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Community and Q&A

Earth Tube used to moderate air source heat pump condenser operating temps

DutchFoxMN | Posted in General Questions on

Long time reader first time posting.

Has anyone tried or thought of using an Earth Tube to pre-heat or cool the air for an outside condensor of an Air Source Heat Pump or Mini-split? 

Earth tubes are new to me and not common in most of MN. I have read about the downfalls of these poorly designed or installed earth tube systems for indoor air quality. Radon is a considerable problem in MN and I see an earth tube system only adding to it. 

I like the efficiency of ASHP or mini-split systems, and the newer systems are now able to provide heat even on the coldest of days. My understanding is that when those below 0 degrees F days are upon us, or any large DeltaT for that matter, the systems aren’t as capable of producing heat efficiently. 

Could the outside condensor be enclosed in a small, insulated, exterior space (similar to a porch or breeze-way) where a few 100′ runs of burried earth tube loop starting and ending with a small fan system to move air through the tubes?

Air from the Exterior, enclosed room that houses the condensor enters the tube is moved by the fan through the earth tubes to either warm it or cool it prior to re entering and being used by the ASHP condensor to make interior heat or cooling more efficient with lower DeltaT values theoretically.


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  1. ohioandy | | #1

    Hey Timothy, it's an interesting idea, but it has been covered here. Previously, the suggestion was doing something similar with a passive solar enclosure around the ASHP outside unit. But it moves a vast quantity of air across its coils; the fan is working hard. At this rate of airflow, an earth tube long enough to allow some heat exchange with the soil would have a very high static pressure; it would require ANOTHER large fan, with a current draw likely to negate any gains at the heat pump. And the soil surrounding the earth tube will very quickly cool so that it is no longer exchanging ANY heat. You're encroaching on the Rube Goldberg territory of geothermal heat pumps. I think ASHP's are already engineered to within a micron of their peak efficiency. As they always say here, spend the extra money on improving your home's envelope, or on solar PV.

  2. PAUL KUENN | | #2

    You may want to ask about the massive earth tube investment the Aldo Leopold Foundation used at the headquarters many years back.
    I'm sure they have some interesting feedback.

  3. Expert Member
    BILL WICHERS | | #3

    There are heat exchangers available to use heat from air conditioner refrigerant lines to heat swimming pools, a kinda-sorta “combined cycle” arrangement. You could probably use one of these heat exchangers and a small circulation pump to use water circulated under the ground to help your system efficiency. A water loop will be easier to install than one moving air, and will probably work better too.


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