The following is not an all inclusive treatise. It is the best we can gather and evaluate.
Below the earth’s surface
It is a well known fact that if you get below the surface of the earth a few feet, the temperature tends to be very even and at a constant 55 to 60 degrees (F), depending on latitude. So, it does not take a genius to understand that if you could move outside air through a buried pipe, you could alter its temperature and then move it into a house where it can warm or cool the home’s interior.
For example, if the outside temperature were -20, outside air passing through a buried pipe could enter the house at a much warmer 50 degrees. And if the outside temperature is 100 degrees, the piped air would still be in the 50s. In theory, a long pipe buried in the ground acts as a heat transfer conduit, drawing tempered outside air into a house where it can mix with and alter the temperature of a home’s interior air.
In reality, this does work and has proven efficacious in some instances. Theoretically, it should work extremely well and probably should be done more. But what about the variables? And, what are the variables?
The dynamics of moving air through a pipe must take into consideration heat transfer, friction of the pipe, velocity of the air through the pipe, as well as contamination, segregation and condensation.
Condensation is of utmost importance because it creates the possibility for mold and mildew. Spores from mold and mildew can be very toxic, or at least allergenic. So, it must be remembered that a pipe buried any distance in the earth, that has air blowing through it, can fill with condensation in a very short time as warm summer, moist air is drawn through it. As the air cools, the humidity condenses and drops out. So, whatever system is designed and installed must consider condensation and properly dispose of it.
On one dome home we built near the Rio Grande River in south Texas, we used a 3’ diameter culvert for the underground air plenum. This culvert was steel corrugated which would effect good heat transfer. It was approximately 200 feet long, that allowed for a substantial contact time. Being three feet in diameter, it had very little resistance to air movement. It was large enough so that it could be manually cleaned in case of mold and mildew.
This culvert was carefully installed on a small slope, so condensation from the air would drain to the outside. It stretched from daylight into the mountain surrounding three buried domes that were the home. Vents were placed in the tops of the domes, allowing the air to continue through the culvert and out the top.
This home stays at a very even temperature year round with no mechanical help at all. In fact, the home is built without electricity. The vents in the top of the dome are 10-foot diameter pipes, rising to the surface and acting as light pipes as well as ventilation chimneys.
Unfortunately, 3-foot diameter culverts are not inexpensive, and most people do not have enough terrain on their property to make the proper amount of slope to ensure needed water drainage.
Mildew and Mold
Where you have water and any food source, you can have mold. Mold is the number one culprit in Sick Building Syndrome. Moldy air cannot be satisfactorily used in a home. There must always be a way to treat and eradicate the mold. This can be done with bleaches and other chemicals, if the proper access is provided in the design. But it’s pretty hard to scrub a 4-inch diameter pipe that is 10 feet below ground. Therefore the eradication method must be one that relies strictly on chemicals.
Amount of Air
At this point in time, we simply don’t know how much outside air is needed for tempering. Since there are no rules to help us, it’s strictly a guess. If the incoming air is meant to be used as fresh air, far less is needed than if it’s to be used for primary cooling.
We are not going to talk about primary heating because the air coming through the earth in the winter is too cold for heating. On the other hand, in hot climates the earth’s temperature can be used as a primary cooling source. It takes a lot of air, a lot of movement, but it can be used. But even in hot climates, humidity levels may still dictate some mechanical dehumidification to provide comfort and eliminate additional condensation.
In my opinion, a four-inch air pipe, approximately 100 feet long will temper the incoming air for approximately 500 square feet of house. But it is not enough to provide cooling. To provide cooling, probably ten times as much air is needed. Again, there are no studies that we can find that give actual numbers. Also, variable conditions affect this so much it becomes almost an art form.
Note: In this treatise we are going to concentrate strictly on tempering the air for quality control of the air within a structure.
Placement of the Underground Piping
The ideal situation is to build the home on a hill which rises 10 feet above its surrounding area. A trench can then be dug from the home, 10 feet down, and then horizontally until it reaches daylight. This horizontal section is placed on a small slope to the exterior, like a sewer line. One inch in ten feet would be very sufficient.
Care must be taken that this flow line is absolutely controlled as we do not want pockets of water building up within the pipe. Therefore, the flow line must be right on grade. This means the air can come into the pipe, flow up the slight incline, drop its condensation as it is traveling through the pipe so the condensation drains out the pipe’s bottom portion. When there is humidity there will be a considerable amount of condensation.
Obviously most homes are not built on 10-foot high hills, 100 feet from surrounding areas. Consequently, we need to consider how to put air pipes in flat land.
We can place a pipe ten feet down and lay it horizontally for 100 feet, curved or straight, and then bring a riser back to the surface. The riser should have an upside down U at its top so rain and debris cannot enter. And it should be screened so critters cannot use it as a back door into the house.
Again, it is important that the pipe is sloped to a collection point. Water will run to this collection point where it must be removed. The collection point can be at either end of the pipe or in the middle. It is left to the installer to decide its best location.
Several pipes can all be drained to one collection point. This can be accomplished by simply installing Ts and cross-connecting the pipes with drain pipes. Drain pipes, in my opinion, should be one inch in diameter. At the collection point, a sump pump can be installed which will automatically turn on and off, pumping the condensation out of the ground and sprinkling it on top.
In areas where there is excellent underground drainage, it’s possible to install a French drain. If a French drain is installed, it must be remembered that once it fills – whether from stray ground water or any other source – it will shut off the air pipe system. Before using a French drain, be certain there is adequate drainage for the water.
A French drain system would not work in our part of Texas. The ground holds water like a bucket. Condensation would build up until it shut off the air pipes and that would be the end of the system.
A drop pipe alongside of our vent pipes is a very simple solution for installing a sump pump. A sump pump can be lowered through the drop pipe. The float switch can be set to keep the bottom of the drop pipe totally clear of water. Obviously this is a mechanical device and will require some maintenance, but modern day sump pumps are pretty trouble free.
As already stated, the inlet ends of air pipes need to be screened for filtering. If we simply take a four-inch piece of window screen and put it over the end of a four- inch air pipe, we will strain out most of the bugs, but we will also restrict most of the air flow. Air does not flow neatly through a screened opening, particularly where the screen mesh size is small.
What needs to be created is a screen box, or a larger surface area for the screening. An area ten times as large as the area of the pipe should be provided. This allows the air to flow slowly through the screens and provides enough air for the pipe. A long roll of screening works very well.
Obviously, an air box would also work as well. We will leave it up to the actual system designer to decide what is best for the situation. The most important thing is to remember that it takes a lot of square inches of surface area for the screen. Consider at least 125 square inches.
[Article originally published October 1, 2002]