A combination of geo and air to air!
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Hello,
I have a very newbie question. I was exploring geothermal using shallow pit geo. However, in Southern California, the ground temperatures don’t really make sense for such a system. The efficiencies are not too different than air to air.
We do have a large crawlspace under our house. It never gets much warmer than 75 degrees – I checked it on one of the hottest days last fall. Half of the house, on the upside of the hill has no vents to the outside. The total square footage for this crawlspace is about 1,000 sq. ft. I could place the compressor well under our porch on the west side of the house. With a short duct, or rather plenum sized duct, I could direct air from this side of the home’s crawlspace that would have its entry on the north side of the house. With crawl ports from the west side of the house to the east blocked off with doors, the heat pump would draw in only air from the north side of the house. The soil would keep the intake temperature of the compressor cooler. Extra insulation under the house would keep the floor from getting much warmer.
All sorts of unknowns here. But, I would guess that the crawl space would regain most of its coolth over night. There is no exposure to sunlight on the eastern wall. It is well underground. A slow fan could run when the temperature falls below 80 degrees, which the nights generally do in our very arid location.
Now I wonder if this is just newbie foolishness or would the air over the coils really enter at a lower temperature after hours of operation?
Thank you for your patience!!!
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Replies
>"The total square footage for this crawlspace is about 1,000 sq. ft."
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>"The soil would keep the intake temperature of the compressor cooler."
That's a common misconception, usually from people who don't do math (or understand much about thermal physics.)
A 1000 square feet of insulative dirt isn't a very good heat exchanger for tapping the thermal mass of the subsoil for heat pump blowers churning through 1000+ cfm. Even the crawlspace were 4' tall (4000 cubic feet) just about all 3/4 ton mini-splits would be driving 15+ air exchanges per hour, bigger minisplits would drive even higher ACH. The temperature in the crawlspace would quickly rise to something VERY close to the outdoor temperature, and the cooling/heating contribution of that soil heat exchanger would be "in the noise" from a measurement point of view, well under 1% (probably less than 0.01%) of the total whenever there is a real cooling or heating load.
That's what I wanted to know. Now I know that digging a ditch for geo would be a waste of time and money and bringing air through my crawlspace would be ineffective at best. Air to air for me! Thanks!
Well, since I have your attention, here's a trickier one. Our house has a couple of 4x6's that are visible in the crawlspace. They presumably extend from the foundation up to the double top plates of the upper floor. We know that the owner/builder had in mind putting more fenestration in the outside wall upstairs, but decided against it because they chose to put in single glaze windows instead of double - this limited their total window/door area to pass California codes (1984). Could these 4x6's be there as a structural solution to satisfy codes for structural integrity if they decided to put in another sliding glass door? We also discovered a steel member in that wall upstairs, a heavy steel stud. We'd be pleased to put in more windows/doors in the wall but have thought that it takes a very heavy duty steel structure to make that happen.
Thanks again!