Underground A/C Alternate Neighborhood 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 to air through the PVC tube wall), in contrast to 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 inside the tubes underground).

That standard system is designed for use by a single house or building. This presentation is for a water-filled application of the same general concept for a group of homes in a neighborhood.

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 these 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.

A Community Approach, Water-Filled

When a street is paved or re-paved, consider installing TWO additional "15-inch diameter PVC sewer mains, reasonably near to being directly underneath the edges of the roadway, and maybe six feet deep. They must be water-tight, which is the whole point of sewer mains! At at least one of the two ends, they have elbows and a cross-connector so they are connected together. At the other end, there could either be a similar cross-connector or both of the mains could be capped off.

For discussion sake, we will discuss here a standard city block, long-ways, 1/8 mile. The usable part, for our use, is therefore around 600 feet long. We therefore have around 1250 feet of 15-inch PVC sewer main. It is all permanently filled with (clean) water, and that water is sealed and not able to mix with anything else. That volume of water is about 1550 cubic feet of water, which is around 100,000 pounds of water.

If we are talking about an area with a climate resembling Chicago, that water will naturally get to around 52°F temperature, which is the annual-average temperature of that location. Using the concept of the water-based versions of this system, and if we say that we should still be able to get a little cooling even if that tank of water had risen to 70°F (to be able to cool house air down to the desired 76°F, and the heat-exchanger differential requirements), then we have an IMMEDIATE cooling capability of 100,000 pounds times 18 degrees, or 1,800,000 Btus of cooling effect.

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A medium-sized house around Chicago needs a maximum of maybe 3-tons, or 36,000 Btu/hr of cooling. This is almost never needed at night, and the maximum cooling capability of an A/C is rarely needed very long, as they are intended to cycle on and off. So, for discussion's sake, say that a house under these conditions might require 10 hours of A/C at around 18,000 Btu/hr. That house would then need a total of around 180,000 Btu of cooling during that entire 24 hour day.

Also, say that we have five houses on each side of the street, for a total of ten houses that might want to participate in this system. Between the ten houses, if all of them constantly are occupied and need the full cooling comfort, we are then talking about a total cooling need of 1,800,000 Btu over that 24 hour day.

Starting to see the point here?

If each house had two simple 1" polyethylene agricultural pipes, one connected to each of the mains (with what is called a saddle connector) then each house would be able to draw some of that 52°F water frim one main, have that water go through a heat exchanger (which could be as simple as a standard car radiator), then a fan or blower in that house would create cooled air as it passed through that heat exchanger. That cooled air could go through the same ducting that standard A/C uses, and even use the same wall thermostat for control. From the occupant's perspective, it will be exactly as though a conventional central A/C was operating! (The second poly pipe is to allow the water that has absorbed that heat from the house air to go back down, to the OTHER main underground). A standard hydronic circulating pump is needed in each house, and that (small) pump is controlled by the wall thermostat. If the house is cool enough, the pump stays off and nothing happens. If the house gets too hot, the wall thermostat switch closes, which turns on the hydronic circulating pump and the fan/blower, and the house gets cooled down to whatever setting the wall thermostat requested.

The cooling even self-replenishes! After all those houses have dumped heat from their air down into that tank, say it has warmed to being 62°F instead of the natural 52°F. That tank has quite a large area of contact with the cool soil surrounding it, around 5,000 square feet. This area acts as another heat exchanger, allowing the warm water to lose heat to the surrounding cooler soil. There are many variables in this, such as the type of soil, its moisture content, the depth, etc, but it is fairly easy to design a tank surface area to be appropriate so that the tank could always get back down near the 52°F by each morning when significant cooling may again be needed. In other words, the system naturally replenishes its cooling ability.

Those ten houses would then forever have a FREE air conditioning system! Yes, there is a slight cost for running the blower and the small pump, but that is a tiny fraction of the electricity used in running the conventional A/C's compressor. If you think about it, this system is likely to require extremely little maintenance (NOT like conventional A/Cs, which are extremely expensive to repair!) Just an occasional addition of some pool-type chemicals to keep algae or anything else from growing inside the tank or pipes or heat exchangers.

The installation expense of this does not seem very terrible. It should certainly be far less than the total cost of ten central A/C systems for the houses! But even if it wasn't, just the fact that all those people would forever have summer comfort, without any giant electric bills! And a real bonus is that there is NO CFC (Freon) needed or used, just standard water! It is hard to imagine a more "green" system!

There are even bonuses in that regard that are not even obvious! All that electricity that is normally used for conventional A/C systems needs to be made. Nearly all of it is made in either fossil-fuel powerplants or nuclear powerplants. An important and detrimental side-effect of making that electricity is a large amount of global-warming heating that is released into the atmosphere. Since this system does not need most of that electricity, there is actually a great advantage regarding reducing contributions to global warming.

This system uses only standard, currently available items, and no new technologies are required. It is hard to imagine that anything could be simpler or more fool-proof.


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.


There is only one possible problem I see with this version. Say that there was one resident on that block who insisted that his house always be cooled really cold. He would then be using up far more than his fair share of this system's capability. In fact, if his use was extreme enough, he might cause the whole system to (temporarily) have reduced capability (because he was dumping so much heat from his house into the tank.) As to the social or political ways that might exist to try to control that, that is not in my realm of expertise. Maybe each poly pipe could have restrictor valves in them, or shutoff valves, but who would have the authority to close one?

I guess there is a potential second problem. Municipal governments like to be able to control and charge for pretty much any service like this that they can. So it seems likely that they will try to figure out how to charge the residents for this A/C system! Again, you're on your own regarding that!

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