Earth Tubes: Passive House Heating and Air Conditioning for Free
Close up view of earth air tube with slit to allow immediate drainage
One of the more controversial elements we incorporated into our Passivhaus project, were earth tubes (also referred to as earth air tubes or ground coupled heat exchangers) as the initial source of our passive house ventilation of fresh air.
For me, an earth tube (earth air tube) heating and cooling system is the most fascinating system of our passive house design. Earth air tubes provide natural heating, during the cold seasons, and natural cooling and dehumidification, during the warm seasons. And they do it for zero operating costs and complement or--during the more moderate months of the year--substitute for active passive house heating and cooling systems which in our case were ductless mini-splits.
Our friend Jason of UltimateAir had installed a pair of earth tubes during his passive house construction and was interested in accumulating performance data on as many installations as he could for a presentation at a PHIUS concerence.
As we were interested ourselves, in contributing to the general knowledge base of passivhaus construction, we decided to assist in increasing the amount of data available by installing an earth tube (earth air tube) system prior to back filling.
Earth tubes have gotten a bad rap because of some "poor" designs and installations in the past. In Jason's case, he installed two 100 foot eight inch SDR35/ABS tubing below ground to provide the air flow rates needed to supply his two UltimateAir ERVs. These pipes sloped or were pitched downward and away from the residence to allow condensation (moisture) to drain away to base at the point of fresh outside air intake.
Our particular site presented some specific excavation challenges that would make such an installation difficult. Our soils were diabase had a tremendous amount of granite rock formation below ground. This made digging a 100 feet five foot-wide trench difficult.
As such, we need to explore an alternative design. Since the foundation area had not been backfilled as yet, we decided to design a system that would encircle the perimeter of the building envelope. This presented another challenge, however. Earth tubes need to be sloped or pitched like a drain pipe to allow any condensation or moisture to be drained away by gravity. Not doing so, can lead to long-term air quality issues as standing water can lead to mold formation. This has been the primary source of failure of some earth tube installations.
Since pitching our earth tube system would be difficult, we had to look at other configurations.
We initially looked at a Rehau EcoAir system. Their system is comprised of a silver-based interior linings, which are designed chemically to discourage mold formation. An advantage to Rehau is that they provide complete systems.
Jason mentioned that we should speak to a gentleman by the name of Larry Larson of Earth Air Tubes, which we did. We discussed out specific challenges and we collectively were able to design a system that on paper would do the trick. An although such a design had never been implemented to this point, it appeared that it would work.
To support the air flow requirements for our ERVs, we designed a system that would allow for up to 400cfm of air flow. This is generally accomplished with two eight inch pipes. Furthermore these tubes need to separated enough from one another to minimize thermal "cross-talk." Unfortunately, we did not have the width in our foundation dig to support their use. Our solution was to create two levels of three six inch tubes each of which would terminate into an eight inch tube the final twenty foot run into our ERVs.
The thermal conductive characteristics of a six inch tube is believed to provide increased efficiency over larger tubing. The aggregate flow rates were determined to be satisfied by this configuration.
The design of this particular earth tube system broke conventional design rules. Our system was comprised of corrugated tubes that were level and serpentined. Corrugation provides for maximum air to tube wall contact and serpentining created natural turbulence inside the tube to permit maximum tube to air contact. Both design elements also lead to increased efficiencies.
Each tube had a small saw cut that ran the length of the entire tube. The idea here is that instead of attempting to drain the condensate away the length of the tube, the moisture is drained immediately below at point of formation to a bed of stone below the tubes. This design should also allow for the tubes to continually "clean" themselves out over time. This home-made DIY system is expected to elegantly address the potential for mold formation.