Thermostat Questions

The hum at the outdoor unit could be the relay-contactor-whatever you call it that turns on the compressor and fan using the 24 v from the thermostat that is still hooked up. That shouldn't be any problem.

If your indoor "furnace" is electric resistance heat, then you probably want to figure out a way to use the heat pump for backup, so the wood plus electric backup doesn't end up costing you more than the heat pump would have by itself. Same thing if you're burning propane or fuel oil depending on the price.

I have a heat pump and OWB. What I did was I installed an aquastat on my supply manifold in the basement that senses the water temp in my OWB's loop. If the water temp is 110 degrees or hotter, the aquastat (via a standard relay) breaks the yellow wire going to the heat pump. The yellow wire controls the compressor in the heat pump. If my OWB's water is warm, the wire that turns on the heat pump's compressor is broken. If the water is cooler than 110 degrees, the yellow wire is complete and allows the heat pump to kick on if the thermostat calls for heat. With this setup, the air handler kicks on as necessary to circulate the heat and the heat pump only comes on if my OWB's water is cool. The AUX heat strips never kick on unless there's a 4 degree difference between house temperature and thermostat temperature or if I manually put the AUX heat on.

If you want wire schematics for my system and thermostat tie-in for my OWB, I can email those to you. I'm using a Bryant heat pump and air handler (Carrier type system). Any HVAC guy should be able to easily read the wire diagrams and figure out what's going on. It is about the simplest way to run a heat pump with electric backup heat and a wood fired boiler together.

You could use a relay to kill the fan and circulator when the water temp is lower than the same set point on the aquastat that kills the compressor to the heat pump when the temp is above that point.