I've been lurking here for awhile, reading all these heat pump threads. They are all very interesting and highly detailed with an immense amount of useful information. I'd like to thank all the contributors for their work
As an engineer, I'm very interested in heat pump technology myself. I've spent the past 2 years getting more and more involved with it.
We had a York ASHP put into our home in central NH about 2 years ago to supplement the home's propane furnace. Its a dual-fuel set up. I think alot of people (not including this forum) misunderstand ASHPs and think they don't work when it gets cold out I even got some comments like this from some of the HVAC people I spoke to when we were looking into it.
According to the manufacturers data the HP achieves a COP of 2.5 at 17 degrees OAD, and 3.7 at 47 degrees OAD with a 70 degree IAD. If the thermostat is turned down (i.e. to 60 degrees) then there's less 'lift' required, the compressor doesn't have to work as hard, and the COP rises from 2.5 to 3.0 at 17 degrees. We turn the thermostat way down when we aren't there (its more of a vacation/weekend home) allowing the ASHP to operate more efficiently.
An ASHP puts out less heat when it gets cold outside, but even at 10 degrees outside its cheaper to operate than the propane furnace. I think if you have a backup heat source already, then an ASHP (mini-split or traditional) is cost effective almost everywhere in the US compared to propane or oil at current prices. At my electric rates ($0.16 / kwh) I figure it costs about half as much over the heating season compared to propane.
A couple other comments:
1) I initially had trouble with my heat pump regularly tripping the high pressure limit switch, and I wasn't getting the $$$ savings I expected. I suspected non-condensables in the system from an insufficient vacuum during install. The tech evacuated and recharged. It was better but I was still getting some trips the next season. I suspected that the system was overcharged (its hard to accurately charge in the winter when my system was done).
2) In the end I decided to take the EPA 608 test and get some tools so that I could work on and maintain my own system. The test wasn't hard and I would recommend it if you want to install or tweak your own system. I recovered some of the charge from my system and was able to fix the pressure tripping problem I had. It brought the high-side pressures down to where I expected and there also seems to have been a noticeable reduction in electric usage. I'm pretty pleased with that.
3) I'm looking to add another small heatpump to a separate part of the house and after reading the "home-made heat pump" thread I'm considering trying to build one. I have the stuff now, but we'll see what comes of that
