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??.
You may be able to use a differential controller such as those used in Domestic Solar Water Systems to switch between both systems. Most are quite flexible and will provide both N/O and N/C contacts.
Without getting to far into it, this should be able to be accomplished with an outdoor stat and possibly some relay logic, or an automated controller. depends upon level of technical ability and dollars able to be spent.