Originally Posted by DickR
I have "geothermal" (ground source heat pump, or GSHP), and I love it. The thing is relatively quiet, and it just runs now and then to keep the house at even temperature. However, mine is a special case. The house is superinsulated, so the heat load is very, very low, compared to "ordinary" construction. GSHP was a good fit for the situation.
I always advocate making the house as stingy on heat loss as is reasonable to do so, before looking for a cheap source of heat for it. Conventional heating systems don't come in the wide range of capacities as do GSHP systems, and thus tend to be oversized for the house. With GSHP, each additional ton (12,000 BTU/hr) of heat load costs extra, due to the increased size of the ground connection and the unit size itself. Accordingly, A GSHP typically is sized to be no bigger than needed, so, yes, recovery from a temperature setback tends to be much longer than with the typically oversized conventional system.
For a GSHP, the ground connection can be in the form of a closed-loop "slinky" loop of black plastic pipe buried 6-8 feet or more in the ground. Depending on soil conditions, 200 feet of coil may be needed per ton of heat load. This takes a certain amount of backyard space, and around the lake this may not be as readily available as in other parts of the country. The other form of ground connection commonly used is a drilled well, the Standing Column Well (SCW) design, as in my case. We were going to have a new well for the house anyway, and with the superinsulated design of the house I knew the load would be small. For such a design, you need on the order of 80 feet of water column between draw and return per ton of heat load. The drilling cost can be considerable, a major portion of the total GSHP installation cost. In my case, the well didn't have to be any deeper than was needed for the house water supply; it's actually deeper than needed for the heat pump. The unit is a two-stage, two-ton unit, and it keeps the house (close to 4000 sqft) at temp in first stage only, through the coldest weather we've seen so far (it's in its second winter).
For an existing house, of conventional construction, GSHP would be a much tougher sell.
Songkrai, what did you mean by "Geothermal has issues in this climate?" One of the nice things about a GSHP is that below 6-8 feet underground the temperature is fairly steady and moderate (around 50 F) year round. If the ground connection is sized properly, that heat source stays at a fairly steady temperature and thus the efficiency and capacity of the unit remains steady throughout the year. In contrast, the efficiency (coefficient of performance, or COP) of an air source heat pump, like the Mitsubishi ones mentioned (good units) drops with outside air temperature. However, any heat pump is more "efficient" than straight electric resistance heating (baseboard, quartz heater, etc.), because a given amount of electric power is used to move some heat from outside to inside. You get, as heat, all of the electric power used by the heat pump plus whatever heat it drags in from outside.
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