I am building a home in the high, arid but fairly warm climate of Carbondale, Colorado. I was hoping to potentially use Earth Cooling tubes to assist in cooling my passive solar home that will be at 6700 feet in elevation. The summer temps can reach and stay in the 90-95 range sometimes for weeks at a time. My thought was to bury a 8-10 inch diameter line in my utility trench that will already be 5 feet deep. If needed, I could go another foot or so deeper. My trench is already going to be approx. 300 feet long which is the distance to the street to my home. My thought was to make a closed loop that would be 600 feet long and start and finish in the home. With a fan pulling hot air from the home blowing thru the loop, the returned air would be significantly cooler than the supply air being pulled out of the house. Has anyone had any experience in this? I have not been able to locate any accurate details on how to construct this, diameter of pipe, what temp to expect the return air to be, needed CFM to effectively cool, etc. etc. I hate to just go it my own and then find out it doesn't work as planned, after the fact. Any advise or comments would be HIGHLY appreciated?? Thanks very much!

Earth tubes have come up again and again, but they are difficult to implement, primarily because of air quality issues surrounding growth of microorganisms in the tube environs.

You are more likely to just get multiple and differing opinions than any real expertise on this subject. So you will likely need to do some first-hand research to get information from knowledgeable people who have actually successfully used earth tubes for many years before deciding how best to proceed. I would recommend starting here:

Build it Solar

The general consensus is that earth tubes don't work well in hot humid climates without some form of dehumidification to prevent water from condensing in the earth tubes. Given that you are at 6700 feet altitude, you should have a nice diurnal climate to use to your advantage. A well insulated and well designed passive solar home located in a low humidity diurnal climate can easily be cooled just using passive stack/chimney effect and/or minimal mechanical ventilation. These approaches are often less costly than the earth tube approach and also less controversial/unconventional should you eventually want to sell your house. We always recommend minimal mechanical ventilation using HRV/ERV in lieu of stack/chimney effect because minimal mechanical ventilation can be accomplished with minimal energy usage and can be easily automated. The building architecture required to accomplish stack/chimney effect is often wasted space which reduces the overall building cooling/heating efficiency.

I have absolutely no experience with these but it seems to me they would make your whole house smell like a damp, unfinished, basement at best.

It's possible to pull this off in high-dry Colorado where the summertime dew points are low, not too far above the deep subsoil temps. It gets to be more complicated in the eastern half of the US where the higher humidity of the outdoor air creates condensation issues inside earth tubes.

There are a number of people using earth tube cooling schemes in CO, but in the end, by the time you've added up the cost of the dirt-work, tubing, and design work, it's not clear that you wouldn't be better off buying a mini-split and a couple kilowatts of PV panels to deal with it.

http://www.earthairtubes.com/resources.html

https://www.thenaturalhome.com/earthtube.htm

If you want to design it rather than hack it, doing at least some of the math is advisable.

http://ceae.colorado.edu/classes/cven5080/earth-tube-EnergyPlus.pdf

I appreciate the input, but in regards to the comments on the humidity issues, this climate is SO DRY. I wish we had some more moisture in the air, but that is just the climate. Anyway, I was just thinking to try to piggyback on the exisiting long trench that I would already have going to the street and dropping in a 600 foot section of pipe (300 out and 300 back) would give me "some kind" of benefit was my initial thoughts. I have to do a 5-6 foot deep trench anyway.

Maybe using that as my fresh air intake might make sense? If I just had a 300 foot section drawing in the fresh air at the other end, surely it would pick up some cool or heated air, depending on the time of year. Looks like in January here it would be about 45-50 degrees and in July about 65 degrees. That would help some and the cost, at least just for the pipe and a small fan, would not be to much. I certainly would not to at resale to advertise that the home was exclusively heated or cooled with this, because the ground at this lot is SO ROUGH as far as rock goes and I am not going to the lengths that I have seen the guys at "earthairtubes.com" go to. That is just not going to happen. I was just trying to get some type of additional benefit out of that trench that I already have to dig. Maybe it is not worth the effort? "Borst", where you guys from? Maybe I could pay you for some consulting?

thanks a bunch for the input, any further opinions given what I have said here? Oh last thing, what is a "min-split?" Pardon my ignorance on the subject please.

Okay, here's the thing. Earth tubes are doable, and they are easier in an arid climate like yours. Problem is that you do run the risk of creating a health problem if not monitored continuously. Do you want to wait until someone in your family gets sick before you shut it down, or do you have the resources to monitor your tube to assure the air quality in all sorts of conditions?

And, it turns out to be a bit of a financial risk, too. I've run the numbers and an 8" tube is about the smallest that can work reasonably. 10" and 12" are better. Have you priced pipe of that size? 600' is going to be pricey. You may install it and find out is is either a disaster, or that it simply doesn't work as well as you hoped. Either way, you probably aren't digging the trench up to recover the pipe.

A "mini-split" is just short hand for an electric heat pump with an outdoor unit and one or more indoor heads which supply heat and air conditioning. They are cheap and efficient. Dana recommends that you use your earth tube money on extra solar panels and a minisplit for the long run.

We got very close to ordering this product earlier this year. http://www.amvicsystem.com/Ecoair
We had the pricing and availability but backed off due to lack of real world information.
We invested in a 3,000 gallon rain walter harvesting system in its place.

A 500' long trench with enough slope to drain any condensation or leakage, using smooth-bore 6"-8" schedule PVC as the conduit can work fine as your ventilation air tempering, and will provide a modest amount of sensible-cooling in the summer. (But for the cost of the pipe & fittings alone you could buy a 3/4 mini-split that would run at very high efficiency, and can provide both heating & cooling.) If you use corrugated pipe you can run shorter since the heat exchange is better, but the risk of accumulating mold-feeding contaminants is much higher, and cleaning it adequately is damned near impossible.

PassiveHouse earth-tube ventilation tempering systems tend to be half that length, but with fatter pipe. Some even run both the intake and exhaust of an active heat recovery ventilation system in the same trench:

http://www.passivehouse.us/phc2012/2012%20Presentations/Morosko_Earth%20Air%20Tubes_%20Denver%202012.pdf

A pretty-good high efficiency 3/4 ton heating & cooling mini-split runs about $1500. In CO snow country rack-mounting it on an exterior wall above the anticipated snow depth, protected from the brunt of precipitation or cornice fall/roof-avalanche by roof overhangs makes sense. As a DIY installation, with the mounting hardware and an hour or two of tech time to charge & test the system (with all the tools you don't want to own, and the experience you probably don't have) you're talking maybe a couple grand. As a full turn-key, at most three grand. Grid-tied photovoltaic has been averaging in the neighborhood of $4-4.50/watt installed, prior to any subsidy. Depending on where you are in CO and what the local deals are, getting the installed cost down to $2-2.50/watt after federal/state/local suisidies are applied is sometimes do-able. A 1-kw PV system would deliver about 800-1000kwh during your peak cooling season, and about 1300-1500kwh, which would more than cover your likely cooling power use with a high efficiency mini-split, for a total PV + heat-pump cost under $5K.

The coefficient of performance of a small mini-split is VERY high during part-load conditions, and would have a much higher efficiency than a fan on an earth-tube in all but the best-engineered earth tube cooling systems. Here is some bench-test data on a couple of popular 1-ton units:

http://www.nrel.gov/docs/fy11osti/52175.pdf

While the daily highs in Carbondale hits the 90s for weeks at a time, the diurnal temperature swings are also high, with lows in the 50s. The summertime daily mean temp is a fairly modest ~70F: http://weatherspark.com/#!dashboard;a=USA/CO/Carbondale

As long as you design and site the house for low late-day solar gains (overhangs on the south windows, little to no west facing windows) your peak loads will stay pretty modest, and a night-time ventilation strategy of simply opening up the windows once the outdoor temps are cool enough, closing the windows in the AM, can probably handle most of the cooling load in a high-R house. Exterior shades on east and west facing windows work too, but not as well as simply reduced glazed area, and using only heat-rejecting soft-coat low-E windows on the east and west sides. Higher gain hard-coat low-E glass on the south side for wintertime passive gain, tempered by overhangs/awnings to limit summer gains is usually a good compromise.

ANY new house can be oriented and sited with future rooftop PV in mind. The current cost while less than half what it was a decade ago, is going to be cheaper by half again before 2020. (The all-in pre-subsidy cost in Germany is already about $2/watt using the same panels/inverters/racks. The higher cost in the US is driven by higher customer-acquisition/hand-holding costs, and deeper permitting red-tape, since it varies from state to state, utility to utility, whereas in Germany it's pretty much the same regulations nation-wide. This is changing rapidly in the US.) If not you, third-parties will be interested in "farming for photons" on your roof, if it's oriented correctly, cutting you a less-than-retail power purchase cost, while they cut a separate deal with the utility, and collect any subsidies etc. That ownership & financing model is expressly banned by regulators in some states, but actively supported by the legal & regulatory structures in CO (and many other states):

http://apps3.eere.energy.gov/greenpower/onsite/solar_financing.shtml

http://apps3.eere.energy.gov/greenpower/buying/buying_power.shtml?state=CO#3rd_party

If you are already digging trenches and want to make use of geo heat, consider using geothermal heating/cooling (with water, not air filled tubes).

Thanks for all the input here guys, well I guess I am looking at just trying to utilize that long trench run I have to make.???

I thought at a depth of 6 feet deep and 300 feet one way, there had to be some kind of fairly straight forward (low risk) heating/cooling supplementation installation that I could do. I already plan on putting in a fairly large PV system, as the rebates in my area are very high and I could get up to a 6kw system for $6000 after all rebates and then the 30% tax credit. I say that is way cheap and hard to turn down. In addition, with the extra power, I will use the PV to power a back up electric furnace to backup my pellet furnace that will be installed with a forced air heating system. Hot water, lights, dryer, and everything else will be electric. All the power I need for the entire house I think I can get off 6kw system, if not a bit less. House is not to small, about 2800 sq ft.

thanks

a back up electric furnace

Don't use PV to make resistance heat. That is near insanity.

Dude that is for a backup in case my pellet furnace ever has a problem. Like an emergency situation..... thought I made that clear in my post. Anyway, paying several thousand dollars to dig a hole and bury a propane tank and then another couple for a propane furnace that I will probably never use is my definition of insanity. Not when I can install a electric coil downstream of my pellet furnace so IF I EVER had a problem, I would have a emergency source of heat which would be the electric furnace till my pellet furnace was back up and going. thanks

I have to ask: have you considered building a superinsulated net zero home using minsplit heat pumps for heating backup (passive solar being the primary heat) and cooling? This would mean spending the money budgeted for furnaces and ductwork on insulation and air tightness, but would be less expensive and more comfortable over time. And all electric as opposed to fuel fired furnaces.

Bob, To be honest I have not looked into the Passivhaus home in detail. I have been told by some people in my area to expect a 25% increase in the cost of the home if I went that route. The home budget is already at about my max. In addition, I hate to say this as well, but if I was planning on being in the home for the next 20 plus years, then I would probably be willing to go out and leverage myself a little more (if the bank would allow). But my wife and I don't know if we will even be there for 7-10 years....... I know that is a problem with America today, always about the short term. However the economics dedicate that I guess.

As far as the passive solar, I do already have about maybe 75% of the home roughly speaking that will be heated via passive solar. My site is great for that and I am taking advantage of that. So I am not expecting a large amount of additional heating to be required by the house. I have a friend that does energy consulting that has already said I am probably at a 40 to 50 on the HERS rating system, without even running the modeling yet. He said its probably going to be lower than that, but we are not done with the plans yet. If I have the PV powering the whole house and my only ongoing cost will be the pellets , in regards to heating and cooling at least.

oh another question Bob, would a mini split fully heat a house in the winter in Colorado? Are you talking about an air to air or air to ground? our mean temp during the Dec-Feb months is 24 degrees.

a home of that size can be easily heated in central New Hampshire with minisplits. I'm referring not to a home built to Passive House standards, but approximately R40 walls, R-60 roof, triple glazed windows, R10-20 under the slab and R24 basement walls. And having an infiltration rate of 1 ACH50.

these are air to air units.

Posted By beetle55 on 22 Dec 2013 01:32 PM
oh another question Bob, would a mini split fully heat a house in the winter in Colorado? Are you talking about an air to air or air to ground? our mean temp during the Dec-Feb months is 24 degrees.

If it were -24F it might be a problem, but in locations with a +24F mid-winter mean temp, better class mini-splits are running with a coeficcient of performance better than 3 (uses 1/3 the power of resistance heaters).  And since it's a heat pump with outlandishly high cooling efficiency COP (way  better than earth tubes and blowers), it's a better expenditure than plunking down coin for both an electric furnace and burying  0.1 miles of pipe. (As I've been saying....)  Both the Mitsubish "Hyper Heating" and Fujitsu Halcyon XLTH are good to at least -15F outdoor temps (up, below zero by double digits.)  The Mitsubishi units kick off about -19F to self-protect, and automatically turn back on at about -18F.  I believe the Fujitsu units just keep on chugging at some un-rated output no matter how cold it gets.  The COP of either when running full-speed at -15F is about 1.8 (a hair more than half the power used by resistance heating.)


The very popular 1.5 ton Mitsubishi MSZ-FE18NA puts out about 22,000 BTU at +5F, dropping to about 15,000BTU/hr at -15F.  The Fujitsu AOU-15RLS2-H puts out nearly as much.  Carbondale's 99% outside design temp. Carbondale is probably somewhere between -5F & 0F, and if you can get your design heat load at -5F down to 20,000 BTU/hr you'd be fine with either one of those.  They make them smaller, but the range is limited on bigger- multiples work though.  It's common to be able to use one mini-split per floor on multi-story houses.  If you have doored-off rooms that are too lossy to stay comfortable with the door closed, electric cove heaters or baseboard as supplemental heat is cheap and works.  

On a deep energy retrofit on a 3-story  (with a full basement) I was involved with ( finished  in September 2012 ) the total heat load on the lossiest floor was about 12,000 BTU/hr at the outside design temp of +5F.  The mean winter temp is comparable to Carbondale's, but the humidity is higher  (=capacity lost to defrost cycles on the mini-splits that you won't have),  and annual solar input lower (even if it had the PV, which it doesn't.) It is heated with on mini-split per floor (Mitsubishi), with no auxilliary heat for the doored off rooms.   It sailed through -8F temps without a hitch.  The only comfort issue reported was on the top floor, where the AM solar gain in June/July for the doored off bedroom on the east side makes it too warm to sleep-in until noon on weekends if the door is closed to the common area with the mini-split head.  (It's fine with the door open.)  The lower two floor's east side bedrooms have quite a bit more AM shade.  It's a 2x4 balloon framed structure with foam cavity fill, and 4" of polyiso between the sheathing and siding, with about an R55-60 roof, U-0.18-0.20 windows, R12-15 closed cell foam on the foundation walls.  It tested at under 500cfm/50 prior to commissioning.

A house like that could be made Net Zero Energy with a 6kw PV array if the occupants were a bit stingier with plug-load power use. (At the moment it houses 8-19 college football players, who aren't exactly energy misers, but nobody has seen a power bill anywhere near $100 yet, at 16-18  cents per kwh which is probably more expensive than your local rates.)  Getting your PV for a buck a watt installed means it should have a sub-decade payback even in relatively low cost electricity locations. Since the savings are all in after-tax dollars, if you use electricity efficiently it could be the best internal rate of return of any safe non-speculative investment you ever make.

Wow, sounds like maybe I should do some serious research into this. I had these thoughts and memories as a kid in the 70's and my dad (electrician) complaining about "heat" pumps not working very good when the outside temps got to cold. I guess things change.

While I don't think I am going to be able to afford triple pane windows through the whole house and quite that level of insulation, I still think my heating load will be fairly low. However 15,000 to 22,000 BTU does sound very low. Any idea on cost of say a home insulated like you have spec'ed Bob versus backing it down by maybe a 1/3rd? I have not got to far into the details of costing yet, just going on what a few GC's around me are telling me. Thanks Bob.

You can tell I am fairly green at the newer building techniques. Last time I built a house from ground up was 18 years ago. I have been living in 70's crap for the most part ever since then. I think a lot more has changed than I first realized. I really appreciate the detailed advice guys, thanks very much!