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, have been the two primary reasons why we have always recommended the air-filled tubes several feet deep. However, there is one disadvantage pf that air-filled-tube approach, due primarily to the tubes only being a few feet deep. As a lot of the heat from the house air is dumped into the soil, the soil tends to warm up, because most types of soils take a while before that heat can be 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 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 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 harder and more time consuming and expensive. 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 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!
Moisture in the house air would condense (as before) in the cool tubes. In this case that moisture would fall to the bottom of the vertical tube and be forever trapped down there, a bad place for things to potentially grow. Therefore, we think that these vertical systems should NOT be used as an air-based system and ONLY be water-filled versions.
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. A cap is installed on the bottom, and sealed properly so there is no leak! 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!
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.
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.
Why do this? Well, 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 than the equivalent water in the inner tube, the outer water starts to flow DOWNWARD (very slowly) 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.
E-mail to: Public1@mb-soft.com