put into the systems in determining EXACTLY how it should be assembled, which turned out to be MONTHS of FULL-TIME WORK in doing the math and Engineering, MAYBE a few more people might pay closer attention, in order to have a system WHICH REALLY WORKS EXCELLENTLY!

This specific presentation is the most COMPACT system which can be built (and still work right!). But this concept cannot really be built as a GENERIC system like the main one or the active one is. It is VERY sensitive to any of dozens of possible errors, which requires that the entire system MUST BE CUSTOM ENGINEERED for each installation. And no point in hoping that I would do that (for thousands of people), I won't! But a good LOCAL Mechanical Engineer or some HVAC people should be able to read this presentation and then get a LOT of data regarding your situation and do the Engineering there. They WILL charge you a few hundred dollars to do that Engineering, or maybe more if your situation is especially unusual.

The FIRST step is to collect data! We will refer to specific numbers which I collected for my house which is near Chicago. These include the type of soil present and its characteristics regarding heat capacity and heat conduction, and the moisture content of that soil, as well as the average height of the water table. It includes the temperature and humidity patterns for the weather in that location, INCLUDING the hundred-year-extremes for both. It includes the size of the building, its level of insulation. It includes some information regarding the family who occupy it, such as whether kids commonly leave doors or windows open. And it includes information on the existing furnace and central air conditioning, regarding the blower and the duct system.

Any one of these factors, if ignored, might make this system not work as intended. There are a number of decisions that must be made regarding air flows and water flows which can greatly affect the performance, which we will explore here.

This Compact Variant involves using WATER in two different circulations, mostly because water has far greater heat capacity and heat transfer characteristics than air does.

It appears that most people simply ASSUME that all will go exactly as they might wish! That the coolness from deep underground WILL very simply capture heat from house air and all will be well! So why do any Engineering at all?

Public Service
Self-Sufficiency - Many Suggestions

Environmental Subjects

Scientific Subjects

Advanced Physics

Social Subjects

Religious Subjects

Public Services Home Page

Main Menu
It don't work like that!

For a heat exchanger to effectively transfer heat from one fluid to another, a temperature DIFFERENTIAL must exist to drive that energy flow. As a starting point, it is common to estimate that a well-designed heat exchanger will require at least 5 degrees differential (F) for proper operation. Maybe you start to see why Engineering is necessary, as this system has THREE separate heat exchangers in it! (ground to water inside buried tubes; that water to a different water flow system which supplies radiators in the rooms of the house; and the transfer to the actual air in the rooms. So we start out with the disadvantage of probably needing at least 15F differential for it to work at all! If the deep soil is warmer than around 60F, this concept therefore cannot provide room air at less than around 75F. That might be cool enough IF you could replace ALL the air in the house every few minutes, but that would require VERY large blowers and airflow! More commonly, a conventional A/C creates air which is around 60F, which then MIXES with room air to provide much larger quantities of the desired 76F air for the house.

So IF the deep soil is warmer than 60F, THIS variant may not be of much benefit!

By the way, the generic underground tube A/C system only has ONE heat exchanger, for this very reason! The actual air from the house is sent down and through the buried tubes to DIRECTLY lose its heat to the cool walls of the tubes down there. That ensures the minimum possible number of heat exchangers, one, so the needed temperature differential to drive that system can be around five degrees (F), where that system can therefore be used in far more climates!

So we now have HOUSE AIR being blown through a water-filled heat exchanger, at an air velocity and mass flow rate that is determined by the (existing) air handler and by the (existing) duct system. I like to use car radiators as the heat exchangers here, for several reasons. They are fairly small, so they could be placed in individual rooms. They are designed to permit relatively free air flow through them, and I commonly like to use the electric fan that is used in vehicles to keep the radiator from overheating, but instead of running that fan at 12 volts, I run them at 6 volts. The result is extremely quiet operation and very little electricity consumption. The low DC voltage also permits compatibility with wind- or solar-sourced electricity! The car radiators also have fairly large diameter water connections, so large amounts of water can be run through them for best performance.

We have that first heat exchanger here, so we can expect to need the first five degrees, down from room air that we desire to provide at BELOW 76F, say at 72F. So we can now expect THAT water flowing through the car radiators to need to be at 67F or lower.

I generally use 1.5-inch standard PVC pipes to connect those radiators to a supply tank (described below) in the basement, where relatively little heat gain occurs due to the large flow rate of the water. So the same water when in that supply tank will also be around 67F. THIS water system is at nearly neutral pressure, and I commonly use car water pumps to CIRCULATE the water from the supply tank to any room radiators. Such pumps require very little power to run because they are not really working against any significant pressure, except for the restriction of the small passageways inside the radiators.

The Supply Tank is not really a tank at all in my applications, but it can be. I generally buy several 10-foot lengths of four-inch PVC water pipe as the tank, and make a skinny tank as long as possible STRAIGHT along a basement wall (where it is out of the way of everything). INSIDE that pipe is a smaller diameter two-inch PVC pipe, which IS the heat exchanger which CONTAINS the 67F water that can be circulated to any of the room radiators. BETWEEN the two pipes is a DIFFERENT flow of water! THIS water is (generally, in my installations) at a very low pressure, in fact at a significant vacuum! Why? Because IF I can find a reliable supply of underground water, or water from near the bottom of a lake or river, I can avoid having to have the third heat exchanger! But then I evacuate that entire system. Why? To AVOID having to have a powerful water pump constantly pumping water upward, only to then let it fall back down a different well! By having this system at a severe vacuum (which only works down to around 22 feet lower supply of water), then NO actual PUMPING action is required! Only a CIRCULATION of water is required, which uses another car water pump! As long as the vacuum is maintained, then water falling down the SECOND well has the effect of PULLING water UP the first pipe! This is tremendously more efficient regarding energy consumption of pumps!

The water described here must not be above around 62F by that requirement of the needed temperature differential in that second heat exchanger.

WHY were there TWO SEPARATE water flows? Because IF you try to run any heat exchanger at such severe vacuum, including car radiators, the very thin heat exchange tubes will immediately slam shut due to the vacuum (or actually due to the ambient air pressure outside it.) The radiator would immediately be garbage and there would be no water flow! TWO water circulations is necessary to keep this problem from happening!

MOST locations do not have an obvious source of large amounts of water from underground or from the bottom of a lake or river, so a trench must be made (as deep as possible, ten feet or more is good), at least 100 feet long, and a water-filled PVC pipe laid down there and the trench re-filled. That buried pipe is the THIRD heat exchanger. At both ends it elbows upward and then it connects to the OUTER chamber in the skinny heat exchanger in the basement.

Since we again need a 5 degree differential to drive this third heat exchanger, we now need to have the soil at no greater than around 57F.

Since the water well temperature map shows Chicago as being around 52F, this means that we have a system which should work fine in that location. There IS still one more consideration, which depends greatly on how deep the third heat exchanger is placed. The hundred-year-extremes of extended heat need to be put in the Kelvin Integral, along with the soil type characteristics and the presence of water down there. The Kelvin Integral gives the CHANGES in the soil temperature over some extended period of time, given some desired heat exchange rate. It is an equation which calculates the behavior of the soil regarding conducting and convecting heat away from a heat exchanger heat source. So where nearly any installation would work great for five minutes, solving the Kelvin Integral equation for 168 hours (if the hundred-year-extreme of heat lasted one week), then it would be known how much the natural 52F deep soil temperature would rise over time. At a ten-foot depth, that solution is only about up to 54F, which is fine. However, IF the third heat exchanger was only buried TWO feet deep, that Kelvin Integral solution gives a result of around 69F, meaning that the cooling system would then stop working!

The remaining aspects of the system to design have to do with the house airflow rate and the appropriate rates of water flow in the two systems. There are many standard thermodynamic equations which are used to calculate these things, and I am not sure there is a lot of value in showing them here. Other than Engineers, no one would likely understand them except for being a bunch of funny symbols!

A central point of this presentation is to show that where it might SEEM OBVIOUS that 52F ground could EASILY cool room air which is at 80F, it can but the picture is a lot more complicated than it first seems!

Consider some things that can happen if you do NOT do any calculations or Engineering:

Say that you used such a tiny blower that it has virtually no effect. The air between the radiator heat exchangers would cool down, but that cooled air would not leave and so no other room air could get cooled. It virtually would not work at all!

Say that you used such a huge blower that room air would all get blown through the heat exchanger every minute or two. Yes, all the air WOULD cool down to very near the temperature of the water inside the radiator. Me3aning that the water inside that radiator would have to absorb immense amounts of heat. Unless all the rest of the system (the other two circulations) were also very great, the water inside the radiator would generally be quite warm. The tremendous circulation of the huge blower would be NECESSARY for it to have much cooling effect. A huge blower generally requires very large amounts of electricity to run. The blower in your conventional central A/C generally requires around 12 amperes of electricity when it is running. So even separate from the even larger amounts of electricity used by the compressor, that blower uses up about 1.5 kiloWatts of electricity, or around 36 kiloWatt-hours if it has to run all day and night, which is around $5 of electricity just for the blower per day!

So there are disadvantages to BOTH too small and too large a blower in pushing room air through the radiator. Good Engineering and some fairly simple math allowed KNOWING the best air flow rate!

When we consider the MIDDLE flow, we have the same considerations. If the water flow is very small, then that flow cannot carry all the heat that is put into the water in the radiator. The starting end of that circulation might be quite cool water, but the radiator end might be very warm. Again, poor performance. And if the water flow is very high, we have similar concerns like the air flow issues. So, again, some Engineering and calculations are appropriate.

And again, the same analysis for the other water flow.

The DESIRE in doing the Engineering is to CHOOSE flows in each loop such that EACH of the three is responsible for about 1/3 of the temperature difference. IF we (in Chicago) know that the basic ground temperature is 52F, then analyzing the hundred-year-extreme heat stretch with the Kelvin Integral, we find that we should expect to have 57F ground available (as a worst possible case). AND if we want the room to be at 76F, then we have a total of around 19F differential to work with. In that case, we would design each circulation to have around a 6F temperature differential.

Whether that sounds easy or difficult, if you know the mathematical formulas, it ain't that hard!

This page - - - - is at
This subject presentation was last updated on - -

E-mail to: Public4@mb-soft.com

Solar Heated House
Home Air Conditioning