EARTH TUBING
Use Mother Earth For Heating & Cooling Your House
C.Bayard 2007
1- What is earth tubing?
Earth tubing is a very simple and basic geothermal system based on an air/earth heat exchange. This system is known in Europe as a ‘Mediterranean well’ because similar systems were used by the Romans in their typical Roman houses. Romans were very advanced in comfort, particularly heating and cooling. They also invented the ancestor of heating-floor which was called Hypocauste. The Mediterranean well is also called ‘Canadian well’ but I could not find the origins for this.
2- How does it work?
Temperatures at the surface of the earth are controlled by the sun and the atmosphere, except for areas such as hot springs and lava flows. From shallow depths to about 60 m below the surface, due to the large thermal inertia of the earth, the temperature is the result of complex balance between the mean atmospheric temperature throughout the year, and the heat flow coming from the earth. In France, if you dig down 8m, this temperature is about 9°C, depending on local conditions. Very close to the surface, the temperature tends to the atmospheric temperature which varies from day to day and from summer to winter.
So, the deeper you dig, the more constant is the temperature, over time. You can see on the following graph of mean month temperature against depth (France) that at about 8m depth, the temperature is nearly constant. Earth tubing simply exchanges heat with this nearly constant temperature area.
The important thing to understand is that there is always a constant flow of air in any house because, for health reasons, you need to constantly replace the air. Here in Europe, we often use what we call a ‘centralized mechanical ventilation’ or VMC in French. It is a centralized fan box in the attic which picks up the air in different points in the house to reject it outside. If you reject air outside, it implies that air is coming in. It generally comes from the ‘leaks’ because a house is not airtight. The idea of earth tubing is simply to force the incoming air to exchange heat with earth PRIOR to coming inside your house. The result is the following:
In winter: the external incoming air at 3°C is warmed by the earth to increase its temperature to approx 9°C then enters the house where you heat it up to a comfortable temperature (19°C). The gain is 6°C.
In summer: the external incoming air at 28°C is cooled by the earth to decrease its temperature to approx 16°C then enters the house where you just don’t need the air conditioning.
3- Special cases
One weakness of the system is that in the mid-seasons, you can have situations were the result is the contrary of expected. Those cases happen when the following formula is false:
|external temp - comfort temp| > |tube output temp – comfort temp|
Where |x| is the absolute value of x
The temperature gain is: G= |external temp - comfort temp| - |tube output temp – comfort temp|
The following graph shows in red the possible temperature gain from the earth tubing and you can see that in certain cases, the gain is negative.
At the end of March, the outside temp is about 17°C so it is better to collect the air outside than taking it from the tube where it is only 13°C. This disadvantage has to be known and understood in order to round it out by disconnecting the tubes or just stopping the fan either manually or better automatically with some temperature monitoring. (More complicated to do by yourself)
A second thing to be aware of is the following: because earth tubing is based on earth thermal inertia, it is efficient in locations where atmospheric temperatures have a large gradient. If the extreme temperatures are varying not more than from 10°C to 22°C, the earth tubing efficiency will be very low. (See the following graph.).
As often, there is never a unique and universal solution to a complex problem.
The previous graphs are only showing the monthly-means temperatures but in reality, you can gain more than that because the tube also attenuates the temperature differences between night and day and from one day to another (see the following 9 months monitoring).
4- How is it made?
The basic elements or sub-systems of an earth tubing system are the following:
- An air intake and filtering device
- One or several earth tubes
- A condensation output system
- A “trough wall” passage for the tube
- An air flow generation device (fan and possibly by-pass valves and auxiliary systems)
- An “in house” air distribution network Even if some specialized companies are starting to propose those in a turn-key solution, most of the things can be done ‘homebrew’ for very low amounts of money.
Air intake: This is the place where the tube is entering the ground. You must build a sort of chimney to avoid water and pollution that could enter the tube. You also want to prevent insects and other small animals from entering the tube. Take care to install the air intake as far as possible from any pollution or odorant sources. Depending on whether you want to increase the cooling effect or heating effect, you should place the air intake in a sunny place or in the shadow of the house.
Tube: this can be one or several tubes coming from the air intake to the house input. The optimum depth (which is a compromise between efficiency and digging difficulty and price) is about 2m. If the tubing is to be done at the same time as the construction of house, an easy way is to put the tube around the house in the trench dug for building the house. In this case, you have to be careful not to place the tube to close to the house to avoid a thermal effect from it.
The length and the diameter are also a compromise between efficiency, price and availability of the tube, area available, head loss for fan, etc. The minimum length for correct efficiency is about 20m. Longer is better. The optimum compromise for diameter is 100 to 200mm. Smaller diameters generate high head loss (which would require a powerful fan); bigger diameters will generate a central zone in the tube where the heat exchange is poor. An idea to round those problems is to install several tubes in parallel. You multiply the length and multiply the earth/tube exchanging surface.
The tube can be virtually of any material compatible with the ground. The heat conductivity has to be the highest compatible with available materials. Stainless steel would be nice but it is expensive. Plastic tubes are often used. A very good compromise is the plastic tubes used for passing electrical cables underground. I don’t know if similar things are available on your side. The advantage is that they are cheap, the external surface is multi-grooved which increases the exchange surface area with ground, but the internal surface is almost slippery which helps to prevent water from accumulating. Please avoid PVC tube because it emits vinyl chloride gas which is poisonous.
The chosen material should also resist the weight of 2m of ground. If possible, use one unique tube without any junction to limit the possibility of water input.
Water condensation: when outside hot air passes inside the tube, it will cool and eventually cool to under the dew point. The result will be water in the tube. In order to avoid moisture and possible bacteriological effect, the tube must be installed with a general 2% slope. The worst case to absolutely avoid is a water trap in a low point somewhere in the tube length. When digging, it is difficult to produce a constant slope. So the easiest way is to dig approximately with a “visual” slope then drop some sand in the trench to level the bottom of the trench to a constant minimum 2% slope.
When the tube is in place, you can cover it gently with more sand which also increases the thermal transfer with earth. Because you now have a slope, one end of the tube must have a system to evacuate condensed water. Depending on the configuration (a house with or without underground level, pitched ground…) you can have this system either at the house end or the air intake end or somewhere along the length of the tube.
When the condensation system is in the house, it is just a small hole drilled in the tube to evacuate water. When outside, the earth tube must link to an “underground reservoir” with some sand and small stones at the bottom in order to evacuate water into the ground. The slope must go down on both side of the reservoir. The easiest way in this case is to fit the air intake on the reservoir itself. The reservoir can be a large diameter tube (approx 40cm) opened on each end. The upper end is then reduced to a smaller diameter for the air-intake.
Trough-wall passage
If possible, it is better to make this passage under the ground level ground in order to minimize the thermal exchange with external air. If the house doesn’t have an underground level, you can perhaps dig under the wall and then get inside vertically from the bottom. (See both input solutions on previous sketch.)
Air flow generation device
The cheapest and easiest is a small fan or turbine connected to the tube. It produces a vacuum in the tube and forces a flow of air through it. Some models are designed to be directly connected to a tube (perhaps with some reduction pipes). If you use one or several existing “picked out from the bin” fans, you can make an “air box” where the input is the earth tube and output is the house network. This can be a wooden box and you can connect several tubes to different rooms with one fan per output tube. This gives you the possibility to switch on and off some rooms. In Europe we can buy some very common and relatively cheap equipment called VMC. It is a box with a quiet fan and several inputs/outputs.
Some are more complicated and expensive because they include an air/air exchanger designed so that the air you draw out of your house exchanges its heat with the incoming air, prior to going out. As mentioned previously, there are some periods when it is better not to use the earth exchanger. You can stop the fans but if you do that, you will have less air replacement. This is not a real major problem because when this case occurs, it means that the outside temperature is near comfort temperature; so just open the windows from time to time to replace the air inside the house!!!
There are some clever and complicated by-pass systems (like shown on the sketches previously). I personally don’t think it is useful but people who sell those earth tubing kits are trying to make more money by selling extra stuff.
Air distribution network
You can use the same sort of tube for distributing the air in the different rooms or you can use some specialized tubing used for central conditioning systems. The important thing to keep in mind is to insulate those tubes in order not to exchange heat before the output mouth. A possible weakness could be, as in any ventilation system, the noise of the fan which is transmitted along the tubes into the rooms. If this occurs; you can try to buy or make a silencer tube to insert between the fan and the output, as close as possible from the fan.
Conclusion
It is cheap, it is easy to make by your own, it is energy saving and planet friendly. Try the earth tubing solution!!!
C.Bayard 2007