Design Goals: The role of the earthtubes is to preheat the fresh air supply to the ventilation system during the cold season. Two benefits are expected: the obvious benefit of replacing exhausted air with air that is warmer that ambient winter air, and the hope that incoming air will always be above freezing temperatures so that the heat recovery ventilator does not need a defrost cycle.
Mold problems and questionable effectiveness are a common theme of Internet discussions of earthtubes. Our system only uses earthtubes for heating air, not cooling, so condensation should not be an issue. Secondly, our tubes are smooth-walled and run continuously downhill, so any moisture that gets into the system should drain out. Thirdly, our tubes are not replacing a heat recovery ventilation system, but supplementing it. The function of the earthtubes is narrow, and specific to what they should do well.
Our Design: Air will be pulled through the earthtubes into our ventilation system at approximately 500 CFM. We used 351 feet of 6" gasketted sewer pipe for the earthtubes. The piping was divided into three 117 foot runs starting from one point at the greenhouse about 3.5 feet below grade. The runs diverge to about 10 feet apart and dive to approximately 7 feet below grade. The pipes emerge above grade at the end of their runs. The whole run is pitched downhill; our sloped site made this easy.
Performance Data: During the winter of 2001/2002, we will collect data on air temperatures at both ends of the tubes to see how effective the earthtubes are at warming the air given our pipe diameter, distance, and depth. We are particularly interested in assessing how well the earth replenishes the heat that is pulled from the area surrounding the tubes over a period of time.
There certainly are cheaper pipe options. For example, corrugated drain pipe would be a small fraction of the cost, and should be considered. We chose the SDR35 pipe because it was the safest option (i.e., no water accumulation in the pipes and can withstand 7' of earth above) and because of the virtual impossibility of correcting any problems in the piping that might arise. We expect the earthtubes to be in service for a long time. Our focus is on the effectiveness of earthtubes in meeting our objectives. If they prove effective, future effort could be directed toward reducing costs and optimizing design.