All About Earth Tubes

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All About Earth Tubes

Buried ventilation ducts can provide benefits — as long as you avoid a long list of possible pitfalls

Posted on May 12 2017 by Martin Holladay
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An earth tubeVentilation air intake tube, usually measuring 8 or more inches in diameter and buried 5 or more feet below grade. Earth tubes take advantage of relatively constant subterranean temperatures to pre-heat air in winter and pre-cool it in summer. In humid climates, some earth tubes develop significant amounts of condensation during the summer, potentially contributing to indoor air quality problems. is a buried ventilation duct. The idea behind burying ventilation ducts — the ducts conveying fresh outdoor air to a building — is that the soil surrounding the ducts will warm the ventilation air during the winter and cool the ventilation air during the summer.

Earth tubes can work, as long as:

  • the duct is installed in a climate with useful soil temperatures,
  • the duct has a large enough diameter,
  • the duct is long enough,
  • the duct is buried deep enough,
  • the duct is installed in a way that minimizes potential condensation problems,
  • the duct is able to exclude radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles..

That said, earth tubes are expensive to install, so they are rarely a cost-effective way to condition ventilation air.

Poorly designed earth tube systems are probably more common than well designed earth tube systems, which is why many earth tubes are eventually capped off and abandoned. If you are considering installing an earth tube, it’s worth learning about what works and what doesn’t.

Get the details right to avoid mold

Earth tubes have a mixed reputation, and it’s easy to find published reports of successes as well as failures. Full disclosure: I’m not a fan of earth tubes. If I wanted, I suppose that I could cherry-pick scary stories from among published anecdotes, but that would be unfair. If you want to install an earth tube, and you do your homework before installing your system, there’s no reason to think that ventilation air delivered by your earth tubes will be damp or mold-infested. Some earth tube systems work well.

The main problem with earth tubes is that even when builders install a well-designed system, the energy savings are so low that it’s hard to justify the high installation cost.

Successful systems

Earth tube systems have been installed successfully in single-family homes as well as large institutional buildings.

Here are links to articles about two successful installations in small buildings:

Here are links to articles about two successful installations in very large buildings:

Deterioration over time

Problems with earth tube systems don’t occur right away. Owners of poorly designed systems sometimes provide enthusiastic reviews soon after the system is commissioned, with disappointment occurring a few years later.

There are at least two ways these systems can deteriorate:

  • If the earth tubes have a small diameter or are too short, the earth temperatures may drop during late winter in a cold climate, or may rise in late summer in a hot climate, causing performance to deteriorate.
  • Condensation in the buried ducts during the summer made lead to mold growth. Problems with mold may not appear until the third or fourth summer of operation.

The moral of the story: early homeowner enthusiasm should be taken with a grain of salt.

Published articles expressing skepticism

There are plenty of earth tube articles expressing what I would call “cautious pessimism.”

A very useful review of the available research on earth tubes was published in December 2011 by the Canada Mortgage and Housing Corporation. The document, titled “Earth Tube Ventilation Systems — Applicability in the Canadian Climate,” noted:

  • “Despite the apparent simplicity of EAHX [earth-to-air heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.] systems, the literature reveals that they are actually very complex systems. They need a proper choice of materials and careful construction techniques to avoid potential infiltration of water and radon, mold problems and accumulation of condensates.”
  • “They require sophisticated controls and may need to be bypassed during some periods. They have a thermal memory: because of accumulation of heat in the ground, the amount of energy extracted or evacuated during a given hour depends on how much was extracted or evacuated in previous hours or days.”
  • “Some proponents of the PassivHausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard have reported anecdotal evidence of air quality problems with earth tubes in northern Europe, especially in Scandinavia, but not in Central Europe, though they aren’t sure why they don’t have these problems in Central Europe. ... This area seems to be incompletely understood so caution should be exercised, especially since places where problems have been reported (Scandinavia) have climates very similar to some parts of Canada.”
  • “There are numerous reports of EAHXs delivering warm air when cold air is needed, or vice versa. For example in the middle of summer it may be beneficial to stop the EAHX at night and draw air directly from the outside if it is cooler. The problem is also prevalent in Fall and Spring seasons. For example in spring, since the temperature of the ground lags behind ambient temperature, the EAHX tends to cool, not warm, the air it delivers to the house. The literature indicates that EAHXs working under such adverse conditions can provide a very significant ‘negative’ contribution to the overall heating or cooling needs of the building.”
  • “There is some evidence in the literature that the heating or cooling capacity of the soil becomes much diminished after some time (depending upon numerous factors such as thermal properties of the ground, flow rate in the EAHX, etc.), and that the heating or cooling capacity measured over a month will be much lower than that measured over one day.”
  • “The literature shows that strictly from an energy point of view, there is very strong evidence that combining an EAHX and a HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. provides little benefit in heating mode. Both devices are trying to raise the temperature of the incoming air; but the temperature rise caused by the EAHX makes the HRV work less efficiently, so the combined energy gain is much less than the sum of the gains of each system working independently.”
  • “This study has shown, through a literature search and interviews with researchers, owners and operators, that EAHXs may have benefits when used under the right conditions and in the right climate, but also that they are very subtle systems which require careful design and operation to be successful. The literature shows that an improperly designed system will not work as expected, or result in poor air quality, etc. leading to disenchantment with the system and in many cases decommissioning: it is often very difficult to fix EAHXs once the trenches are back-filled. The literature also shows that controls, air quality, and thermal memory of the ground are but three of the areas to pay close attention to when considering an EAHX. It also demonstrates that an EAHX can be redundant when used in conjunction with heat recovery ventilators (HRV), and that the economics are rarely favourable.”

Another cautiously pessimistic article is one I wrote in 2012, Belgian Passivhaus is Rendered Uninhabitable by Bad Indoor Air. After describing how soggy earth tubes contributed to a serious IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness. problem, I quoted building scientist Hugo Hens: “Ground pipes are potentially a risky technology. Though solely promoted as energy savers, their impact on energy consumed is too minimal [for] the possible risk they create in terms of degrading supply air quality.”

Things to consider

If you are eager to install an earth tube system, in spite of the caveats, keep these points in mind:

  • Climate matters. You’ll want to consult a soil temperature map to determine whether you live in a location where an earth tube will work (see Image #2, below). You certainly don’t want to install an earth tube in south Florida, where the temperature of the soil 5 feet below grade is 77°F. An earth tube system makes more sense in Massachusetts, where the deep soil temperature is likely to be 47°F.
  • Use large-diameter pipe, make it long, and bury it deep. Earth tube systems are expensive, so homeowners are tempted to cheat on the specifications. An 8-inch duct is better than a 6-inch duct, a 100-foot duct is better than a 40-foot duct, and a 5-foot-deep trench is better than a 3-foot deep trench.
  • Exclude radon. Some designers of earth-tube systems have used buried perforated pipe, so that condensation can drain from the system. The only problem with this approach is that it can allow radon to be mixed with your ventilation air. Bad idea.
  • Choose the right duct material. Some earth tube systems use 4-inch or 6-inch-diameter PVC pipe (because it is widely available and inexpensive). There are at least two problems with this approach: a larger diameter pipe would be better, and some green builders worry that PVC may offgas undesirable chemicals. Polyethylene or polypropylene pipe is better than PVC. You’ll probably want smooth-walled pipe, not corrugated pipe, if you want to assure that collected condensation will drain. And if you are worried about mold, you’ll probably end up buying polypropylene pipe from Rehau — the kind with an antimicrobial silver lining. (Guess what: it’s not cheap.)
  • Slope the pipe to drain the condensate. Your ducts need to have a continuous slope so that condensate can drain. The usual approach is for the earth tube to slope toward the exterior, so that condensate can drip outdoors. If your site doesn’t allow that approach, you’ll have to slope the earth tube toward your basement, and include an indoor trap and cleanout fitting.
  • Plan ahead for duct cleaning. If you have a 100-foot long buried duct at risk of developing mold, you need to come up with a good plan to clean the 100-foot duct every few years. If the duct has a diameter of 4 feet, you can send a child into the duct with a brush. If the diameter only measures 8 inches, you’ll need an elaborate brush with extension rods or a system using liquid cleaners. One of the few companies that sells components for earth tube systems, Rehau Ltd., provides this advice on its web site: “According to the specifications of VDI 6022, appropriate cleaning intervals and precautionary inspections (sanitary control) should be conducted. Country specific regulations may have different requirements in terms of frequency of inspection. Cleaning and inspections costs should be factored into your total investment plan.”

These systems are expensive

In a presentation on earth tube systems, Jason Morosko, the vice president for engineering at UltimateAir, shared cost figures for three residential earth tube systems. The least expensive system cost $1,770. The next-most-expensive system cost $5,729. The most expensive system cost $10,400. The average cost was $5,966.

As you might guess, variations in excavation costs explain this wide range.

Energy savings

Estimating the energy savings associated with these systems is quite complicated, since the energy savings vary with changes in the outdoor air temperature and the earth temperature. Needless to say, both of these temperatures change all the time.

According to optimistic estimates, a residential earth tube system can save $300 or $400 per year. Skeptics tend to come up with much lower numbers. Using an optimistic estimate of $400 annual savings would mean that a $5,966 earth tube system would have 15-year payback — not counting expenses related to cleaning the buried ducts.

Glycol ground loops

If worries about condensation and mold make you leery of installing an earth tube, you can consider another way to condition incoming ventilation air: using a glycol ground loop connected to a fan-coil unit installed in a duct upstream from your HRV or ERV.

For more information on these systems, see Using a Glycol Ground Loop to Condition Ventilation Air.

Martin Holladay’s previous blog: “All About U-Factor.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Malcolm Isaacs
  2. Image #2: BuildItSolar.com

1.
May 12, 2017 10:22 PM ET

Overly pessimistic
by Malcolm Taylor

Surely you could send a child with a brush into a small diameter pipe than four feet?


2.
May 13, 2017 4:35 AM ET

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
You're right. I forgot about all the times I crawled into culverts when I was a kid. It was fun.


3.
May 13, 2017 8:04 AM ET

Response to Martin and Malcolm
by stephen sheehy

It's important to plan ahead for cleaning earth tubes. If you put a rope in during installation, all you need to do is find a child who's a tight fit, tie the rope to his feet and pull.


4.
May 17, 2017 11:42 AM ET

earth air tubes- JM
by Ultimate Air

Nice article Martin. I'm on year six with my earth air tubes... with no cleaning. I can't scientifically say there has been no problems- but I can say that anecdotally.... I have had no problems. I'll have to find someone with a scope to inspect them I guess. But everything seems to be working fine. And my tube cost was the cheap one. At the beginning of the season is when you have the highest 'capacity'. Moving from winter to summer... ground is still cold... and you are using it to 'cool incoming air'. Summer to winter- ground is still hot and you are using it to 'heat incoming air'. There is roughly a 16 degree temperature swing from beginning to end of season. Could be more or less- depending on tube depth. At $1800- I would still say it is worth it. It eliminates the cost of preheat on a H/ERV... both the up front equipment cost, and the energy use cost over the life. Another interesting point- all cost comparison is based on indoor set temperatures... 68-72 F... If your goal was instead to 'survive' - they would have much higher value as passive means to keep your home from freezing. - Jason Morosko


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