Earth Cooling Tubes
Earth cooling tubes or earth warming tubes (also known as ground-coupled heat exchangers) utilize the earth's near constant subterranean temperature to warm or cool air for residential, farm or industrial uses. They are often a viable and economical alternative to conventional heating, cooling or heat pump systems since there are no compressors, chemicals or burners and only blowers are required to move the air.
Earth tubes are regularly used in Europe to pre-heat (or pre-cool) air for the whole-building heat recovery ventilation systems that are used in buildings designed to the German Passive House standard.
Most systems are usually constructed from 4 to 18 inch (100 to 450mm) diameter, smooth-walled, rigid or semi-rigid plastic or metal pipes, buried 6 to 10 feet (1.5 to 3m) underground where the temperature is typically 50-70 °F (10 to 20 °C) all year round (in the northern hemisphere).
There are basically three configurations, an open 'fresh air' system, a closed loop design, or a combination:
- Closed loop system - Air from inside the home or structure is blown through a U-shaped loop(s) of typically 100 to 500 feet (30 to 150m) of tube(s) where it is moderated to near earth temperature before returning to be distributed via ductwork throughout the home or structure. Larger diameter tubes need less total length. The closed loop system is usually more effective than the open system since it cools and recools the same air.
- Open system - outside air is drawn from a screened intake in the yard through, typically, 100 feet (30m) or more of straight tube into the home.
- Combination system - This can be crafted with unidirectional check valve dampers to allow either closed or open operation depending on the season and/or fresh air ventilation requirements. Such a design, even in closed loop mode, could draw a quantity of fresh air when an air pressure drop is created in the house by a fireplace chimney draft or attic fan.
Earth cooling tubes are, by their subterranean nature, usually located to penetrate the wall of a house's basement where they interconnect with a standard system of ventilation ductwork for distribution of the air throughout the home. As such, they may be better suited for new construction than for retrofit into an existing home, especially if there is a limited outside yard space to work with.
One drawback often cited to earth cooling tubes is the possibility of condensation buildup in the tubes and subsequent mold growth. A properly designed system that maintains a constant downward slope from the (upper) warm air intake end to the(lower) cool air outlet end should provide adequate drainage of condensation that can be handled by a floor drain, much like a conventional home air conditioning system. Also, it is important not to use corrugated or ribbed tubing as this would create traps where moisture would assuredly accumulate.
While some possibility of mold growth might still be possible, smooth round tubes should be much easier to clean than conventional rectangular metal ductwork found in the average home or office where mold growth is, of course, a common and accepted problem.
The efficiency of earth cooling tubes can vary widely depending on tube depth, climate, soil type, soil moisture content and ground level sun or shade. Generally sandy, dry soil with little or no ground level shade will yield the poorest benefit while dense clay or damp loam with considerable shade should perform well. Careful site selection and making provisions for adequate soil moisture, perhaps via landscape drainage or soaker hoses could be advisable. Damp soil absorbs and dissipates the warmth drawn off the tubes better than dry soil.
In the context of today's diminishing fossil fuel reserves, increasing electrical costs, air pollution and global warming, properly designed earth cooling tubes present a sustainable alternative in reducing or eliminating the need for traditional heating and ventilation systems.