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Guest Blogs

My Earth Tube Story

Buried ventilation ducts represent an absurdly simple and cheap source of limitless free energy

Buried ventilation ducts take advantage of the fact that the earth is warmer than the air during winter months. When ventilation air is pulled through buried ducts in January, heat from the surrounding soil warms up the air before it enters the house. The concrete blocks help protect the PVC ducts from damage by heavy excavation equipment.
Image Credit: All photos: Malcolm Isaacs
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Buried ventilation ducts take advantage of the fact that the earth is warmer than the air during winter months. When ventilation air is pulled through buried ducts in January, heat from the surrounding soil warms up the air before it enters the house. The concrete blocks help protect the PVC ducts from damage by heavy excavation equipment.
Image Credit: All photos: Malcolm Isaacs
The buried ventilation ducts were routed near the septic tank so that the ducts would be warmed by heat from the tank's wastewater. Backfilling the earth tubes. The ventilation ducts were run under the slab to the far side of the house. The total buried length of the three parallel ducts was 90 feet. The three PVC ducts penetrated the slab at the location of the future mechanical room. The buried ducts were connected to the fresh air intake duct of a Zehnder HRV. When necessary, a 1-kW electric resistance heater downstream of the HRV is used to raise the temperature of the supply ventilation air to approximately 113°F, turning the ventilation system into a “micro-furnace.” The house is located on a steep, rocky site — not the ideal terrain for installing an earth tube. The blue line shows the outdoor air temperature in degrees Celsius. The red line shows the temperature of the incoming ventilation air after it is tempered by the 90-foot-long earth tube.

I saw my first “earth tube” back in 2004, on a tour of row houses in Darmstadt, Germany — a tour which had been organized by the Passivhaus Institut (PHI) to show international visitors some examples of Passivhaus construction. As a visiting Canadian engineer specializing in residential energy efficiency, this was a novel and, for me, unheard-of way to temper incoming ventilation air from extremes of heat and cold.

As Dr.Wolfgang Feist, PHI founder and our tour leader that day, explained, “The efficiencies of this approach are extremely high… These earth tubes generally work very well, but they have to be installed correctly or you can have problems.”

Condensation and mold

Bad news travels fast, and it only takes one “problem” house to undo a lot of good work. During the past ten years in Europe, more than one earth tube was incorrectly installed, leading to pooling of condensate in summer, local mold growth, and major downstream problems with the incoming air quality.

What followed was a lot of bad press. Suddenly opponents had a legitimate complaint with one of the more unusual aspects of the Passivhaus approach (even though you certainly don’t need an earth tube in a Passivhaus building). As a result, many potential clients simply refused to have one installed, despite the excellent track record of such units.

Another approach: brine loops

Instead, designers and builders turned towards low-pressure geothermal “brine loops” for preconditioning incoming ventilation air. The typical loop consisted of a 300-foot length of 1-inch plastic pipe placed around the foundation or under the slab, with a brine or glycol solution circulating through a heat exchanger upstream of the HRV.

These systems don’t need a heat pump, also work well, have a reasonable cost (typically under $2,000 installed, or far less if you build your own heat exchanger and controls), and there’s no danger of air contamination.

I built a house with earth tubes

But my 2004 visit to the Passivhaus Institut predated all these debates about earth tube viability; I returned to Canada with all the enthusiasm of the newly-converted, determined to build myself a passive house — and right away. My understanding was that I needed an earth tube, so that’s what I did. To the best of my knowledge it remains one of only a handful of such installations here in Eastern Canada. So — does it work?

We learn most from our mistakes. I’ve now taught the international Certified Passive House Designer course across Canada and the U.S. for four years, and my own house often serves as a case study of how not to build a Passive House. So, if you want some advice: pick a suitable site with good solar gain; don’t rush into things; don’t assume you know everything; get expert help when you need it and plan every detail before you begin.


EARTH TUBE Q&A


Q. Would you generally recommend earth tubes? What are the biggest problems with installing one?A. I’d recommend this approach to anyone who lives in an area of climate extremes and who will take the time and trouble to install it properly. There are certainly difficulties sourcing components, and the whole process would be far easier, for example, in Austria, where this technology is relatively common, and the right components can be found more easily. I’d recommend a significant slope with a good accessible drain. If you’re digging a straight trench with a backhoe then over 100 feet you will drop 2 or 3 extra feet, so be aware of that and plan accordingly. Q. What was the cost of the earth tube itself?A. I spent around $500 on plastic pipe and fittings, but I still need to build a proper housing for the downstream air intake. Since my site was steep and had difficult access I also used $100 or so of used concrete blocks to protect the pipes from being crushed by trucks and heavy equipment during backfill and leveling. Labor was not much more than a person-day, and landfilling cost was negligible, since I completed the piping during construction, before any backfill had commenced. Q. Are all soil types suitable for earth tubes?A. Clays and silts will have more geothermal heat capacity than light or sandy soils, but perhaps the biggest issue is water: it makes no sense to me to install this technology in areas with a high water table, where it will be put under hydrostatic pressure. Therefore part of the installation procedure can (and perhaps should) involve a pressure test, to check whether the joints are airtight and water-tight, which they need to be. Q. What is the approximate COP of your system?A. The fan draw is around 50 watts, so with an average air temperature increase (delta-T) in winter of (say) 36 F° [20 C°], and a flow rate of 60 cfm, and assuming a heat capacity for air of 0.0182 Btu/cf/°F, then the geothermal “heating” power of the earth tube system in winter works out to around 660 watts. That corresponds to a not-too-shabby COP of 13. Q. How can you be sure of avoiding contamination?A. These days its cheap and easy to check underground pipes through a ground camera — I’m told a contractor can get one of these for his tool kit for $1,000 or less. It’s the same camera used to check for roots and leaks in septic system pipes. Q. Won’t the ground freeze up over a full winter for a shallow system like yours?A. After six weeks of running this system in very cold winter weather there was no sign of this happening — the lowest supply air temperature entering the HRV that I measured was 36.7°F [2.6°C] at -17°F [-27°C] outside. My guess is that the bedrock which lies just below the pipes and under the house acts as a high-conductivity heat sink, and this mitigates the relatively shallow pipes. I have 9 inch of EPS insulation under my slab, so there is minimal heat flow downwards from the house, and it must be said there have been over 2 feet of snow on the ground around here since early December. In addition, I like to think that all our warm drainwater going down into the septic tank provides a significant extra temperature lift for the incoming air, as the ventilation pipes lie right against it. Q. Can you clean the earth tube?A. Sure — I can easily mix up a bucket of warm water with bleach and just pour it in. The steep slope down to the condensate drain will handle that — no problem. Q. Is radon infiltration a potential problem?A. This is another good reason to seal the joints properly, and if there were a rupture in the pipe then yes, there could be a potential issue. But I was pretty careful with sealing every connection and protecting the pipes well during backfilling. Nevertheless, occasional radon testing at very low flow rates might well be a good idea.


I chose the wrong HRV

One of the mistakes I made back in 2006 was to install a Canadian-made VanEE HRV in my new, close-to-Passivhaus. Sure, I did my due diligence and picked a model rated by HVI at 86% efficiency. It was the best I could find and it cost $1,350 with the control unit.

I subsequently found out this unit was completely inappropriate for a Passivhaus. Far from being “high-efficiency,” the HRV produced serious noise in the house and cold drafts at every supply outlet (by cold I mean a supply air temperature of between 41°F [5°C] and 44.6°F [7°C] when the exterior temperature was 14°F [-10°C]). In a Passivhaus, where comfort is paramount, this is totally unacceptable, and when I did the calculation it corresponded to an overall ventilation system efficiency of around 50 – 55%.

The result was that I hardly used my HRV for 7 years — it got turned on only to disperse cooking smells or other odors in the house. Measurements of indoor air quality during that time showed CO2 levels which were often well above the accepted ASHRAE and international guidelines, especially in the bedrooms.

My house has a tested airtightness level which exceeds the 0.6 ach50 Passivhaus threshold, but at 1.0 ach50 it is still remarkably airtight compared to conventional construction. The situation was highly unsatisfactory, and as a result I never even bothered to hook up my earth tube — it remained capped for those 7 years, sticking up through the slab in my mechanical room, an apparent waste of time and money.

In my defense, back in 2006 there were no Passivhaus-quality HRVs available in North America, but thankfully that’s changed today. As Passive House educators and advocates we’ve been helped enormously by the efforts of Zehnder America to bring high quality Passive House-Certified units to the US and Canadian market. This winter I ordered a Zehnder Comfoair 200 HRV in an attempt to improve the sorry state of my indoor air quality. It made sense to hook up the long-neglected earth tube at the same time, and see if it all worked as theory suggests. The little-used VanEE unit was removed and replaced with a European Zehnder HRV, and I also hooked up a data logger and temperature probe on the incoming airstream.

The tubes should have been buried deeper

Good practice suggests you should install an earth tube well below the local frost line, ideally at a depth of 5 or 6 feet, and that you need 100 feet of run at that depth for an average single-family home. My own 2005 installation hadn’t achieved anything like this — for a start, my tight budget was unable to accommodate a specialized 8-inch diameter pipe, so I went with an array of three 4-inch white PVC plumbing pipes*, which could at least be found at the local hardware store.

Next, my site was a steep rocky slope, so it was obvious that nowhere could I bury these pipes much deeper than 24 inches. And lastly, even running the pipes under the slab to the far side of my house, I could barely manage 90 feet of total length. Partly to offset these issues, at the intake end I ran the ventilation pipes right up against my septic tank, which was being installed at the same time, to take advantage of residual ground heat in that area.

The system has (belatedly) been commissioned

I finally hooked the earth tubes up to my new Zehnder unit in January, 2014. I didn’t have high expectations, and I wondered whether the HRV intake fan actually had the power to draw air through three 90-foot pipe lengths, each with several bends and elbows.

Yet, as of March 2014, results have been spectacular. Despite one of the coldest winters for many years, the earth tube has consistently provided a dramatic temperature lift to the incoming ventilation air. On the coldest morning of my monitoring period, at an outdoor temperature of -17°F [-27°C], the supply air was entering the HRV at 36.7°F [2.6°C], for a delta-T of almost 54 F° [30 C°]. That’s consistent with the past month of measurements, during which I’ve seen the supply air temperature entering the HRV fluctuate mostly between 37.4°F and 39.2°F [3°C and 4°C], depending on the outdoor temperature.

The data log plotted below for the 10-day cold weather period starting February 26, 2014, provides more detail.

Feeling smug

Results from just two winter months have convinced this skeptic that the earth tube is an absurdly simple and cheap source of limitless free energy, in much the same way as the sun shining through a window.

This past month, the houses in my town all had continuous infiltration of -4°F [-20°C] (and colder) air, while I had a flow rate of 60 cfm entering the house at an average temperature of 39.2°F [4°C]. It’s hard not to feel smug.

I can’t wait to see the earth tube performance in hot, humid weather. My guess is that air at, say, 86°F [30°C] and 80% relative humidity will be conditioned down below 68°F [20°C] and 40% RH. And in my installation, the underground condensation will flow right back down the 45° slope to a drain sump.

In a very high performance house such as this, with good summer shading, this will be all the air conditioning I’ll need. And the real surprise is that this system runs off around 50 watts of fan power. Would I do it again? Absolutely.

* Yes, I know that PVC is hardly ideal as a ventilation pipe material, and that polyethylene would be desirable, but in this case the pipes have cured for over seven years and have already seen thousands of cubic meters of airflow.

Malcolm Isaacs is a civil engineer with over 20 years’ experience as a consultant in Canadian low-energy construction techniques. In 2005, he designed and built the first house in Canada to Passivhaus specifications. He now works full-time on developing Passivhaus construction solutions for clients in the Ottawa area, and in teaching Passive House techniques as a member of CanPHI’s training team.

23 Comments

  1. User avater
    Brian Knight | | #1

    Interior sidewall condensation
    Thanks Malcolm for sharing your successes and areas for improvement. I think the biggest risk and problem with earthtubes is the condensation that forms on the interior walls of the pipe which attracts dust and pollen resulting in mold and mildew. I dont see how pouring cleaning fluid down the pipe could address this concern.

    Ive heard some folks install strings or ropes to pull a cleaning pig/plug through to better clean the interior surface. Not exactly the kind of maintenance routine I would ask of most of my homeowners. Any thoughts on this and/or how other people handle this problem detail?

  2. Jin Kazama | | #2

    what is the evaluated total additional cost?
    I am not sure it is worth the risk of air contamination ... usually when u detect that there is a problem
    with this type of stuff, it is already too late .

    Using geo loops made out of pex with simple pump and a small furnace rad as an exchanger right at the HRV input to the house probably costs around the same?? and health concerns are much lower.

    Using a simple solar thermal panel and a water mass accumulator through a simple rad would also be a simple possibily ( can also be used to pre warm cold water going to tanks )

  3. User avater
    Dana Dorsett | | #3

    Really? How did you even measure that?
    "This past month, the houses in my town all had continuous infiltration of -4°F [-20°C] (and colder) air, ..."

    It's pretty rare to have (smooth bore & insulated for low heat exchange) infiltration paths that deliver the random infiltration at the outdoor temperature. Almost all natural infiltration involves heat exchange along the infiltration & exfiltration paths. It's nowhere near the efficiency of a full counterflow heat exchanger such as you'd find with an HRV, but it's also a mistake to assume that heat exchange is even close to zero. (Just one ways that most heat load calculation tools overestimate the true loads.)

  4. Brent Eubanks | | #4

    Two questions
    You describe a ~30F temperature rise with the old HRV and no earth tubes. With the new HRV and the earth tubes, you are getting a 54F delta T. However, I'd like to know how much of that is due to the new/better HRV and how much of it is due to the earth tubes. Did you measure incoming air temp ahead of the HRV?
    (Also note that isn't even a fair comparison of the HRVS, since any heat exchanger will perform better with colder incoming air).

    Also, if incoming air at 41-44F was unacceptable, how is incoming air at 39F (with the new system) OK? It's better performance relatively, sure, but the final test is whether or not you are comfortable and that depends on the absolute incoming air temperature.

  5. Marc Labrie | | #5

    Any comments about the CMHC study
    Hi Malcolm,
    Thanks for sharing your experience with earth tubes and north american hrv's. As you know we're building a near Passivhaus and we've already selected the same Zehnder hrv. We're in a similar situation with potential to tap on a septic field heat via earth tubes but had discounted the idea after I read this CMHC study http://www.cmhc-schl.gc.ca/odpub/pdf/67558.pdf?lang=en

    I value your expertise in this field and would appreciate your comments on this study?

    Thanks for your contribution to energy efficient housing. Marc in Moncton

  6. User avater GBA Editor
    Martin Holladay | | #6

    Conclusions of tthe CMHC researchers
    The conclusions of the CMHC researchers were as follows:

    "This study has shown, through a literature search and interviews with researchers, owners and operators, that EAHXs [earth-to-air heat exchangers, or earth tubes] 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."

  7. Jin Kazama | | #7

    Dana or others ...
    When an HRV manuf states efficiency of 75% , is it that we get back up to 75% of total energy or more than 50% equilibrium ?

    I still have hard time understanding how a passive transfer situation can yield more than 50% energy recovery ( as in equilibrium ) .

  8. Dick Russell | | #8

    HRV Efficiency
    "I still have hard time understanding how a passive transfer situation can yield more than 50% energy recovery ( as in equilibrium ) ."

    When two fluid streams exchange heat in a device (heat exchanger), the more surface area the device has, the closer to some thermal equilibrium condition that is reached. The most heat transferred would be in a purely countercurrent flow exchanger, in which an infinite area exchange surface would let the outlet temperature of one stream reach the inlet temperature of the other. For two air flows of equal mass and with no phase change (condensation), each outlet stream would reach essentially that of the other inlet stream. If you were to plot the stream temperatures vs position along the flow path for a countercurrent flow exchanger, you would see two essentially parallel lines, some distance apart, always with the warmer stream temperature some distance above that of the colder stream, coming closer and closer together as you add more surface area. As long as there is a positive temperature difference, heat will be transferred to the colder stream.

    The actual configuration results in different outlet temperatures. A shell-and-tube exchanger, with U-tubes on the inside of a shell, may well be cheaper to build for the surface area required for some industrial processes, but getting much better than equal outlet temperatures is hard to achieve efficiently, unless one uses multiple exchanger shells in series; the more shells used, the closer to true countercurrent flow you get.

    Typical in HRVs, for cost and size reasons, is the crossflow exchanger, which has its own limitation on heat transferred. Some units have two crossflow units inside the cabinet, such as the one I have (Lifebreath 195ECM). As with shell-and-tube exchangers, the more crossflow units in series you have, the closer you get to true countercurrent flow. Mine claims to be up to 88% efficient, however that is defined.

  9. Malcolm Isaacs | | #9

    Answers to Various Questions
    Q: Interior sidewall condensation? by Brian Knight
    A: I agree, this is the most important issue to be aware of. It would be cheap and easy to install a wire or non-corrosive rope to facilitate access through some kind of pulled cleaning brush (i.e. chimney sweep), or you could simply pull through and back with a hose/pressure spray (no danger of getting the hole blocked here). If you had this system figured out up front then annual cleaning, necessary or not, would take 30 mins max. I might try this! Most clients I know would be quite OK with this as a precautionary measure. Another option could be to install a pollen filter on the air intake, with large X-sectional area so as not to drive up static pressure. The Zehnder unit certainly has this "F7" filter already fitted inside the unit, as compared to my old VanEE, which had a wire mesh filter mosquitos could pass through with ease.

    Really? How did you even measure that? by Dana Dorsett
    If we accept that infiltrating air rises in temperature from -20C as it passes through the envelope, as you say, then an unfortunate caveat is the total demolition of the wall U-value and a massive rise in transmission heat loss as a result. That old Second Law again - there's no free lunch, the heat has to come from somewhere if the air temperature rises.

    Two questions by Brent Eubanks
    I don't think you've understood the piece: I refer to air coming OUT of the HRV and into the house at 5 to 7C, which is similar to the air temperature going IN the HRV from the earth tube, even at -20 degrees. It comes out of the HRV at around 20C. Therefore I have no need for defrosting at all, at any temperature, nor any power input to heat the air, just to move it using a 50W fan. No moving parts - it is a most elegant solution if you install it right.

    Any comments about the CMHC study? by Marc Labrie
    It's interesting that none of CMHC's "researchers" has actually installed or used one of these systems - everything I see here is anecdotal. I think it's worth pointing out that both CMHC and "good practice" in these parts suggests that you install a domestic HRV which continuously cycles into defrost mode in cold weather, thereby dumping all your kitchen and bathroom smells into the living areas. That is considered good practice here, while it is illegal in much of Europe. I have not claimed that earth tubes should be installed everywhere, but I do observe that nobody in the North American HRV industry is giving us a better solution than this for our truly high-performance houses, or even attempting to do so. My advice to you, Mark, is to install a low-cost system like mine, with a good slope, access for drainage and cleaning, and just see how you like it!

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Malcolm Isaacs
    Malcolm,
    Your language is still confusing to me. In your article, you have written, "On the coldest morning of my monitoring period, at an outdoor temperature of -17°F [-27°C], the incoming air was entering the house at 36.7°F [2.6°C], for a delta-T of almost 54 F° [30 C°]. That’s consistent with the past month of measurements, during which I’ve seen the intake air temperature fluctuate mostly between 37.4°F and 39.2°F [3°C and 4°C], depending on the outdoor temperature."

    In these two sentences, you tell us that the range of temperatures for the "incoming air entering the house" was 2.6°C to 4°C. Please tell us whether you were measuring:

    (a) The temperature of the supply air leaving the HRV, or
    (b) The temperature of the supply air entering the HRV.

    If the answer is (a), then we really don't know the contribution of your earth tube. What readers would really be interested in knowing is the temperature of the air entering the HRV (compared to the outdoor temperature).

  11. Jin Kazama | | #11

    Dick Russell
    thanks for the quick explanation,
    i had never pictured the crossflow exchange in my head yet,
    and you triggered just that.
    Now i understand i think.

    So in theory, with 2 counterflow of air, the length of the exchanger should be tuned
    in accordance to 1- its exchange rate efficiency ( design/material etc... ) 2- the maximum delta T of the flows, so that it would reach near complete exchange before the end of the exchanger ???
    am i right to assume the "basic of the theory " is in this direction ?

  12. Jin Kazama | | #12

    aftertoughts..
    that would explain why most higher CFM HRV are very large as they need more total surface of exchange to be able to achieve a desired efficiency at the high CFM ????

  13. Tim Naugler | | #13

    Measured Incoming air temp
    I have recently spoken with Malcolm about his earth tube. I believe Martin that the measurements in question that Malcolm gives in his article are (B) the temperature of the supply air entering the HRV. I feel relatively certain about this as we discusses results comparing the earth tube to our simple geo preheat system (similar to the one mentioned by Jin Kazama) I think it's a great option, for the right client, lot, situation, etc... I'll be interested in how effective it proves to be during the summer months as well.

  14. Sonny Chatum | | #14

    Ah, Martin, Why did Malclom's language confuse you?
    Martin, you can't hardly say it any clearer than Malcolm actually did: "the incoming air was entering the house." "Entering the house" comes prior to entering the HRV, unless you would install your HRV outside.

    Of course, that doesn't mean you can believe the claimed magnitude of delta T. I don't. Malcolm says, "nowhere could I bury these pipes much deeper than 24 inches." What?! That's not even below the frost line in Maryland, and all of a sudden 90 feet of earthtubes are taking air temperatures in the negative teens F and turning them into plus thirty-some F by the time the air enters the house. I think that is F-ed up by too much.

    I've seem some actual measurements of below ground temperatures. Malcolm is correct in saying that you need 5-6 ft of depth, not 24 inches, in order to achieve a usable, year round ground temperature. So, why would anyone believe that 24 inches is giving the claimed delta ?. It must be an amazing septic tank. I believe it would take more than 90 feet of pvc pipe running right through the middle of a major sewage treatment plant on the Friday after Thanksgiving in order to get that kind of quality btu transfer into pvc and then into the airstream.

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Sonny Chatum
    Sonny,
    Some readers might think that air "entering the house" might refer to air that enters the living room and bedrooms through supply air registers.

    I believe that the phrases "supply air entering the HRV" and "supply air leaving the HRV" are less ambiguous that the phrase "air entering the house." But, as an editor, I'm always striving to avoid ambiguity, so this confusion may simply be due to my editor's glasses.

  16. Malcolm Isaacs | | #16

    Answer to Questions
    Sonny - you're correct, the temperature refers only to air coming out of the earth tube before it enters the HRV. Otherwise there would have been no point in my article! I did post specific Q&As after the piece to describe my own surprise at the performance of the system. You may state that you "don't believe" these values, but I was taking temperature measurements on an hourly basis for 6 weeks during the depths of winter, and these readings are IN FACT what happened, with pipes that are IN FACT buried less than 24", albeit under plenty of snow. I agree that the 30C temperature lift was a great surprise, but as a scientist I then try to figure out how to explain this reality. That I've tried to do. What I've also learned is that most people have pre-conceived ideas about "frost depth" but zero actual measured data. With early snow covering my area since November, it turns out that there may be very little frost penetration this year, in an area much colder than yours.

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to Malcolm Isaacs
    Malcolm,
    Thanks for your clarifications. I have changed the wording slightly in your article to remove any chance of ambiguity.

    I agree completely with your observations about frost depth. I have seen pipes that are buried 3 feet freeze in northern Vermont (because the pipes crossed a plowed driveway, and were unprotected by snow) -- and at the same time, in the nearby woods, with snow three feet deep, it was possible to dig through the snow and find unfrozen soil (and happy evidence of voles and moles) under the snow.

  18. Sonny Chatum | | #18

    Response to Malcolm and Martin
    Malcolm, if you have tried to explain such a large, claimed delta T, you certainly have not done it here. I would hope no one would stumble on this article and immediately start shopping for pipe, based on what is given in the article.

    I am capable of a somewhat vigorous treatment of the heat transfer problem, but have no interest, because I already know the answer would be quite different than the one you have implied. I have only a general interest in the beauty of solar energy and that it should b e used whenever practical. Solar energy is much more diffused than you imply. Further, heat transfer from earth to pvc to a largely and necessarily laminar airstream is fairly poor.

    Malcolm and Martin: Both of you have thrown out some examples of the R value of some snow. While providing some insulation for the buried tubes, an analysis would reveal that it is trivial for the big picture. If you plot below-ground temperatures versus time over the course of a year, you would get a sinusoidal curve with an amplitude that may be somewhat affected by this snow cover, but not by enough to achieve what you claim. Further, as you have already noted, the temperature at 24 inches deep would be a very poor area to use. To get a constant year-round temperature in your area, that would make any sense to use, would be somewhere in the 5-6 foot deep range. Sure you can throw out that frost depths are not scientific---from code, which is just a generalization, just a concensus determination, just a conservative number. I just used that for a quick and easy demonstration of how far off you are.

    Oh, Martin, I now believe you were correct in trying to ensure clarity. Unfortunately, the entire presentation suffers from considerably more cloudiness than just that element.

  19. Joe M | | #19

    Earth Tubes are a great idea
    I believe the results given in this article are pretty good considering the depth and length of the pipes. The results would improve dramatically if the pipes were 6 feet in the ground. Depending on the climate of the location. Regardless, all one has to do is look at the "umbrella" system being used to draw the heat up from further down in the Earth.

    Just run the pipe in a straight run and very deep. Then insulate the ground above it in layers. Like they are doing on Earth bermed homes. The heat in the Earth will move closer to the surface since the frost line will be considerable warmer. Thus, tapping into more heat in the tube.

    Humidity has always been an issue for any underground home or greenhouse. Bringing outside air into the tube solves the problem 'mostly'. But thats not enough to avoid problems. So yes. Some kind of maintenance needs to be put in place. Condensation doesn't simply run down the pipe at a slope. It would have to be nearly vertical like a downspout to drain it off. So the only way I can see to fix this is to either manipulate the humidity of the incoming air (not likely) or just wipe out the pipe (feasible).

    If its -17F outside and my house is 36F inside. Imagine just how easy it would be to heat it with a stove or furnace. Or even some other heat source. The savings in heating would be dramatic. Well worth the cost and effort of this Earth Tube installation. It's very logical and proven. Heat Pumps are overrated and have only dominated the market due to people having zero know-how. I think you could incorporate a "hot water" system that would make the tubes even more effective. Here's how:

    It has been demonstrated that you can slowly run water through an open sided pipe lying on the ground, running across 100ft to heat the water. Using materials like foil and other thermal heat catchers. The sunlight heats the water. Even in the winter. If you are a homesteader that uses a mountain fed spring for a water source, you could channel the runoff into this thermal pipe. The water temperature would rise as it slowly moves down. There would be a regulator to slow the flow. The heated water would run down into the ground directly alongside the Earth Tube for the full distance. Finally running out into a drain tile under the ground and moving on out. This would increase the heat output of the Earth Tube. Would it affect the humidity? I don't know. Possibly. But probably not any worse than what is already present.

  20. Bob N | | #20

    Thinking about trying it
    So if I follow there were 3 - 30' sections of 4" PVC.

    Why not place them under the slab of a passive solar build?

    where I am the earth temp below is about 60 degrees, that heat transferred to the intake has to big a big boost when it's cold out, we get into the 20's in the winter.

    then again do I need 90' with the warmer earth temps of my location? would 2 - 30' runs work?

    If all 3 were given a 1/4" slope to area where they come up thru the slab by HRV an inspection cap of some type can be made to see if any water was present, if so cleaned out.

    A wire with a brush or sponge to wipe thru 1 or 2 times a year should be an issue,

    Always have the option to cap it all off and reroute the intake directly outdoors.

    The other possibility is a brine loop, but that needs a small pump running and an ex-changer of some sort.
    Any details on those two parts?

    look forward to your feedback

  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to Bob N
    Bob,
    The main reason that builders don't install earth tubes under a slab is the danger of freezing the soil under the slab in winter.

    You mention "in my location," but you didn't tell us where that location is.

    Most experts warn builders to stay away from earth tubes, because of the many possible problems with these systems.

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

  22. Bob N | | #22

    the frost line is only about
    the frost line is only about 10", it is located in the NW corner of GA

    I'm planning on using wing insulation so don't think there's a risk of freezing under the slab esp with the slab being the heat mass

    Oh Martin that link didn't work?

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Bob N
    Bob,
    I'm fairly sure that in your climate zone, installing an earth tube under a house slab won't freeze the soil. Of course, you still want to install a continuous layer of rigid foam directly under your slab.

    Here's the thing, though: the frost depth has nothing to do with whether or not an earth tube can freeze surrounding soil. Even if an earth tube is installed below the frost depth, introducing a continuous stream of winter air will cool the soil touching the earth tube. In many climates, that means that a cylinder of ice will begin to form around the earth tube. (Imagine an earth tube in a very cold climate, where the air entering the earth tube is often below 0 degrees F.)

    As far as I know, the link works. If you can't read the article, it's not because the link is broken -- it's because you forgot to subscribe to GBA. You can always sign up for a free trial if you don't want to pay for a subscription.

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