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An earth tube is a tube that is buried in the ground which heats or cools air moving inside it. Earth tubes may be a closed circuit where they take air from inside a structure, circulate it in an underground loop before returning it to the structure, or an open circuit which takes air from outside and brings it into the structure, or counterflow open circuit when air is brought in and indoor air is taken out in sepearate earth tubes. The air can be moved passively with convection or actively with fans (air blowers). Whether the air is heated or cooled depends on the outside air temperature relative to the indoor temperature, and importantly the temperature of the soil at the depth the tube is buried. This article focuses mainly on open systems.
- 1 Synonyms
- 2 Basic theory
- 3 Components and considerations
- 4 Cost
- 5 Return on investment
- 6 Problems and disadvantages
- 7 References
Changes in air temperature
The temperature of the air at the surface of the earth is subject to constant changes due to weather, day/night cycle and seasonal cycle. For example, when it is a sunny day the air temperature is warmer than when it is cloudy. Air temperature also drops at night. Seasons are created by the tilt of the Earth's rotational axis as it orbits the Sun. In temperate and polar regions, for part of the year some locations will be tilted towards the sun. Days are longer and more heating energy is received per unit of surface area. In the northern hemisphere this is May, June, and July and for the southern hemisphere this is November, December, and January. 6 months later, the same location will be tilted away from the sun and the days will be shorter. Equatorial regions do not experience significant changes in daylight hours year round.
Changes in soil temperature
The temperature of the earth responds to these changes in the surface temperature, as heat is gradually conducted through the soil. However earth takes time to warm up and cool down, and so the temperature of the earth lags behind any temperature change of surface air (i.e. earth acts as thermal mass). The deeper in the earth you go, the more depth of soil temperature changes have to permeate through, and the constant changes in surface air temperature start to get blunted and merge together. Most importantly this means that the soil temperature in the warm season will be cooler than the surface air temperature, and conversely the soil temperature in the cold season will be warmer than the surface are temperature.
Deep earth constant temperature
At a certain depth, even the temperature change between winter and summer average out. This is sometimes termed the "deep earth constant temperature", or the "amplitude correction factor". Below this depth the temperature of the earth starts to gradually increase since there is also heat rising from the interior of the earth. The exact measurements of this deep earth constant temperature vary (possibly according to geographic location). For example, in Montana, USA, 6 meters down the temperature is stable year round at 7 degrees Celsius. In the UK, this is between 8 - 11°C at approximately 15 meters down. The depth may also vary dependant upon soil moisture level (4.25m, 5.5m and 6.7m for dry, average and wet soil respectively).
Heat exchange between air and earth
As earth is passively moved through the earth tube (by convection), or actively moved through with fans / air blowers, any temperature differnece between the air and the earth surrounding the tube will begin to equalize. Indeed, if the air travels through a long enough tube, the tube air temperature will trend towards the temperature of the surrounding soil.
In the warm season, the external air is warmer than the temperature of the soil below the surface. External air that enters an earth (cooling) tube will be cooled to an extent. This warmed air then enters the interior of the structure, providing cool, fresh air.
In the cold season, the temperature of the soil below the surface is warmer than the surface air temperature, and so the opposite occurs: the air in the earth tube is warmed before it enters the interior, providing ventillation and reducing the need for other heating measures.
Components and considerations
There is no standardization in terms of materials of design. This is perhaps why some work and others don't, and why opinions about earth tubes vary considerably.
Open systems feature an external air intake by definition. Also termed the collector, this may take the form of a vertical air intake tower. Intake towers require a rain hood. A filter or screen to prevent insects, animals from enterring the tube is advisable. It is sensible to locate this away from odors and pollutants. The intake should allow for removal of condensation.
If the earth tube is intended for cooling then the collector chould be located in a shaded area near a lake or river. If the earth tube is intended for heating, the collector would be better placed in full sun away from any large water body. The intake of a counterflow system should be positioned such that it does not suck in the exhausted, stale from the outtake. The intake can be orientated towards the prevailing wind to encouraged passive air entry.
Polyvinyl chloride (PVC) pipes are common since they are cheap, and will not degrade when buried (at least not for a long time). Concrete has been used, and while it is resistant the forces exerted on it by being buried and will not rust or degrade easily, concrete wicks moisture. Steel has also been used, which may be galvinized to prevent rust.
The R-value of the tube material is a consideration, i.e. the lower the value the better since good thermal conductivity is desired. Thickness of the tube wall also affects thermal conductivity, with thinner tube conducting heat between surrounding earth better than a thicker one, although thinner pipes reduce strength. Weaker tubes may be damaged after the trench has been backfilled, as earth settles. Placing hard packed gravel beneath the tube to help support it.
Internal tube surface
The internal surface of the tube can be lined with anti-microbial material. It should be smooth rather than corrugated to prove less resistance to air flow.
Most systems tend to use tubes of diameter between 10 cm (4 inches) and 45 cm (8 inches).
Finacial cost is variable. One researcher concluded that a typical systems cost between 2000 and 3000 Canadian dollars (in 2001). A major cost can be excavation of the earth. There may be an ongoing cost if the system has active controls, or components such as fans.
Return on investment
One researcher concluded that the average time taken for the energy savings to cover the initial cost was long (e.g. 9 years, or 10-20 years).
Problems and disadvantages
Moisture and micro-organism growth
Moisture and mold may be encountered in earth tube systems. While some state that this is due to poorly designed, installed, operated and maintained systems; others conclude that the risk of deterioration of indoor air quality is significant and the technique cannot be justified given the limited energy savings provided.
Rainwater may pool inside earth tubes, if designed poorly. However moisture in the air can also be deposited on the iside of the tube as condensation as air moves through. This is most likely when the air is hot and humid and the earth is cool. Earth cooling tubes may therefore benefit from dehimidification, particularly in warm and humid climates. Moisture can lead to growth of micro-organisms, particularly mold inside the tube. This mold releases spores which are carried on the air into the home. This can lead to a musty smell, and even cause health problems. Airborne spores of penicilium, aspergillus fumigatus, aspergillus versicolor and aspergillus niger were detected indoors in a PassivHaus with earth tubes in Belgium. The occupants had to move out due to chronic health problems which then resolved.
- Earth Tube Ventilation Systems - Applicability in the Canadian Climate. Didier Thevenard. Canada Mortgage and Housing Corporation 2011
- Down to Earth - An ‘Exhumination’ of Earth Tube Heat Exchangers. Robert Bean 2010. Originally published in HPAC Canada, hosted on healthyheating.com
- What is a Ground-Air Heat Exchanger? Ziger / Snead Architects 2010
- Earth-Sheltered Houses: How to Build an Affordable Underground Home. R Roy. New Society Publishers, 2006
- Passive annual heat storage: Improving the design of earth shelters. John Hait. 2013
- Ground source heat pumps: development of GeoReports for potential site characterisation, issue 1.2. Ian Gale. British Geological Survey 2005.
- Mechanical and Electrical Equipment for Buildings. Walter T. Grondzik, Alison G. Kwok 2014
- Passive Solar Architecture Pocket Reference. D Thorpe. Earthscan from Routledge, 2018
- Belgian Passivhaus is Rendered Uninhabitable by Bad Indoor Air. Martin Holladay. Green Building Advisor 2012
- The Solar House: Passive Heating and Cooling. Daniel D Chiras. Chelsea Green Publishing, 1 Oct 2002. p 177