Underground A/C Alternate System

There is an alternate way to use the Free underground air conditioning system. Where the standard system sends the actual house air through the sealed tubes to be cooled, a two-step system can also be used, where all the tubes are filled with water instead!

This version has one important difference from the air-filled version. It MUST use STANDARD (called schedule 40) PVC plastic piping rather than the very thinwall PVC FIELD TILE that we suggest for the air-filled version. Unfortunately, this thicker pipe is somewhat more expensive, nearly double! The reason that it MUST be used for the water-filled version is because the joints and couplings are much more assured of being absolutely water-tight.

There is a reason why the standard system is favored by us! Since it sends the actual house air through the cool underground tubes, there is ONE heat transfer that must occur, from that air to the ground surrounding the tube. When the exact same system (same dimensions, same size tubes, same depth, etc) is all filled with water, then TWO heat exchanges must occur. The first is from the house air to water passing through a heat exchanger which is in the air path of the forced air furnace system, and the second is from that water to the soil through the tube walls.

In some climates, this is not a problem, but where the soil is generally pretty warm, the performance of the system can be less. Every time a heat exchange is to occur, a difference in temperature is required between the two materials (air or water or underground soil). The greater that difference is, the better the heat exchange works. This is why your car heater does not seem to work very well soon after you start your car, because the 100°F is driving a heat exchange to 70°F air in the car. Once the engine is up to its normal 195°F temperature, the differential is much greater, and even the little heat exchanger in a car heater can give plenty of heat.

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In the case of 85°F house air and 53°F ground, the heat exchanger needs to be pretty big (all the tube area described in our main presentation) but it works excellently. If that is the ground temperature, using a two-stage heat exchange still works very well. Not quite as good as a one-stage would do, but still very well. In the case of our system, it even has an advantage! The water inside the tubes gets cooled by the ground 24 hours a day, every day! This generally results in the water being only a few degrees above the ground temperature, which then allows the second heat exchange to work well. The large amount of water trapped inside the tubes is important in this effect. If really small diameter tubes were used, so there was only a few gallons of water in the system, the water temperature would be much higher than the ground temperature during operation, and the second heat exchange would work far more poorly.

In practical use, with the same 4" diameter tubes we recommend and the field pattern, there are around 500 gallons of water sealed in the system! This generally permits between 80% and 85% as much cooling performance as with the air-only system. Like before, since the materials are so inexpensive, it really makes sense to just dig a couple extra trenches and put in 15% or 20% more pipe!

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The main advantage in using this approach is that the house air is never underground! It gets cooled exactly like it does with any other air conditioner, in a heat exchanger in the air path inside the house. Whatever humidity condensation will occur simply occurs on the outer surfaces of that heat exchanger (as with all standard air conditioners) and it get collected and disposed of with a standard tray and hose arrangement. This eliminates any remote possibility of any puddle of standing water existing inside the underground tubes, as a result of insufficient drainage slope in those tubes, since the humidity from the house air loses its moisture down inside the tubes. Therefore, no possible mold, mildew or other growths inside the tubes. Some people might buy a small quantity of swimming pool chemicals to mix into those 500 gallons of water trapped in that system. Since that water never gets very warm, and is never exposed (being sealed in) to air or any other sources of such biological organisms, the chemicals should essentially last forever and never have to be replenished.

There are two main disadvantages of this approach. One has already been discussed, the fact that two separate heat exchanges must occur and that they each need sufficient temperature differential to operate well. The second is that, in addition to the house furnace blower, which must be working, an additional pump must work to circulate the water through the system. Since we are so high on saving energy (and dollars) we see this as extra operating expense.

Now, for people who do not have a forced air furnace, and have hot water (hydronic) heating instead, this version is perfect! The same original hydronic circulating pump can push the water through the underground tubes, with just a simple solenoid valve as though it was an additional "zone" of the water distribution system. No extra operating cost would be involved, and installation is extremely simple and inexpensive!

But for people with forced air heating, we REALLY encourage using the one-stage air-only approach, as long as you are careful about sloping all the tubes so they would drain, just like house gutters. One of the nicest things is that, once the tubes are installed underground, nothing out there would need to be changed to use either water or air! The only differences are inside the house!

Instead of the tubes all coming together into an "airbox" they would be similarly joined together in a "waterbox". Yes, an immense number of PVC tees could do it, too! The (two) waterboxes could be made out of concrete, painted/sealed on the inside to be absolutely water-tight. Each box would also have a separate 2" PVC pipe connection, which each would connect to the two sides of a standard car radiator which is installed in the airpath just above (after) the furnace, and standard flexible radiator hoses would connect the radiator connections to the PVC pipes. A circulating pump having at least 1.5" inlet and outlet connections would be installed somewhere along the PVC lines. This provides a sealed chamber which contains the water, all the underground tubes, these PVC pipes, the pump and the radiator. As long as you have no leaks, no water or anything else can get in or out!

The circulating pump would turn on by the usual relay that normally turns on the air conditioning compressor, so the standard wall thermostat works perfectly for controlling this version. The wall thermostat is already set up to turn the furnace blower on (but not the burner) when air conditioning is called for, so turning on this pump is the only (very simple) wiring necessary.

There are two details that you must add! One is a tee and valve somewhere near the highest point of the PVC piping, so you could attach a water hose to first fill the system. The second is another tee near the same place which simply goes straight up several feet with a cap on it. If you have seen the water piping near a washing machine, there are usually these odd-looking extensions there. They are meant to collect any air in the system and to provide a cushion for expansion and contraction of the 500 gallons of water in the system. Water doesn't expand or contract very much, but it does a little, and you should provide for it. Regarding washing machines, those tees are installed to prevent "water hammer" a loud and potentially damaging surge of water pressure when the washing machines water solenoids open or close. Our use is somewhat different, but it still should be done.

As we said, if you use this water-based version, we recommend that you increase the amount of field tube lengths by 15% or 20%, to make sure that you would still get excellent cooling!


Yet Another Version

There is yet another variation of this general concept! No trenches or buried tubes would be involved in a yard. Instead, part of a (concrete) basement floor would be removed, and the space for a water storage tank provided down there! For most climates, we suggest a 10-foot square space, four feet deep, all several feet BELOW, UNDER where the basement floor is now. The bottom of such a tank is therefore quite deep, ten or twelve feet.

That tank would provide TWO nice functions! First, the 3,000 gallons (or 25,000 pounds) of water in it would normally tend to be pretty close to the natural deep ground temperature, in our examples, around 52°F. With a circulating pump and the radiator as described above, this water provides INSTANT air conditioning of around 25,000 * 20 or 500,000 Btus of cooling. That's an always available 15 hours of 36,000 Btu/hr of cooling! In many climates, that would provide for several days of total cooling. However, it has another benefit. Once you have used the air conditioning and the water in the tank has warmed up to say 60 or 65°F, there is a natural replenishment of cooling that occurs, where that (warmed) water loses its heat downward into the ground. With this size tank, this adds about 384,000 Btus of additional cooling in each 24-hour period. Even if you had the A/C running continuously at full bore, the tank would tend to auto-cool itself to essentially keep up!

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