Earth Tubes Q&A

Earth tubes (earthtubes, or earth-air tubes) are underground tubes that use geothermal energy to cool or heat temper the air for your home. It works like cheap air conditioning because you can build it yourself for several hundred dollars and it is FREE to run (no electricity needed). Being completely passive, this is a sustainable technology based on designs that are 3,000 years old and still used today around the world to cool homes.

This is a basic Q&A (Questions and Answers) compilation based on things people have asked me about earth tubes, or that I have learned along the way, which you may want or need to know. All of this, plus more, is covered in the rest of my book: DIY: How to Make Cheap Air Conditioning Earth Tubes

This page (which is the entire last chapter of my book) is divided into 7 more-or-less equal parts…

Part 1

How much do earth tubes cost to make?

The cost is usually about $150 USD per earth tube.

How much do earth tubes cost to run/use them?

Once you build them, the price to use them after that is FREE because they are completely passive and have no moving parts.

How many earth tubes do I need?

You will need about one earth tube per average room, or two for large rooms such as oversized living rooms or great rooms.

Can earth tubes be used for any kind of building?

Different sizes/styles of earth tubes (ground-coupled heat exchangers) have been used for residential houses as well as commercial and even industrial buildings. They are usually smaller for homes, and easily built, but grow in diameter and length (and sometimes complexity) in order to be efficient enough for larger buildings, however some larger systems have been tried but failed to produce the desired effects while others have been successful. I teach small-sized residential earth tube construction for owner-builders here in this book.

Are materials to build earth tubes hard to find?

Earth tubes can be made using common materials from your local hardware store—thin-walled PVC pipe, PVC primer/glue, elbows, couplings, screen materials, and utilize inexpensive electric tools like drills and a jigsaw (or hacksaw).

Can earth tubes really replace an air conditioner?

Yes! If they are built according to the specifications in this book (size, length, type, use, and maintenance, etc.), they should readily replace any air conditioning use you may have now. It does depend on the energy efficiency of your home, however. A mobile home/trailer may only find a small drop in temperature inside, whereas a standard constructed house will get more relief, an energy efficient home will find even more of a reduction in air temperature from the summer heat, and a superinsulated and/or passive solar home would find the highest air temperature differences between inside the house and outside ambient air temps. This amount of efficiency can range approximately 5-30 degrees Fahrenheit (3-16^oC) between these types of homes.

One can always offset this by adding extra earth tubes to make temperatures cooler in the house (if the house is less energy efficient), which should help, although the cost would need to be compared with the cost of simply adding more insulation, caulking to tighten air leaks, etc. to see which is more cost effective and will reap you the most benefit. Or you can always do both (increase energy efficiency, while adding earth tubes for cooling), which would help it from both directions.

Part 2

How do earth tubes work?

Earth tubes draw air in from their inlet (furthest away from the house/building), which is approximately 100’ (30 metres) away from the outlet (where the air enters the building). When all the windows and doors are completely shut, except for one or two up high, convection causes the warm air to exit the house, therefore drawing in air from the earth tubes.

As air flows slowly but continually through the tubes, the ground’s stable cool temperature (during summer) cools it, causing moisture from the air to form/condense into droplets inside the tube, which drain out holes into gravel. The air flowing into the house is cooler and dryer, and keeps the temperatures inside the building lower than outside temperatures.

In winter, this process can help heat the air (called heat tempering) to warmer temperatures than the outside air, although other systems are typically needed to finish heating the home the rest of the way (a furnace, woodstove or fireplace, passive solar design, electric or gas heater, masonry stove fireplace (or Finoven), or other method).

Do earth tubes need fans or pumps?

When designed properly earth tubes do not need any pumps or fans or electricity to “run” as they are completely passive systems. If a system “needs” to be more complicated by requiring such devices then it is probably designed wrong or may be more costly to run and may require more maintenance (e.g., corrugated tubing instead of smooth walls restrict air flow and cause puddling of water and may grow mold, or too many or the wrong kind of bends in the tubes can cause issues with natural air flow).

If a fan is absolutely required you can use a solar PV powered fan, otherwise it is going to cost you to run it. A contractor I know suggested using a pipe reducer and a fan to force cooling of the air, but fans pushing air through the tube too fast could deplete the ground temperature affecting the tubes and cause an inefficiency due to thermal lag (the earth’s temperature cannot keep up pace with a forced system that takes more than the earth can replace in the same amount of time). I definitely do not recommend using fans unless absolutely necessary, and then only low-level fans.

What are temperature differences inside and outside the house?

A standard older or newer house will have a 10-20 degree F (5-11 degrees C) drop in temperature inside compared to outside. We had a passive solar straw bale house, which was superinsulated and designed to keep the sun out in summer, so experienced a 30-degree F (16^oC) drop in temperature, even though it did not have much thermal mass (other than partial dirt floor and partial wood floors) built inside the house yet.

Can earth tubes be used with new or existing houses?

Yes, earth tubes can be designed into a new house plan (prior to breaking ground, preferably), or they can be built and the house retrofitted to accommodate them. This might be more challenging to retrofit for adobe or rock or other solid walls, however, but not impossible. You will want to discuss options with a structural engineer first.

PART 3

What is the air quality like from earth tubes?

When earth tubes are constructed properly, used correctly, and maintained properly, then the air coming into the house should be as the same quality going in from the outside (although certain types of air filters can be added, if desired, but may restrict air flow). A few studies have shown that earthtubes in certain areas of northern Europe have some issues, but not in central Europe, but whether the former is due to regional types of mold or bacteria, or user error, or a faulty design of the tubes passed from one person to the next, has not been thoroughly investigated or documented.

Radon should be minimal if the tubes are designed correctly, and any house in a high-radon area should already be designed to rid radon build-up anyway. All homes—especially energy efficient homes built tight—should have an adequate ventilation system, in which case many times earth tubes can provide for proper air exchange rates.

Mold and bacteria are not typically a problem in the types of earth tubes that we constructed (17 years ago and still working great as of this writing) and that I teach to build here, as long as they are properly built, maintained, and used. I cannot vouch for other types of earth tubes designed by others in different regions of the planet, although the concept is adoptable.

Can you clean or sanitize earth tubes?

People have tried multiple ideas to sanitize earth tubes but without much luck. If there are issues they need to be fixed first (obstructions removed, leaks sealed, etc.). Some people have suggested running bleach-water through them, but this would not work on cleaning the walls at all and the water would exit tubes if there are drainage holes. Others have considered connecting a rope and dragging a soft bleach-soaked ball through them (risking getting stuck). Tubes generally will not need to be cleaned at all if they are built correctly and used properly. The ones we built in the late 1990’s have never been cleaned and still work fine.

The only sure thing I know of, without all of this fuss for removing/killing possible mold, bacteria, smells, biological issues, or the like is by an ozone generator placed at the tube outlet (inside the house) and directed solely through the tube to the outside for any manufacturer-recommended time. Do NOT breathe ozone as it can damage the lungs when the generator is running. Ozone is used to kill mold and odors in houses.

Do you need a sump pump?

Typically the earth tubes drain condensate from the tube into a gravel bed, so no, a sump pump is not needed. There are, however, some designs that utilize a sump pump, but it is not usually required with the type of earth tubes that I teach to build here.

Can you use earth tubes year around?

If the earth tubes are buried well below the frost line (at least 6-10+ feet (2-3+ metres)) then they can probably be used year around—in summer to cool the home, and in winter as a heat tempering tool, and also to feed a fire (woodstove or fireplace or furnace), if desired.

 PART 4

How do earth tubes remove condensate?

There are small holes drilled into the bottoms (only) of tubes where the condensation drains into a gravel bed. In some designs non-perforated tubes are sloped away from the house and the condensate may go into a deep gravel pit, cistern for collection, or a drain sump may be used.

What are average soil temperatures year around?

It depends on where you live and what kind of winters and summers you have. Generally speaking, the deeper underground you go, the more stable the temperature. The air moving through the tubes in the earth at deeper depths, will follow those temperatures closely. Ambient temperature below the frost line, at an 8’ (2.5 m) depth may drop to 41 degrees F (5^oC) in winter, and rise to around 65 F (18^oC) in North America. It is not usually logistically or economically feasible to place earth tubes below the 10’ (3 m) level.

Can earth tubes be used in a humid climate?

The short answer is yes, although there appears to be some misconceptions floating around the Internet about whether or not earth tubes can work in humid climates. I think the misunderstandings come from poor designs, based on everything I have read.

We visited local homes in Nebraska and copied the designs of their earthtubes, which had been used for five years, and even 10 years—the owners said they worked great in that humid climate. We made our own tubes when we built our straw bale house, then finally moved in and used the earthtubes (mid-late ‘90’s). In 1999 we sold the house, and to this day the current owner continues to use the earthtubes and has never had a problem.

In fact, when it is 99% humidity outside and 99 degrees F (37^oC), it is a cool and dry 70 degrees F (21^oC) inside the house—a 30-degree (F) difference! So yes, if designed properly, earth tubes can absolutely be used in hot humid climates. If someone is having trouble and it is not a design problem, or user error, and if the heat and humidity levels are extremely high all of the time, then it could probably be remedied by adding extra earth tubes.

How long should earth tubes be?

We were told that, for completely passive earth tubes (no moving parts), 70 feet (21 m) might work okay, that 100’ (30 m) is ideal, and that 130’ (40 m) is better, but that after that the temperature does not really drop enough to make the extra cost worthwhile. We used 100’ tubes and they worked well for us, as did they for other people in our region. How much land do you have? Are you utilizing a fan (if so longer tubes are essential)? Is it much more costly to add a few 10’ sections to the tube? If not, consider going up to 130’ long, but I personally would not recommend going less than 100’ in length unless you simply do not have the room in your yard, in which case you might have to add more tubes to compensate.

PART 5

How wide should earth tubes be?

We were told to use 4” (10 cm) diameter tubes, which worked excellently for us. 6-8” (15-20 cm) tubes could be used for extra air flow or to feed larger rooms (or more than one room) but these tend to lose efficiency after this size for homes (commercial buildings may find larger tubes useful in a different application, however).

What pipe materials are needed?

We were advised by our contractor/teacher and earthtube owners to stay away from metal, concrete, gutter drainpipe, weeping tiles, or corrugated pipes at all costs as they could have mold, bacteria, air quality, corrosion, or other issues associated with them. I have heard of people using metal tubes that had an antimicrobial smooth surface on the inside with luck but I have not used them. The successful earth tubes we witnessed working are what we copied exactly as they built them, and that utilized 4” (10 cm) diameter thin-walled PVC sewer pipe (unused). Thin-walled pipe is needed for greater conductivity and thermal exchange between the air and the ground temperatures. Thicker-walled pipes do not allow for proper thermal transference.

How deep do earth tubes need to be buried?

Although 20 feet (6 metres) and beyond has the most stable ground temperatures, it is not always feasible to dig this deep, and is costly and may not be worth the effort. Half that deep is probably ideal, but anything below 6’ (3 m) is fine, typically. If you want to use the tubes during winter they definitely need to be well below the frost line. We only used ours in summertime and had very conductive (clay) soil, so only buried ours 2’ (.6 m) down and they worked very well for us, which actually surprised me.

What kind of soil is best?

Highly conductive soil is best for thermal transference, which would be clayey soils. Regular “dirt” is good. Sandy soils are poor conductors. Some gravel or rock mixed in the soil (if it exists in-ground already) is okay as long as it does not compromise the tubes’ integrity.

Wetter soil will remain the coolest, while dry soil will tend to be warmer. I cannot vouch for the wisdom of this, but regarding physics principles this would be a good idea… one could potentially dig a deeper trench and place one tube over a second one underneath (at least a foot (30 cm) apart on center, filled with dirt and then the gravel layer just under the top tube) to take advantage of the wet cooling effect from the top tube’s dripping condensate, which will leach down and wet the dirt over the bottom tube, therefore making the bottom earth tube cool more effectively.

PART 6

Does the ground temperature change?

Any time you draw thermal properties from the ground it is subject to change. Systems that use fans to blow air through the earth tubes tend to overuse the earth and draw from it faster than the temperature can be replaced (called ‘thermal lag’, which is the time that is required to remove or add heat from mass), thereby reducing efficiency of the ground in its ability to bring a steady supply of cool air to your house during summer (or warm air during winter). This is why I recommend passive cooling earth tubes (no forced-air fans or moving parts) because it does not deplete the ground’s ambient temperature faster than it can replace it.

When earth tubes are designed passively the air moves very slowly, a natural process through convection, and properly cools through the physics principles of condensation and thermal transference (heat loss). SLOW-MOVING AIR is key! The thermal lag is met in near-equal measure in a passive system, so ground temperatures remain more stable than with “forced” or active systems. Hence, passive cooling via earth tubes without fans will continue to cool or warm the house longer than trying to force it via mechanics. Active systems cost more and require longer tubes and/or extra tubes to compensate for robbing the ground of its thermal properties too soon or much.

Are earth tubes a ground-coupled heat exchanger?

Basically, yes. The earth’s subterranean temperature is fairly stable over time, with lower swings than we find in the air. Thermal lag keeps this a slow process compared to ambient air temperatures, in fact, but the earth’s temperature will rise and fall depending on the season. A ground-coupled heat exchanger captures heat and/or can dissipate the heat to the ground. This is how geothermal systems work.

If air for a building is blown through a heat exchanger for heat recovery ventilation (HRV) it can be called ‘earth tubes’ and is a type of heat exchanger for harnessing natural geothermal energy. However, HRVs can also be (and are usually) mechanical (MHRVs) and used in ductwork along with a furnace, so be aware of that. Earth tubes are also called earth-air heat exchangers (EAHE/EAHX), or a variety of other names, depending on where you live.

Ground-coupled heat exchangers can also use water or a heat transfer fluid (like antifreeze) in a closed-loop tube system underground, which raises the temperature of the earth underneath the home rather than blowing air through it into the house. In areas with mold or radon issues that cannot be bypassed or adequately controlled, this type of closed-loop system may be beneficial.

Earth-air heat exchangers of different types have been used in conjunction with solar chimneys, cooling towers, and qanats in hot arid or semi-arid regions for thousands of years (in Persia and other areas in the Middle East). Earth tubes are similar to geothermal heat pumps or downhole heat exchangers, but are generally far easier to install (owner-builders can typically do it with the proper knowledge, whereas more costly and complicated electric systems require a professional installation and can be costly).

Part 7

Can earth tubes be used anywhere?

I would say that earth tubes could probably be used in many, if not most, places around the planet, but in extremely cold climates they would have to be buried well below the frost line. The tubes would probably get damaged in permafrost zones, and also would not work in latitudes that remain frozen year around. However, for the rest of the habitable places on the planet, they may be useful.

Regional differences do occur so check for what types of earth tubes are used in your local area by asking passive solar home builders, checking online blogs or forums, asking local natural building groups, and son on.

Be aware that earthtubes also cannot work in areas with a high water table, such as Florida, where the water table is a mere few inches (10+ cm) to a few feet (1-2 m) down below the ground’s surface—this would cause the tubes to be underwater and would fill up with liquid so no air could pass through. However, they can be placed within an extra-thick earth berm above ground, or in a hill or mountainside, if it suits your circumstances. If you dig and hit bedrock, this may also be a deterrent to using earth tubes, or if the ground is too rocky (if you can dig a trench, then soil infill could be used over the tubes, however).

Outside of these situations, earthtubes typically work well in either dry, hot, arid climates or hot humid climates. You must have sufficient space to dig the trenches and install them, however. Thermal conductivity of the soil (clay being best, sand being worst) is also a factor in how well the tubes will work, as is moisture content.

What is the environmental impact of earth tubes?

Contrasted with the high utility bills and use of fossil fuels, air pollution, energy use, and materials involved to build, maintain, and use forced-air HVAC (heating, ventilation, and air conditioning) systems that typically cool or heat a home (especially in North America or developed nations), earth tubes are a godsend in regards to reducing effects on global warming (not to mention your pocketbook).

People spend up to a few hundred USD/CAD per month to heat and cool their homes; whereas, for what they might spend on utilities for one or two seasons, they could install earth tubes and benefit from the cost savings (which are FREE if the earth tubes are completely passive with no fans or mechanical systems associated with them) for the lifetime of the tubes or the house. Earth tubes are a passive cooling technique utilizing geothermal energy and are a sustainable alternative to standard systems for cooling (and heating).

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Also, here is my new paperback book, published as of 2016… I found it difficult to find how-to books for the owner-builder for making–geothermal energy–earth tubes, so I wrote this for the people who need very detailed information on how to build them yourself: DIY: How to Make Cheap Air Conditioning Earth Tubes

You can also read the first section of the new book via the LOOK INSIDE section (click on the book cover) at Amazon before purchasing it. This new book is 186 pages and is greatly expanded (packed full of useful plans and information and lists, as well as the Q&A above) compared to the small free version of the book I wrote back in 2001.