EarthTube - A pipe in the ground exchanging heat with the soil, forming the intake for the HRV system. The soil temperatures (about 15deg C year-round) preheat the incoming winter air, improving the HRV performance quite a bit. It also cools the incoming summer air. In the summer, one would bypass the heat exchange in the HRV unit and simply bring cooled summer air from the earthtube directly into the house.
Needs to be about 8" (200mm) diameter pipe buried 6-8ft (2m) in the ground, about 150ft (45m) long, sloped toward the HRV. At the end of the slope, there will be a little well where condensate can collect and be removed. A pipe with anti-bacterial coating on the inside would be good. PH-Luft is a free online software which calculates the efficiency of the soil-to-air heat exchange, based on soil type, air flow rates, incoming air temperatures, and tube length and size, and so on. The Passive House certificate accepts the results of this software in the certification process. PH Luft is in German - bit of an issue to me, but struggling along a little bit with it.
Was told to avoid PVC for the earth tube. Of course, it has the dubious reputation of being very environmentally unfriendly both in manufacture and in use - though there are two main types of PVC, one of which has little or no plasticizer - a major factor in eco-conscious debates. The one with plasiticizer is the bad one, I think. Trouble with this is that PVC pipe is commonly available, cheap, and easy to work, creating strong, tight joins with solvent alone. Mechanically, I find it to have excellent characteristics for both the earth-tube and interior air distribution. Alas, chemically, there could be issues, though I must say, I would like to find out more about this.
The recommended material is HDPE. This stuff has to be joined by fusion-welding using a purpose-designed welding machine - I think one can see this process in Youtube videos, and its not available at the hardware store. I found Oxford Plastics makes a variety of this type of pipe, and Corix are the distributor/installer/welder, in our area. 90deg Bends and Tees, and caps area available in the larger sizes, 8" diameter, and larger still. Pipes come in straight lengths up to say 55ft (as long as a truck can take), which would make sloping the pipe in the installation easier - if you've ever worked with coiled pipe you'll know what I mean. One issue I have is that the welding process leaves a slight bead on the inside of the pipe - this will collect water and be a place for the formation of organic matter, defeating to a small degree the slope to a collection well. Of course, solvent-welded (glued) pipe will also have small grooving effects at the joints.
New Comments June 2012:
We didn't end up installing an earth tube due to costs and complexity, but also because enthusiasm for them has declined a bit.
In addition, the risks of earth tubes are not zero, though some say they are fine. Have a look a this article: