That is a known known... My heat pump can still extract heat from the outside air down to something like -4 degrees (F), but clearly as I observed there is a lot less heat to harvest at 15' then in the 30s'. But another known is typical temperatures for the region (15' is pretty abnormal).terkio wrote: ↑Sat Feb 20, 2021 2:46 amThat is the big trouble with air air heat pumps, the efficiency drops at lower air temperature.
That is a bit paradoxical, it is good when it is not so cold outside.
I could have fired up my old in wall resistance heaters, but I don't like paying for 1x resistance heat when I can enjoy using a several X heat pump.
As I probably already shared my last month's electric bill revealed that I was using roughly 50% of my typical neighbor's electricity consumption and 15% less than the most efficient 20% of my neighbors.
In use the outdoor heat exchanger is driven even colder than outside ambient (its how they work), from time to time I can hear it cycling to deice itself. I do not notice it blowing cold air inside. It is a mature and competent design. Net net it is reducing my energy bills a bunch.
It is even worse, at very low temperatures; The outdoor heat exchanger can freeze, so to present that the heat pumps runs cycles in reverse, cooling inside to warm up outdoors.
I am all to familiar with ground sourced heat pumps, as noted installation is way more expensive, and unneeded for the rare very cold events where I live. Probably smarter to oversize the heat pump for the application, and in my case live with it slipping a few degrees. Did I mention I'm cheap?
An alternative to the air air heat pump is to use deep underground temperature instead of outdoor ambient air temperature. This is still subject to thermodynamics second law issue but to a lesser extent because of better temperature conditions.
More efficient, but more costly at installation.
A friend of mine who built a huge house outside Atlanta had to dig a pretty deep water well and he put the heat exchanger for his heat pump down deep under his house in the well water. The ground water probably stays in the 40s year round (guess).
Since I have standing water in my crawlspace right now I probably wouldn't have to dig very deep or large hole for an effective ground based sink, but still not worth it IMO. For a modest investment I could add a second heat pump for my back bedroom and turn off the main house heat pump at night... that would probably reduce my consumption even more, and provide extra capacity for rare weather events.
Now I need to try to burn some of those pine logs I have in my yard. My neighbor got his phone line fixed this morning.
I had a professional tree guy trim my pine trees a couple years ago, looks like I need to call him again. I came very close to losing power this time.