John Mueller - Earth Tubes Exchange
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Here's John Muller's entry in the Civil Engineering Forum --
from Civil Engineering Forum I have seen one and built one at the residential scale of construction. Factors to consider include the soil moisture content as mentioned by others because this significantly affects the thermal conductivity of the soil, heat load, and other similar engineering concerns. The temperature difference between the soil and the surface is a function of the depth, both in the magnitude and the time lag compared to the surface temperature. I suggest you look at some reference materials on earth sheltered construction for more detail on this, but at 30' (9m) depth, the time lag is typically 6-months and the delta-T over the year is close to 1-deg C.
Practical concerns are: Introduction of toxins, unwanted odors, etc. from the soil (so probably a lined earrth tube is better). Control of water condensation during humid warmer weather along with the probability of mold growth if water is standing (continuous slope to appropriate type of drain). Ability to clean if needed. Protection from entry of unwanted animals, insects, etc. And, preservation of the air entrance location from damage or removal by people in the future who may not know its purpose.
My empirical one was with 10" dia (254 cm) PVC pipe with rubber gasket joints so it would remain flexible in the soil (SDR-35 in the USA), encased in compacted clay about 5' (1.6m) below the surface, 80' (26m) long. It started outside with an above-ground vent in the corner of the building, insulated from it, ran out into the yard where it turned and came back under the basement floor, and ended inside by connecting to the return air plenum of the central heating and cooling unit. Thus, whenever the air was being circulated in the building, there was a mild positive pressure equal to the pressure loss due to friction in the return air system. All air leakage points in the building were therefore exfiltration only. All exhaust fans had a generous source of earth tempered fresh air into the building. The low point in the earth tube had a small drain pipe attached, which went to a nearby sump pump pit.
There are many methods of passively heating or cooling buildings, but the local climate you have will be the biggest concern in deciding which one(s) to use. Best wishes in your work. Contact me, if you want at firstname.lastname@example.org.
John Mueller, Kansas City, USA
This is our email exchange.
We found your posting (see below) on Earth-Air tubes in the Civil Engineering Forum most helpful. Just wondering if you have any further data or lessons learned with your Earth Air tube system?
We're planning to use the tubes at our Green Garage project in Detroit. Here's links to our Green Garage project and our Earth-Air tube design so far.
Thanks for any help you can provide.
A minor math error in my post--metric diameter should have been 25.4cm. The one i did in my daughter´s house is working great. When they had a problem with excess indoor humidity after a bgroken sump pump pipe flooded their basement, a contractor suggested they install a whole house dehumidifier at a cost of over $1000. I asked if the air tube was connected & found it was not. They reconnected it and reported the excess humidity and window condensation was gone within 24-hours, at a cost of $0. ADS tubing in the larger sizes is made with a smooth inner core and is quite rigid. I am puzzled about why you are not suggesting PVC in this application--educate me on this, if you want. There are many other plastic tubing materials available, such as high-density polyethylene and others. I chose the SDR-35 because it was widely available and therefore fairly reasonable in price. Earlier reply sent by Regards--John Mueller
show details Feb 15 Follow up message
Thanks for your reply. Another couple questions?
- Have you done comparison's of the ambient air temp going in to the tube versus the air temp coming out?
- Specifically how did you handle the condensation in the tube? It was sloped and had a drain...but how much condensation did you get? The condensation went to a drain and then where did it go? In to a dry well?
Regarding the tube material, we have not made a final decision. We do know the PVC is one of the most toxic materials made during the manufacturing process and has cause significant health issues for the workers. Here's one of many articles on health issues of PVC. We're trying to learn if other plastics are less damaging.
Also, another tube builder is suggesting that the ridges in the tubing high-density poly help with creating turbulence, which help break down the air layering effect in smooth tubes.
Any thoughts you have would be welcomed. Thanks again for taking time to respond.
Tom Brennan The Green Garage
- What is the length of the tube?
- Have you made any temperature measurements?
show details Feb 18 by John Mueller
My work has been entirely empirical. As a person familiar with the scientific method, i know it would be wise to do studies of the air flow, temperature differential, etc. Indeed, this would be desirable for determining the effectiveness and possible improvements to this design. Since my daughter (for whom the home was built) is a high-school sciences teacher (physics, chemistry, astronomy, aquatic biology, oceanography, etc.) it is quite possible that she would have some students and access to appropriate equipment for such studies. Hmmm.
Thanks for the link on PVC. I read parts of it and quickly scanned the remainder. Seems like plastics of all forms have serious problems for our future.
Regarding tubing with ridges, any ridges, if they are circumferential, will increase turbidity and mixing of air, but will also become locations where water can be trapped and thus become incubating locations for molds or bacteria. This is why I chose a smooth-walled product.
Regarding slope and drainage, I chose a practical value of about 1/8 inch per foot as a minimum slope and bedded the pipe in sand or pea gravel, tamped down thoroughly around the pipe. The low end was under the basement floor and was connected directly to a sump pump well. When the basement was being excavated, we ran into an old stole-lined well (filled) on the site, along with stone foundations that matched no home outlines known to area residents for the previous 40+ years. Since it was in the original plat of the town, these probably dated back into the mid or late 1800´s. The basement excavation was very wet, with ground water level being close to the bottom of the excavation (water being in it for a week or more after rains). In that location, there was no such thing as a potential "dry well" for the pipe to drain into. Indeed, a couple of times, when the inflow into the sump exceeded the capacity of the pump, the air tube also filled for many feet of length. This had the unintended effect of becoming a surge reservoir for the sump pit.
In some areas, radon gas migration into a very tight home is a real design concern. I don´t know the status of this in the Detroit area, so I hope you will take this into account when you design the drainage for the earth-air tube. --John M.
Excellent. Thanks for a very thoughtful email. We better understand your design concept now.
Ground water levels will be a real limiter. We're hoping to get that determined soon.
Again, thanks so much for helping to make the Green Garage more sustainable.