Offgrid Underground Air Conditioning

My wife and I did something similar here in central British Columbia. We buried two 4" PVC pipes six feet deep, side by side and running out seventy feet from the house. Fans pull outside air in, sending some to our bedroom and some to the kitchen. In the winter, minus 30C air is warmed by the earth to plus 10C. In the summer, plus 30CV air is cooled to plus 15C. It works very, very well. If I had to do it again, I'd go a little deeper (maybe 8-10 ft) and make it longer. We do sometimes get a bit of water in the pipes but haven't had any issues. If the run was sloped away from the house and drainage provided right where the pipes turn up to the surface the system would be self-draining. Anyways, six years later, the set-up works quite well.

So, a consideration to note on the earthships here in New Mexico, the geothermal tubes are open to the outside for a few reasons, one is to prevent too much condensation build up, and the other is to allow a cross breeze to be your main air mover instead of a fan, so you use little to no power at all. Most earthships will have an attatched greenhouse, or at least a roof peak vent. All the hot air in the house will move to the upper most vent in the greenhouse/house and leave, pulling in fresh air through the geothermal tubes, pulling in that cooled (and typically drier) outside air, with no moving parts, although in some cases a fan may be used for extra movement if needed. These are also used for very light heating in the winter, as we have moderate winters, much like Texas. This will still work in this set up, but just as others have said, condensation is worth considering, but most AC systems have to deal with that anyways. These systems work the best in highly insulated buildings with very little temperature fluctuations

I've considered that sort of system before. My concern would be condensation in the cooling pipes. It only takes a little water to create a big mold problem. I think it would be a good idea to run a dehumidifier in the house to limit the amount of moisture that gets into the lines. Dehumidified air also feels cooler on the skin because it's easier for evaporation to occur. Also, if you're cooking or showering in that structure, make sure that humid air is properly vented as immediately and completely as possible so that humid air doesn't get into the lines. Now that I think about it, I think a better system might be to cycle the same closed-loop of air through the ground with a furnace-style heat-exchanger that you pass the house's internal air over to cool it. This way, no humid air can ever get into your sub-surface lines, and if any condensation occurs on your heat-exchanger, you can just clean off any mold that might occur.

I had an eccentric friend that buried 4” pvc 6 feet deep . He ran it about 75 yards from his home to his creek fed pond. He ran it to the middle of the pond and elbowed it to exit the water in the pond. He ran it to a box with a cheap box fan. The fan was pulling the cooler air into his house. His house was a dome home built in Mantachie, Ms, by Joel Nichols.

Just a thought, probably not important but you decide. Blowing warm, humid air into cool underground pipes might cause condensation in the pipes. Condensation may cause mold growth. The air circulating in the pipes and through the potential mold growth may disperse mold spores throughout the structure and into the lungs of anyone breathing the confined air. I would consider a redesign personally. I would close loop the air in the pipes and install a radiator in the cabin to push the warm air through. Not as effective but maybe safer. Even better would be to replace the air in the pipes with something with better thermal efficiency like water with some antifreeze depending on climate and usage. Good luck with your decisions.

"Hotter than Satan's ass" is my new favorite phrase.

Great plan and execution. Couple of cautions. Run a check on radon from the fan area. And remember in humid weather the pipe will literally fill up with condensate and pump humidity into the cabin. Those two factors ruled out a similar system for my house near raleigh, nc but i am able to pipe the cooler air through my ac to reduce costs there. Good luck and keep on innovating.

I have worked designing systems similar to this... but lack money to build them. One thing you might not have allowed for is condensation in your underground pipes. The warm air from the house will have humidity that might condensate inside the pipe and eventually the whole pipe will fill with water and become stagnant. Another thing you might not have considered is the dirt near the surface has no water in it... so... even though the temperature of the dry dirt is 50F... it might not hold very many btu's of coolness. If you can bury the pipes down where the dirt is always wet, it might work better. Another problem is the corrugated pipe. That kind of pipe does not have smooth walls.... so... the air moving through the pipe will have turbulance and will not flow as well. If anyone is thinking of doing this... you should do some TESTING of temperatures below ground BEFORE you start. I installed temperature probes at various depths in St. Louis, Missouri. My results are as follows. Temperature at 20ft down is about 60F Winter and Summer! It doesn't change. As you get nearer the surface, then temperature changes depending on the season and the depth. Down here in central Florida, I measured the temperature at about 10 ft down and it is 70F. If you live in the NORTHERN U.S.A. it would be likely 50F. I wrote a whole PDF book about these subjects including a couple methods of cooling your attic from 150F down to about 105F with NO moving parts/NO electricity! Anyone who wants a free copy of the whole book I will email it to you as an attachment... My name is David Mundy and anyone can email me at this email address: [email protected] Ask for the "U.G.A.C book" and I will send it to you.

I would only run this in a closed system. You could use an HRV to do this. You'd run the underground loop across one side of the coil and then the interior air across the other. This would massively reduce mold, mildew, radon, etc issues. HRVs can transfer like 50-75% of the heat so it would work super well

Dude! Finally, an off-gridder who made a sensible and cheap solution at last based off geo-thermal cooling! I love it, great detail and explanation. One question though - have you thought about doing any filtering to prevent dust build-up? I get that it's a cabin and you don't expect HEPA air indoors, but don't discount the additional work a dirty fan motor has to do - your power consumption will take a hit.

Cool setup! I'm looking into the efficiency of a recirculating water system going to a radiator with a fan blowing over it. My soil temps are a bit higher here.

Brilliant design. We do something similar in an earth sheltered home and livestock housing in the Ozarks. It is completely passive, where warm air rises through vents to the outdoors and cooler air is drawn in from sunken pipes. We often have to shut the vents to keep it from getting too cool. This is based on the vent design of my unairconditioned familly home in Florida. The intake vents were all along the bottom of an outside facing wall, sheltered by foliage. Lots and lots of foliage. The upper vents let warm air escape and drew the cooler air in. If there had been sunken pipes we could have gotten better than the average summer temperature of around 78 degrees indoors when normally it would be nearer to a hundred degrees. It made the northwest facing side of the house much more bearable without HVAC. Downside in Florida was the wear and tear on the small screens and the humidity and moisure affecting the jalousy mechanism of the vents.

"hotter than Satans ass" lolol. thanks for the early morning laugh

At the very beginning he states the approximate ground temp of 52 degrees, then said he used 100' of pipe. Best case senario, 100' plus the length above ground. Realistically, he likely bought a 100' roll of the 6" pipe he used (mentioned later when talking about the fan that pulls the air through the pipe) so 85' +/- buried. I'd guess not much over 250 sf of living space to be conditioned so I'd use that as my starting point....except I only watched it because it seemed a pretty simple, but not easy project, unless you happen to have a backhoe. Kudos to Uncle Dave and his low budget cooling project. I am mildly curious about mold and mildew being sucked into the cabin, though.

If you put the air intake closer to the ceiling you'll find it much more effective and efficient. As level is evacuating near the same temperature air as it's imputing. Heat rises cool air falls In thinking about it the air output should be head high also that reduces the hot zone by increasing the cooler area. Im impressed because I am guestimating how much site development will cost ie;solar and battery storage. Ill need and a fan is cheaper and easier to run than a ac unit . Could run a fridge on the savings in energy.

That's a great system! I'm glad that you're already aware of the potential for mold growth. One way to mitigate that risk would be to pick up an ozone generator and once a year, sanitize the air in the cabin and pipes. I would remove anything biological from the cabin that you don't want dead (people, pets, plants, etc.), close up the cabin and with the AC blowers running, fill the cabin with ozone. I'd let the ozone generator run for 24 hours, then let the ozone dissipate for three days after, opening the doors on the third. This is how I've removed mold from basements particularly and older structures (especially before upgrading it with a vapor barrier and vinyl siding), and in a system that is likely to produce mold, the ~$750-$1,000 for the investment in the ozone generator could well save the occupants from a major health hazard in the future. Anyway, I'm enjoying your channel and you've got a new sub!

I had an idea like that could be more than 20 years ago but I could never talk my family into doing it? Don't have a backhoe,, but the neighbor does! Another idea that would definitely help in old style rafter buildings is to sheet off possibly even insulate the bottom side of the rafters, not in between. Create a manifold at the top on the south side of the building, then an insulated pipe to take it down to the air vents in the rooms. Probably a little huskier fan would be needed, but you could have warm air during the day during winter time.

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Impressive, many thanks.

I never thought of anything like this. One thought I had was using a small pump to pump ground water through a heat exchanger to cool air circulating through the house and let it drain back to the well, but I suppose there would be a contamination issue there.