Image Credit: Image #1: George Sullivan
More Q&A Spotlight
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.
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33 Comments
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.
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.
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.
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.
Geothermal tax credits?
I thought those were for installing energy star rated refrigeration equipment, GSHP's.
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.
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.
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."
Josh
I agree. A pox upon Martin and his clear-eyed logic. I want my dreams back too :)
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.
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!
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.
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?
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.
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.
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.
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.
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
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!
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
What a great discussion! Until I just ran across this yesterday, I had no idea my questions had spurred this article.
I never did end up building the home - I realized that my energy would be better used toward other ongoing projects, so I sold the land I was going to build on and doubled down on other pursuits. I simply realized I couldn't possibly do all the things I wanted to do, so the home got scratched off the list.
I had built my present home back in 2001, a geodesic dome, and enjoyed the process so much I had decided it was time to do it again, but this time with a conscious emphasis on building a net zero home - or as close as I could get. Instead, though, we are staying put in the geodesic for now.
I’m late to the game...I’m putting up a 54’x80’ FarmTec building and plan on using 8 4” lines 500’ long going into 2 18” manifolds. I live in Southern Ca. And extreme heat is my issue for the worm farm I am starting for my 4,000 acre vegetable ranch. I hope I’m not in the category of “loved it, until I didn’t” years down the road. Because we are below sea level, we use 4” perforated pipe spaced 75’ apart to keep our water table down. I tied my cooling pipes into my drainage system.
I was going to install an earth tube in my 34 x 96 foot greenhouse. I want partial heating in the winter and early spring. Just above freezing is OK. Propane crushes the cost effectiveness of partial heating. How did your experience work out? I am leaning toward a glycol loop given the variability in effectiveness. I don't have a basement in the greenhouse, so the slope would be away from the building raising cleaning questions. That said, while introducing mold to a greenhouse is a very bad idea, I don't have the same indoor air quality considerations of a home. The greenhouse is much leakier. How'd your design go, lessons learned? Any other comments/ideas very welcome.
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