Earth to Air Heat Exchangers (or Earth Tubes if you want to sound less scientific about it) are really only part of your home HVAC system. In an ideal world, the Earth Tubes would heat/cool and dehumidify all the air you would need for your home via passive means. In reality, you are going to need fans to augment the air flow and the air coming from your earth tubes is likely to need some additional dehumidification. It may also be a good idea to mix it with “return” air as part of your whole home circulation. You may want to add filters, heating/cooling coils, etc. In some ways, the rest of the HVAC system would be like a normal home, but it could all be sized much smaller due to the advantages of drawing the intake air thru the earth tubes.
Here we explore some of the considerations and adjustments that may be needed when combining earth tubes with more traditional HVAC solutions.
One of the main concerns about underground living is cool walls leading to condensation of humid air and, eventually, unhealthy mold. Hot humid outside air drawn directly into an earth shelter will condense on the cool interior surfaces. However, if the air is drawn in thru cool earth tubes, the condensation can happen there instead. Of course, you don’t want mold in the earth tubes either, which is why you want them made out of nice smooth HDPE pipe and sloped to carry the water away.
If the temperature outside is a balmy 95°F with a Relative Humidity (RH) of 100% and it passes thru a 60°F tube, the cooler air can not hold as much moisture even as it maintains the same relative humidity. Once it enters the home, it is mixed with other air and warms up to 70°F. Since the temperature has now increased, the RH is only 70%, which is much better than outside, but still a bit high for comfort. If the earth tube temperature was only 51°F and then the air was allowed to warm back up to 70°F, the RH would only be 50%. This idea of dipping the temperature of the air as low as possible to force the water out of it, and then raising the temperature back up again is precisely why Air Conditioning (AC) units are so good at dehumidification. They usually drop the entering air temperature well below the comfort level in order to force the moisture out of it before it enters the home and mixes with the “return air” to warm up again. Note, these units do not attempt to cool and reheat all the air, only the entering air is cooled and it is then mixed with the warmer air that we are trying to cool. When humidification is wanted without cooling, the evaporator and condenser are both placed in the flow. The evaporator reduces the temperature of the air, condensation is drained off and then the condenser uses the heat it just removed upstream to warm the air right back up again. This is all a lot easier if you dehumidify the air as it enters the home because you have less volume to worry about, so be sure to dehumidify before mixing with the return air that you are re-circulating thru your home.
Fans are needed to help move air around for ventilation or other reasons. If you are using a forced air heat system, you will need larger fans to transport the heated air thru the home and then return much of it to the furnace to be “reheated” and used again.
Most forced air HVAC systems for above ground homes use 85% return air and only 15% fresh air. I plan to use radiant floor heating, so my fan requirements will be significantly lower, my house will be quieter, less dusty, etc.
(more to come)