Short story long, I had two mini split heat pumps installed this summer, mostly for the AC but I started using the heat portion when the temps dropped some, so far they have kept up great although I don't have warm floors and free hot H2O 
I have been putting off lighting it partly as an experiment with the heat pumps and also the fact that the ambient air temps have been strangely warm.
It got down to 18* last night, I had turned the circulator for the OWB on yesterday afternoon, I have a HX in the plenum of the hot air heat system.
Lit her up this morning, now I have warm floors and free hot water
I have been putting off lighting it partly as an experiment with the heat pumps and also the fact that the ambient air temps have been strangely warm.
It got down to 18* last night, I had turned the circulator for the OWB on yesterday afternoon, I have a HX in the plenum of the hot air heat system.
Lit her up this morning, now I have warm floors and free hot water