Posting this in Residential vs. pro as may be of more general interest.
Wondering if anyone has looked at the issue of switchover between air-air and wshp, but do realize very few likely have dual source independent HP systems.
Have 4 T RHeem air to air HP running >3.5 COP for 60F air (hardly ever use the AC in PNW) and a just installed water source heat pump that runs almost 6 COP well water this time of year with 57F water (yes, did need heat last week!). From well measurements last year, expect well water to drop as low as 52F in the winter.
Looking at best way to automatically switch over from air-air hp to wshp. Obvious way is just an outdoor thermostat that switches to wshp when air temp is less than water temp (could not find a differential air/water TS, so will have to figure out how to cobble that together unless someone knows a source).
Another idea is for wet bulb/dew point sensor to switch to wshp whenever the air-air can get frost or ice up.
Maybe easier to figure which is best once we get past December and have an operational cost data base on relative cost - may turn out best to just always leave the air-air off??.