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Old 03-05-16, 08:53 AM   #18
Mobile Master Tech
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Sam, +1 on Jeff & Steve's comments. I will add that the efficiency of an open source heat pump depends heavily on the energy needed for pumping. Your ground & groundwater temp is probably around 47F where you are (confirm on a map), and you will surely drag the temp down below 40F in your heating dominated climate. You will need to plan for at least 2gpm per ton of capacity.

If your water table is consistently less than 25ft down from your pump location, you can use a shallow well pump and the lift isn't too bad. If not, the energy required to pump 8 or more GPM from lower depths really adds up. You only need 2 boreholes, saving installation cost, but the average efficiency gets dragged way down by a pump powerful enough to meet peak flow requirements. If you were going to pump that much water anyway to fill a lake or somesuch then it doesn't matter much.

The beauty of a closed loop system is because it works like an elevator with a counterweight. The energy required to pump your fluid up the pipes is returned to you as it flows back down. The only "wasted" energy is the amount required to overcome flow resistance. If designed properly, that amount can be very low.

Concerning frost on ASHP coils: I really think it's mostly a nonissue now. The worst possible conditions would be near freezing when it's misty and 100% humidity outside. During these conditions I've never seen frost on the coils or moisture draining from the unit indicating there had been. Air can hold only 1/10th the moisture at 32F as it can at 95F.

I and others have talked a lot about raising the efficiency of heat pumps. Check my posts, especially starting HERE. The important thing is the amount of temperature "lift". If you don't have a high water table so water doesn't steal your heat away, you can have a perimeter of shallower boreholes around your home, using solar thermal collectors (dedicated or behind PV panels) to raise the soil temp under your home to 73F or so during the off season. 135F output from a water/water heat pump using a 73F source is more efficient than getting 105F water from a 40F source. You are now passively heating the home from below in winter but not continuing to heat when the house is above 73.

135F water is hot enough for retrofit hydronic heat with transfer plates (the warm floors I mentioned) as well as the ideal (and safe from Legionella) temps for DHW. Combining systems and using a buffer tank so you can size for average load instead of peak load could get you free of your energy companies.

I will never see -15F in Atlanta. Does anyone have experience with a modern inverter drive heat pump in those temps? Does it shut off below a certain temp as Sam's did, or does the output just dwindle down? At least the COP would always be greater than 1 and therefore better than resistive heating, since you get the energy consumption of the compressor and the indoor fan back, unlike cooling mode where you are trying to get rid of heat.
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