Ground Source Heat Pumps

Ground Source Heat Pumps use the more even temperatures of the soil for the "outside" loop. The coils (pipes) are dug into a series of trenches near the home. The advantage here is in the fact that in most places, soil temperatures are quite steady year around. This allowed the heat pump to work efficiently even when the outside air temperatures fall bellow freezing.

One problem with these systems occurs during very dry periods. Moisture in the soil is needed to transfer heat between the coils and the soil. When the soil is too dry, the amount of contact between the coils and the soil can be too low for proper operation.

I asked Bob Davis about Ground Source Heat Pumps. Bob is a Technical Consultant with Ecotope, and a real expert in the energy and heating field. Here are his comments:

"On ground source HPs, my general reaction is that they CAN work well, but there are many reasons why they often don't (poorly done excavation, incomplete understanding of groundwater/earth heat supply/rejection capability, failure to keep track of the differential between cooling/heating capacity, duct leakage, etc.) The premium the homeowner pays for the system ($8-10k over same size air-to-air installation) is extreme; the ground source CAN be twice as efficient as the air source system but that's not a given. So I advise people to proceed cautiously on getting these systems. In general, the most enthusiastic GS customers are well-to-do; they want a Cadillac system and are willing to pay for it; their primary concern isn't necessarily a more efficient system.

"I've looked at a few of these systems (mostly in Montana) and found a range of installation quality and performance. I recently looked at one in Snohomish County (Washington) that was working as designed, but the installer had cut it really close on sizing and the backup heat was coming on a fair bit. The house and ducts were relatively tight; I think the installer had just failed to realize that a "3 ton" system only delivered 29,000 Btu/hr at the entering water temp that was available. I generally specify systems that are undersized relative to most contractors' experience, but this is a case where the next size up would have probably done better, in terms of efficiency." - Bob Davis, Ecotope

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Heating Your Home