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Q&A Spotlight

Do Earth Tubes Make Any Sense?

A homebuilder wonders whether he should revive two energy-saving ideas from the 1970s

Image 1 of 2
Twenty-first-century earth tubes: Although few builders incorporate earth tubes in their HVAC systems these days, some do. This commercial installation was designed by George Sullivan of Net Zero Analysis & Design.
Image Credit: Image #1: George Sullivan
Twenty-first-century earth tubes: Although few builders incorporate earth tubes in their HVAC systems these days, some do. This commercial installation was designed by George Sullivan of Net Zero Analysis & Design.
Image Credit: Image #1: George Sullivan
This graph plots energy use associated with defrost cycles in BuildingGreen founder Alex Wilson’s Zehnder’s ERV system.'
Image Credit: Image #2: Alex Wilson

Daniel McKinney is reaching four decades into the past for two important features of a new house he plans to build. Both notions were mostly discarded after early attempts at energy efficiency led builders in new directions, but McKinney thinks they may still have some merit.

“OK, here’s the basic idea,” he writes at Green Building Advisor’s Q&A forum. “I would like to use an earth tube system to bring fresh air into a very tightly sealed home. I’m also designing the home with a ‘solar stairwell,’ a stairwell that’s exposed to the sun with big windows, with the back wall of the stairwell being made of dark-colored cast concrete.

“During cold months,” McKinney continues, “the concrete wall acts as thermal mass, gathering heat during the day and shedding it during the night. Exterior louvered shades above the big windows would keep this wall from getting direct sunlight during the summer. So, the question is this: Would it make sense to duct the earth tube air vertically through the concrete wall?”

An earth tube, which is simply a pipe buried in the ground, alters the temperature of incoming air because soil temperatures well below grade don’t change much seasonally. Incoming air is warmed in winter and cooled during the summer. The other part of McKinney’s plan, a heat absorbing, high-mass Trombe wall, was a common feature of many early passive solar homes. The appeal of both of these ideas lies in their simplicity, but many builders and designers now think their flaws outweigh any potential benefit.

Who’s right? That’s the topic for this Q&A Spotlight.

No, this is the wrong approach

Earth tubes and Trombe walls are dated concepts, says GBA senior editor Martin Holladay. Don’t bother.

“You are entertaining two ideas from the 1970s,” he says. “Both ideas have been substantially discredited, but, like the walking dead, it seems that it takes more than a stake through the heart and a bunch of garlic to keep these ideas where they belong.

“Briefly, the added cost to build a Trombe wall or install an earth tube are so high that there is no way that any conceivable future energy savings will ever be enough to justify the investment,” Holladay continues. “In the case of a Trombe wall, the details you describe may actually increase rather than decrease the annual energy load of the house. Earth tubes are expensive to do right — you need a very large diameter duct, buried very deeply, in a very long trench — and are subject to problems with condensation and mold. They don’t save much energy.”

Stephen Sheehy wonders why McKinney would consider installing earth tubes when he already is considering mechanical ventilation. Further, there is the prospect of a decline in air quality over time.

“I guess what scares me most is that the tube is pretty much impossible to clean, so over time the ‘fresh’ air is likely to get less fresh,” Sheehy writes.

Early inspiration from Canada

McKinney isn’t alone in his interest in earth tubes. Brad Hardie also was intrigued by the technology as he designed a net-zero energy house in New Hampshire. Hardie points to a 2014 guest blog at GBA by Malcolm Isaacs, a Passivhaus pioneer in Canada, who installed earth tubes at his own house.

In researching the topic, Hardie wrote to Isaacs to ask whether he’d changed his mind about earth tubes since he installed them, and whether Hardie should incorporate earth tubes in his own design. His plan was to use the tempered incoming air to lower energy demands by piping the earth tubes directly into a Zehnder heat-recovery ventilator instead of installing a Zehnder ComfoFond to moderate outdoor air temperatures.

“I have been considering the Zehnder ComfoFond as an add-on to the HRV, but have to admit the idea of using earth tubes has me so much more excited,” he told Isaacs. “Is there anything you would change, add, or omit if you could do it all over again? Some other info: I’ve got a fairly level site, with well draining sandy soil, and I will be doing the excavation myself.”

The short answer was, no, Isaacs said. Earth tubes still look like a good idea.

“A couple of colleagues have built much more high-end earth tubes than mine recently, with ‘proper’ components,” Isaacs replied, “but I feel that if you’re vigilant and careful then you don’t need custom products to do this — the [earth tube] approach is remarkably resilient. I did everything ‘wrong’ but it still works very well, so I might use a better quality [polyethylene] pipe, but otherwise not much would change next time.”

That view is shared by Jason Morosko, a certified Passive House consultant who has been living with an earth tube system for four years. From his experience, McKinney might be able to gain between 500 and 3,500 BTU/h of “free energy” from earth tubes. He invested less than $1,200 in his system of two 8-inch pipes 100 feet long.

“Are they worth it?” he asks. “Well, in my climate my [energy recovery ventilator] would require preheat to ventilate at cold outside conditions (times below 12°F). Hence, I would have need of some type of duct heater to keep incoming air warm during cold snaps. Hence, the duct heater is generally an electric resistive device, meaning large instant amp draw for short pulses. Not a lot of energy — but if you are looking at an off-grid house with solar electricity, you do not want spikes of energy use, which are bad for batteries and storage capacity. The load happens at the worse condition (climate timing speaking).

“So that is something to consider,” he continues. “Given my cost and experience of four years thus far — yes, it was worth it. Financially, maybe break even at year 4?”

Taking necessary precautions

The idea has obvious appeal, McKinney adds. Fresh air at a temperature that has been moderated by the earth looks a lot better than ambient air when it’s 10°F below zero.

“I don’t think that the requirement that a system be designed well should disqualify the idea out of hand,” he says. “A poorly designed and constructed roof can spell disaster for a new home, but does that mean that the idea of a roof is a bad one?”

Excavation costs, an argument against earth tubes, will be lower because he plans on doing the work himself, and he can make the earth tube trenches do double duty by using them for utility and water lines as well as earth tubes.

Further, he could use a material that’s more thermally conductive than plastic, and design the system so that condensed moisture can’t collect or cause mold growth.

“Obviously, the pipe must be pitched at an angle that will allow condensation to flow and not puddle, and that condensate must be able to drain at one end or the other,” McKinney adds. “I’ve seen designs where the tube pitches down toward the building and the condensate drain is in the basement. This seems like a good idea, allowing for that line to be cleaned, and allowing access directly to the earth tube, as long as the desired depth can be achieved and still have the end of the tube accessible inside the basement.”

Installing ultraviolet lights at one end of the tube would be another hedge against the contamination of income air by mold, bacteria and fungus, he says.

“I am quite fond of more than a few ideas that were born in the ’70s,” he says. “That decade did generate some useful stuff.”

Making the right choice for the type of pipe

McKinney has been considering a type of pipe called PCD, or polyvinyl coated ductwork, which is a G-90 galvanized steel pipe primed with epoxy and finished with a 4-mil polyvinyl coating. If buried above the water table, McKinney says, it would provide much more thermal conductivity than PVC pipe.

It’s not the thermal conductivity of the duct itself isn’t the limiting factor in heat transfer, writes Charlie Sullivan, but the thermal conductivity of the dirt and the thermal transfer between the air the earth tube.

“And in other resects,” he says, “the PVC/galvanized steel sounds like the worst possible combination to me: the steel will eventually rust out, and in the meantime, the PVC will outgas into your ‘fresh’ air. I would want a smooth-walled seamless polyethylene tube. (Polyethylene is hard to glue, and so regular HDPE drain pipe would not be ideal — the joints could leak soil gas into the system.)”

For these reasons, Sullivan suggests a smaller-diameter buried line in which a glycol mixture is circulated, eliminating the potential for mold that comes with an air system.

“Then you spend less money on pipe and take advantage of the better heat transfer with the liquid, which you then connect to a liquid-to-air heat exchanger,” Sullivan says. “It’s pretty clear that that’s not cost-effective, but if you are wanting to do a DIY system and you aren’t too concerned about cost effectiveness, you could probably DIY one of those for less than the Zehnder system.”

It’s true, Holladay adds, that a glycol loop brings with it a lower risk of low indoor air quality. But the bottom line is that neither a glycol loop nor an earth tube approach is cost-effective. “They both cost more to install than can ever be justified by future energy savings.”

Try it, and then let us know

McKinney seems confident that earth tubes are still relevant, and seems unwilling to let go of the idea. With that in mind, Holladay suggests he just give it a try.

“Here’s my advice: go ahead and install an earth tube,” he says. “Keep track of how much it costs to install the earth tube (including an estimate of excavation costs, backfilling costs, and landscaping costs). Install some monitoring equipment for temperature and relative humidity [RH]. You want to measure the temperature and RH of the air right before it enters the HRV. Ideally, you also want to log outdoor temperature and outdoor RH.

“After you live in your house for two years, write a blog for GBA about the experience,” he continues. “And remember — if you have mold problems, you can always cut off the earth tube in your basement with a hacksaw and cap it. Then you can install an air intake for your HRV somewhere else.”

Our expert’s opinion

GBA technical director Peter Yost added this:

I must admit that my building science background creates an almost knee-jerk reaction against earth tubes: To what gain would you intentionally run your fresh air system through underground ducts? And certainly early work with earth tubes was rife with indoor air quality problems, either from the get-go or over time.

But there are fully-engineered earth tube systems, such as those being designed by George Sullivan of Net Zero Analysis & Design Corp and by Jason Morosko of UltimateAir that I think should not be dismissed out of hand.

Sullivan has done more than a half-dozen earth tube designs, tempering the air exchange in his ERVs with tubes running through 55°F soil temperatures. “In Climate Zone 5 and colder zones, it’s worth it because of the combined heat and cooling effect,” says Sullivan. “In really cold climates, the defrost cycle avoidance is a big plus.” Sullivan has developed his own modeling program to design and predict his earth tube systems.

Another consideration is the defrost cycle associated with ERVs and HRVs, which vary by device. For example, the graph shown in Image #2, below, shows the energy associated with defrost cycles from BuildingGreen founder Alex Wilson’s Zehnder’s ERV system.

Finally, a word about PVC and CPVC outgassing: While it’s true that both PVC and CPVC pipe outgas, these “rigid” plastic pipe products should not be confused with PVC shower curtains, PVC vinyl siding, etc. These more flexible PVC products contain plasticizers, such as phthalates, which are much more volatile and of much greater concern.

[Editor’s note: In an earlier version of this article, GBA published an inaccurate report of energy savings data from an earth tube installation. The error was mine. GBA regrets the error. — Martin Holladay.]

30 Comments

  1. Malcolm Taylor | | #1

    Depth
    Peter Yost describes the Iowa' projects tubes as running 15 to 21 feet deep. Was this depth necessary for performance, or were they able to get it through some serendipitous circumstance? Excavating to 20 feet is either not possible or extremely costly in most areas.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Malcolm Taylor
    Malcolm,
    I had the same thought you did. Digging a 110 foot trench that varies in depth from 15 feet to 21 feet is quite tricky. If you follow OSHA requirements to avoid trench collapse (and possible death to workers in the trench), as you should, this is a massive job, and quite expensive.

  3. Charlie Sullivan | | #3

    Combined with radon system?
    I decided to see whether the project web site said anything about how they excavated the 15-21 foot deep trenches, and I didn't have any luck with that, but I did find this description of how the garage exhaust and radon systems are tied together:

    "Garage Earth Tube Exhaust is combined with Radon Collection and Venting from Footing Drainage System. The combined ventilation system exhausts all of the toxic gases from an extremely tight building envelope, and is driven by a wind ventilator on the end of the earth tube exhaust."

    If I'm understanding that right, that means that when there's no wind, the radon collected by the sub-slab perforated tubing will be as likely to go into the garage as it is to go out where it's supposed to go. If this is the system that is being held up as an example of how it can be done right, Peter Yost's knee jerk reaction against these systems sounds like a pretty reliable indicator. I hope they are doing radon monitoring and that it turns out that the radon system wasn't really needed, or that I'm wrong about how it's set up.

  4. Rick Van Handel | | #4

    Wow. Those depths do sound
    Wow. Those depths do sound extreme. At depths exceeding 20 ft a Professional Engineer needs to design a protective system to keep trench walls from collapsing. Also the pipe sidewall thickness needs to be increased to handle soil weight. Installing at around an 8' depth would be less than half the cost. If it were me, I would install smothe wall HDPE pipe with gasketed joints. And pitch it away from the structure.

  5. Chris Jorgensen | | #5

    Geothermal tax credits?
    I thought those were for installing energy star rated refrigeration equipment, GSHP's.

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

    Response to Chris Jorgensen
    Chris,
    I agree with you. I think that anyone who takes a geothermal tax credit for an earth tube is skating on thin ice. (But we all know the basic rule about filing a tax return: any claimed deduction works, as long as you don't get audited. But if you get audited... you have to prove that the deduction is valid.)

    The basic problem with the earth tube deduction is that for any type of geothermal equipment to be eligible for the 30% tax credit, the equipment has to be qualified by Energy Star. Energy Star provides labels for ground-source heat pumps -- but not for earth tubes.

  7. Josh Manders | | #7

    I was in love with the concept, till I wasn't
    For what it's worth:
    About 8 years ago I really got into earth tubes. I mean REALLY got into them. It started when I got thinking about hospitals and how they often exhaust 100% of the conditioned air. They also have significant fresh air requirements. If that air could be per-conditioned via earth tubes, that would equal some fantastic savings while still making it safe and alleviating concerns about contaminated heat recovery wheels. Right!?

    At first, I found lots of great information especially from Europe. I researched a lot and stumbled upon case studies like http://www.zigersnead.com/current/blog/post/earth-tubes/04-06-2008/1045/ . I started looking at tube construction (antimicrobial copper, HDPE with inner lining...) and contamination issues. Newer pipe construction and install techniques seemed to address many issues but other problems kept creeping up. Ultimately, what really sealed the deal is the impracticality of it all. From installation, maintenance, repair issues, contamination, proper sizing (both size and surface area/heat exchange properties), longevity (materials & site) -- it just doesn't logically pan out.

    Only a couple years ago I was emotionally triggered because someone on this forum (not naming names :-) ) made the assertion that the practicality of residential solar water heating in most of the country was coming to an end. How could this be?!? Solar water heating is one of our more efficiently brilliant ideas humans have come up with and yet when you actually take a look at cost of it all (cost, materials, install, prolonged maintenance..ect) the numbers aren't panning out. That isn't to say that there aren't a bunch of copper crickets somewhere still cranking out hot water after 20+ years. And certainly, just because a great idea isn't practical doesn't mean it isn't worth trying. A prime example might be Facebook and Google's adventures in relocating server farms underwater in the ocean -- doable? Yes. Practical? Not really.... or at least.... not yet.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Josh Manders
    Josh,
    Thanks very much for your comments. Your intellectual journey on this topic has been similar to mine: "I was in love with the concept, till I wasn't."

  9. Malcolm Taylor | | #9

    Josh
    I agree. A pox upon Martin and his clear-eyed logic. I want my dreams back too :)

  10. John Spears | | #10

    Got a well?
    We use our 200' deep well to pull 55 degree water through a water to air heat exchanger for free cooling in summer and pre heat ventilation air to our ERV in winter. Water is returned to the well at the top of the water column. Works great and none of the issues of an earth tube.

  11. Brad Hardie | | #11

    Well that costs a lot!
    John,

    I just so happened to look into having a well drilled (my friend drills wells, and it wasn't going to cost retail), but it was still $4500-6000. Double the ground loop costs for the Zehnder. I'm on village water, so I wasn't drilling a well for water use.

    I bet if you needed the well though - that would be the way to go for sure!

  12. Charlie Sullivan | | #12

    Well in winter
    John, in the winter, do you run your well water directly through the water-to-air heat exchanger? A hazard with that is that if something goes wrong and the water stops flowing in the middle of winter, the cold outside air going through the heat exchanger can freeze the water and burst the pipes in the heat exchanger. A way to reduce the chances of that happening is to run a glycol mix through the water-to-air heat exchanger, and cool or warm the glycol mix with a water-water heat exchanger coupled to the well water. You could still freeze the water in the water-water heat exchanger with the glycol, but you can use a flow switch to stop the flow of the glycol if the water ever stops flowing. Of course, by the time you do that, the system is getting complex enough that Martin will need to once again remind us that it's unlikely to be cost effective, but I wanted to warn about that potential freezing and bursting hazard.

  13. Andy Kosick | | #13

    ventilation rate?
    I feel like I'm missing something, did I just read that a 10,000-square-foot Passivhaus (an oxymoron the likes of which deserves its own discussion) is being ventilated at a rate of 15 CFM with a $10,000 system?

  14. Carl Mezoff | | #14

    Earth tube problems
    Aside from the problems mentioned by others, back in the late 70s we found in monitoring some experimental projects for ERDA that often the tubes stop working after a short while due to the poor conductivity of the soil adjacent to the tube wall. If your soil is relatively non-thermally conductive (as most non-saturated soils are) the 2" of soil adjacent to the tube wall rapidly approaches the temperature of the air passing through the tube, at which time your tempering effect stops to provide any significant benefit. You can make use of the earth's temperature only to the extent you can conduct heat through it.

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

    Response to Andy Kosick (Comment #13)
    Andy,
    You make an excellent point -- 15 cfm is a very low ventilation rate. Some quick math shows that the reported annual energy savings of $1,669 is highly unlikely -- more like impossible.

    In the example under discussion, the earth tube raised the temperature of the incoming air from 10°F to 45°F. That's an increase of 35 F°.

    Since we know the specific heat of air (0.0182 Btu/cf/°F), we can calculate how many BTU it takes to raise the temperature of one cubic foot of air from 10°F to 45°F:
    35 x 0.0182 = 0.637 BTU

    So, 15 cubic feet of air would require 0.637 BTU x 15 = 9.555 BTU

    If the fan is delivering 15 cfm, that means that the earth tube is providing
    9.555 BTU/minute, or 13,759 BTU/day
    (assuming, of course, that we are talking about a day when the outdoor temperature stays at 10°F for 24 hours).

    Converting from BTU to kWh, we get:
    13,759 BTU/day = 4.03 kWh/day

    Let's assume that this earth tube operates at this level for 6 months out of 12 (unlikely, because 10°F is a pretty low outdoor temperature, and because there won't be very much heat transfer during the "swing seasons" of spring and fall).

    That gives us a (calculated, not measured) heat energy saving of 733 kWh per year -- actual savings will be far less.

    If this much heat is delivered by a heat pump operating at a COP of 2.5, it will take 293 kWh of electricity to deliver 733 kWh of heat.

    If electricity costs 15 cents per kWh, that means that the earth tube provides savings of $44 per year (optimistically) -- which is far lower than the reported savings of $1,669 per year.

  16. Andy Kosick | | #16

    Response to Martin Holladay
    Martin,
    Your generous math above is enough to dissuade me from using an earth tube but I think you've forgotten a very important thing. It was said that the earth tube is ducted directly into a 96% efficient ERV. The savings of the earth tube in this case would only be the additional gain above what the ERV would have operated at drawing ambient air. Even if it is eliminating defrost cycles and saving some electricity, there has to be addition electricity usage to pull the required air through 110 feet of pipe. Either way the savings has to minuscule.

    Correct me if I'm wrong because it was before my time, but I always thought the original intend of the earth tube (and perhaps its only possible justification) was not to precondition air for an ERV but to be combined with operable clearstory windows and stack effect to provide summer cooling without using electricity at all.

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

    Response to Andy Kosick
    Andy,
    I'm waiting to hear back from Peter Yost to find out how many calculation errors were made by Full Revolution Farm, and how many reporting errors were made by GBA. We will, of course, own up to any errors in our reporting, and will make corrections as soon as possible.

    One apparent error is that most of the $1,669 in calculated savings apparently comes from heat recovered by the ERV core, not heat supplied by the earth tube. If my hunch is correct, GBA has compounded the error by our unclear reporting.

    That said, skepticism over earth tube savings claims appears to be warranted.

    Lots of people have experimented with earth tubes over the years. Some people duct these tubes directly into their homes. Others connect the earth tubes to an HRV or ERV. Neither approach makes much sense to me.

    The additional static pressure attributable to a 100-foot buried duct -- static pressure that would need to be overcome by extra fan energy -- depends on the diameter of the duct, of course, and the air flow rate of the fan. But in many cases, a long earth tube will increase the electrical load on the fan motor.

    P.S. GBA's report that the earth tube at Total Revolution Farm was responsible for $1,669 in annual energy savings was erroneous. The error was mine. GBA regrets the error.

  18. James Morgan | | #18

    New to me
    I just met my first earth tube. Renovation project, 4,200 sf house built in early eighties, tube installed directly into two story glass roofed sun space with tile over suspended slab floor. Looks like the sun space was never actually separated from the conditioned space which probably accounts for the need for four complete ducted a/c systems. Shudder to think what the energy bills must look like. The tube was apparently sealed off after a couple of years. At least one crawl space duct is lying open on the dirt disconnected from its register. Step one will be to encapsulate the crawl space and replace ductwork and mechanicals. Step two will be to air seal the second floor ceilings, upgrade insulation to the extent possible and seek options for external shading of the roof glazing. Oh, and a gut remodel of the second floor master bathroom suite

  19. Graham Irwin | | #19

    Earth Tempering for Passive House - YES!
    While I share concerns about condensation in earth tubes (particularly in humid climates), ground tempering of incoming air to a HRV/ERV system in a Passive House can be very helpful for performance, both in summer and winter, and it's something too few North American practitioners are examining, IM(H)O. Whether you end up with an earth tube or a "brine loop" (PEX tubing in the ground filled with antifreeze that's pumped through a heat exchanger) in more extreme climates, this can be the lowest hanging fruit on the building performance tree. Reaching around the low hanging fruit to pluck the higher stuff gets expensive, even if the lower hanging stuff is a bit muddy. Sorry, couldn't resist the pun!

  20. John Spears | | #20

    Response to Brad and Charly
    This design is intended to use the existing water well and well pump. A new dedicated well is too expensive for this. All the lines are buried below the frost line so no freezing problem. The water is supplied from the indoor plumbing, controlled by a solenoid valve and the heat exchanger is indoors. The extra cost is in the return line to the well, the heat exchanger, a solenoid valve and some miscellaneous valves and pipe. You will need to check the local code to see if they allow a "Standing Water Column" geothermal heat exchanger.

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

    Response to John Spears
    John,
    The water in this type of system is at risk of freezing -- but not because the water pipes aren't buried deep enough. The water in the heat-exchange coils is at risk of freezing when very cold outdoor air blows across the coils.

  22. David Inch | | #22

    10000 ft and 15 cfm... not likely
    The Home Ventilating Institute recommends that an HRV or ERV provide at least 0.35 air changes per hour.

    That means that the recommended air change in a 10,000 sq ft home is about 500 cfm. 15 CFM is only 900 CFH... about 1% of the volume of that home. Why would anyone even bother with an ERV if they were going to ventilate 10,000 sq ft at a lower ventilation rate than even the leakage from a Passivhaus?

    There must be an error in the 15 CFM number...

    If the recommended /minimum/ of 500 CFM were used and if the ground source loop was able to keep up with the energy draw, the annual savings would be 33 times higher than the $44 calculated above and would bring the savings up to more than $1200. That would give a payback in the range of perhaps 4 to 5 years.

    I am contemplating this same arrangement for an imminent build... no major expense for my ground source loop... it will simply be laid out under the gravel below the basement slab and protected with screenings ... the ground has a high water table and is on a slope so heat transfer or btu availability shouldn't be an issue. I would think it would remove the need for a dedicated air conditioning system for the relatively few really hot and humid days that occur around southern Georgian Bay.

    Addendum.... I got distracted by the reference to ground source using brine farther up in the discussion. I had considered the earth tube concept but decided the risks were not worth the effort, but using a sort of "passive" ground source like brine to a comfosoft exchanger did make sense to me.

  23. Michael LeBeau | | #23

    Recycling ideas, phobias and mindsets
    It's good to see we haven't stopped the merry go round of rehashing. The technical, and IAQ, concerns discussed here will continue to be relevant as long as new riders keep getting on the carousel.

    I share all of the concerns about IAQ risks in earth ducts. We shouldn't have to argue about those issues for very long. I have seen remarkable results in indirect glycol loop in our one 7 climate though. An HRV can be in defrost mode half of the time in the depth of our winters. That reduces ventilation, increases fan energy and creates noise problems with systems that use recirc defrost on high speed.

    I do find one often repeated opinion problematic in this thread though. That is the worn out argument that a proposed alternative to the status quo "will never pay for itself". That mindset ignores the fact that we are living in an economy that keeps energy cheap by refusing to value the huge environmental impacts of our fossil fuel based energy mix. We have to change that, but if we do - and that is not certain, the replacement mix is certain to cost more than what we have today. The economics of alternatives will change accordingly.

    We simply have to get past refusing to accept that we need to invest more in needed change and soon. It will very likely cost more in upfront infrastructure investments and in ongoing energy costs. To insist that nothing that costs more is ever worth doing is being part of the problem instead of part of the solution.

    There are many reasons to challenge technical notions but digging in our heels and repeating the mantra that nothing is worthwhile doing if it costs more will keep us heading for the cliff in our PZEV on our way to our NZERH.

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Michael LeBeau
    Michael,
    It makes perfect sense to invest in energy conservation measures that cost more than the investment returns in energy savings -- as long as this type of investment is made with eyes wide open.

    Before a GBA reader invests $2,500 in a buried glycol loop, it's useful to consider (a) whether it might make more sense to invest that $2,500 in some other measure that saves more energy, and (b) how much electricity is required to operate the glycol pump each year. These aren't particularly complicated calculations (assuming that we have enough monitoring data to work from) -- and they are useful, because there are many, many ways to spend $2,500 on a job site.

  25. Michael LeBeau | | #25

    Hi Martin,
    The pumps that I

    Hi Martin,

    The pumps that I have used consumed 30 watts. When operating they avoided the operation of either a 1000 watt duct preheater or HRV fans (back then) using 150 watts on high speed recirc for defrost (plus not ventilating during defrost). The installations took advantage of already excavated areas next to basement footings during construction. The duct / coil boxes were simple locally made units although not as elegant as some of the products available today. I doubt the whole systems cost much more than $1,000 installed (5-10 years ago). In this climate there is also added value. Spot checks have shown incoming -20 F air warmed to ~40 F keeping the HRV core from frosting, delivering more comfortable air and reducing noise from mandatory high speed defrost operation. The climate context matters. Your mileage may vary.

    I wish very much that I had installed a ground loop when building. I suffer with loud inefficient HRV system defrost. I live off grid with an oversize PV system (10 kw), heat water and cook with electricity much of the year, and still could not run an electric resistance duct heater as an alternative during a lot of the dark cold winter months when defrost is most required.

    Investing that money in other places to save energy does not address the needs of delivering ventilation services comfortably, reliably and efficiently. It is not a board game where we only get one or two tokens.

  26. User avater GBA Editor
    Martin Holladay | | #26

    Response to Michael LeBeau
    Michael,
    I live off-grid too, so I understand your dilemma. That said, you and I fall into a very special category. Most Americans live in grid-connected homes.

    Anyway, thanks for sharing your data. Data collection is good. Some homeowners think that an investment in a glycol ground loop makes sense for them. Others, not so much. We all learn from net-zero homes that try out different technical approaches to problems.

  27. Tim C | | #27

    PCM for thermal mass
    If you need thermal mass to temper your air temperature & avoid defrost cycles, there are other option aside from using the ground outside of your home. A tank of PCM (a cheap one, like water) in your utility room could achieve the same purpose. I don't have the time at the moment to run the math on it, but I suspect even a fairly small tank would, uninsulated, be able to shift the defrost heat to your main heating system, or insulated, be able to average short changes in outdoor air temperature.

  28. Michael LeBeau | | #28

    Response to Tim C
    Tim,

    I actually did do the math some time ago regarding using some energy from a 325 gallon solar thermal storage tank I have, for this purpose. Since we cannot run straight water through a duct coil here when this needs to happen my idea was to tap the tank portion of the glycol loop that brings harvested solar energy in to steal a little heat from the bottom of the tank. There isn't much solar potential to be had in the first half of the winter here but I recall that the energy math wasn't too bad to just temper the air. The complexity of modifying my solar thermal system and adding potential leaks in what is a higher stress, higher pressure loop put me off that idea in the end.

    The HRV recirc defrost systems are essentially using some ambient energy from conditioned air in a temperature based timed cycle to thaw the core. However they stop ventilating during that cycle so it is not ideal.

    The amount of energy required to temper outdoor air to 30-40 F to avoid core frosting in this climate (northern MN) is not trivial. I have always appreciated the HRV's that can reliably bring -20F air to 60F (between defrost cycles that is). Straight up, however, those with recirc defrost already use some house energy to perform defrost. To add a tank and a glycol loop with heat exchangers and pumps to rob house energy to avoid defrost would probably be tough to get around to. An uninsulated tank would get cold and create condensation also so there would be many layers of fun to be had.

  29. Andy Kosick | | #29

    Thanks
    I just wanted to thank you for following up on the data from Total Revolution Farm. I must admit to having a brief love affair with earth tubes myself, so if anybody does have data on the savings for earth tubes and/or glycol loops compared to standard HRV operation I would enjoy seeing it. From what I'm seeing above it sounds like the elimination of defrost cycles is the real benefit.

  30. User avater GBA Editor
    Martin Holladay | | #30

    Response to Andy Kosick
    Andy,
    The best available monitoring data I could find, after an extensive effort to track down data, show that a buried glycol loop system saved about 45 cents per year. You can read all about it in my article: Using a Glycol Ground Loop to Condition Ventilation Air.

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