[ChetT]

I've been thinking about just this problem for my Maine home. I have 3 zones of underfloor water based radiant heat. Given my location, an electric boiler made the most sense as a heat source. However I figure that a minisplit heat source would reduce my electric bill by at least 1/2 and maybe more. I think merging a single zone minisplit with my existing system should work just fine.

Can't you just add a heat exchanger between the two systems? The compressor on the minisplit could be controlled with a contact from the circulation pump from the heating system. The minisplit head unit would be set for max temp and therefore calling for the compressor but the compressor would only come on when the circulation pump is running.