We don’t often associate “earth” with a fresh air supply, yet earth and air do make a great team in a high-performance home. The concept of passing air through earth as a means of tempering it is not new; the temperature of the earth is relatively constant once you get down 6+ feet below grade. When the ground temperature is warmer than the outside air in the winter, or cooler than the inside air during summer, we have an opportunity to harness some cheap geothermal energy!
During the heating season, fresh air is essential but enters the home at a price. In a leaky home fresh air comes in at outdoor temperatures; this is why we need to get the air-sealing right… If we control where the air comes in, we can pass it through the HRV and harvest heat energy from the stale outgoing air. I’ve talked about HRVs in a previous post, and mentioned the importance of their efficiency, but the fact of the matter is, the colder the outside air, the cooler it will be even after going through the HRV heat exchanger.
We need a constant supply of fresh air, so we can’t turn off the HRV during cold spells; we can however, improve HRV efficiency. When building a new home, we typically have to excavate for the footings and foundation, so why not plan ahead and run some pipe beside the footings before back-fill? In a Passive House, these pipes are known as a Subsoil-Heat Exchanger or SHX for short. Earlier SHX implementations used earth tubes – essentially a large diameter intake for the HRV installed below grade. Although effective, the Passive House Institute no longer recommends earth tubes. If not perfectly installed, they become a trap for moisture. Where there’s stagnant water for long periods of time, there is mold and mildew; certainly not an environment suitable for transporting your fresh air supply. I’m a big fan of simple, and having no moving parts makes an earth tube very simple indeed; yet in the interest of guaranteeing healthy indoor air quality, perhaps a bit of complexity isn’t so bad: The alternative to moving air through the earth to pick up some of its heat, is moving a liquid. The large diameter earth tubes are replaced with 3/4″ PEX tubing and a highly efficient circulation pump moves food-grade propylene-glycol (protection from freezing) through the pipes and then through a liquid-to-air heat exchanger. This way, the fresh incoming air is warmed by traveling through the heat exchanger as opposed to the ground. The circulation pump runs on a thermostat so it is activated only when the incoming air is colder than ground temperature.
In the summer, the SHX is set to run when it gets a bit warm inside. A separate heat exchanger is installed after the HRV, and subsequently has a cooling effect on the fresh air distributed to the house. This heat exchanger requires a condensate pan to catch any moisture extracted from the humid summer air.
The soil is also important for SHX efficiency; if the PEX tubing is in a porous soil or (even worse) gravel, there is little surface contact and performance will suffer. In a dense packed soil such as clay, there is much better contact. From a SHX perspective, we have ideal conditions at Kingston Passive House… clay on top of limestone bedrock. I’ll post pictures as the SHX components are installed.