Quote:
Originally Posted by buffalobillpatrick
This is sourced from the solar tank.
"From 80* down to 35* I plan to source this to the heatpump evaporator. The destination from the condenser is the 120 gallon DHW / buffer tank."
I want to keep the 120 gallon DHW / buffer tank in the 120-145* range.
So it sounds like I need a R410a compressor?
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You'll probably be better off with two separate heat pumps and two storage tanks, one for the central heating/hot water first stage and one for the hot water second stage. The first one operates with a water temperature of 80-100F, so a R22 compressor would be just fine. The second one needs a R410a compressor as the condensing temperature can be very high, but much smaller. Keep in mind that the GE Geospring uses a refrigerator compressor, so you don't need very much, especially when you have the big one doing most of the work in cold weather. A 5000BTU/hr compressor (like one in a cheap window A/C) would likely be more than sufficient.
Exactly what temperature do you want for the first buffer tank? You said that when the solar tank is 80F or more, you want to bypass the heat pump entirely. Therefore, I used that to imply that you wanted 80F as a minimum. The second buffer tank (can be much smaller, as in 40 gallons or less, whatever size turns out to be most common and cheap) is what brings it all up to the temperature you set for DHW. Note that washing dishes is about the only thing that needs 140F water, so you'll get great efficiency gains by using a lower temperature (say 110F) for other uses and a digital control system to select the temperature. (I suggest something like the Raspberry Pi as it's cheap but easy to network.)