Earth tubes are earth coupled heat exchangers or EAHE (earth-air heat exchangers) that use the constant temperature of the ground to preheat or precool the incoming air of a building.
The term heat exchanger conjures up advanced technology but it actually has been used for ages. The Persian Empire used “solar chimneys and earth tubes” to cool their homes in the hot arid climate.
There are three basic design configurations for earth tubes or EAHEs.
The closed loop, which uses a “closed system” of roughly 30 to 150 metres of thin wall four-to-24-inch PVC piping buried at three metres, takes the air from the building and moves it through the underground system to either pre-heat or pre-cool the circulated air as need and season dictate.
The next is the open system, which simply uses a long, straight 30-metre run of tubing to bring tempered and filtered outside air into the home.
When combined with a heat recovery ventilator (HRV), it can reach efficiencies of 80 to 95 per cent. Filters for the intake of the system are recommended to meet MERV 8+ (minimum efficiency rating).
The third is a combination system.
This configuration uses dampers and control units that allow outside air to be brought in when clothes dryers, bathroom vent fans, or fireplaces drop the air pressure in the building.
When conditions require simple heating or cooling of building air temperature, the closed loop is utilized to pre-warm or cool the circulated air for an HRV.
Ground tubes are most effective where there is a large temperature differential such as in the extremes of summer and winter.
They are not so effective in areas where ambient air temperature is stable; Florida would be one such climate.
Considerations for earth tube installations are trenching costs, as optimum depth is between 1.5 to 3.5 metres, the ability to clean the tube, and condensation collection and disposal.
It is not recommended to use water or sewer line trenches because of freezing concerns.
Use of 45-degree elbows, and cleanouts in the piping to facilitate “pigging” with a large foam sponge (pig), which is recommended as mould or fungi can take up residence in the pipe if relative humidity is high and ground tube air flow is low.
The condensation concerns can be addressed by adding slope to the line; ideally to a collection sump in the home, where it can be pumped out if necessary.
Sealing the pipe from the influx of ground water, however, is of utmost importance, as the most efficient ground tubes are placed in dense, damp, well-shaded soil.
Here in the more temperate region of Canada, heat and cooling generate some of our highest operating costs.
Earth tubes offer low operating cost alternatives to aid in the heating and cooling of our homes.
An open or combination system can provide super insulated buildings (with their air-tight construction) fresh filtered outside air and in doing so provide a more livable home space while reducing cost.
Lorne Oja is an energy consultant, power engineer and a partner in a company that installs solar panels, wind turbines and energy control products in Central Alberta. He built his first off-grid home in 2003. His column appears every second Friday in the Advocate. Contact him at: lorne@solartechnical.ca.