I recently had three mini-split heads installed, two downstairs and one upstairs, primarily for A/C. These are Mitsubishi units, rated down to -15C (or perhaps F, but it doesn't really matter for my purposes).
I've been considering using them for heat, either primarily or as an assist to my 25 year old gas boiler for a forced hot water radiator system in a reasonably well-insulated 100 year old house.
I pay about $1.50 per therm for gas, and about $0.11/KWH for electricity. By my calculation, that comes out to a $3.22 equivalent price per therm for the mini-split solution. Even if the old boiler is not terribly efficient, I can't believe it would make up the difference.
My current plan based on this analysis is to use the mini-splits for A/C only and my existing gas forced hot water for heating. Am I missing anything?
Edit: The units I'm concerned with have SEER ratings of 23.1 and 21.6, and HSPF ratings of 12.5 and 11.7. The models are the Mitsubishi MZ-GL15NA and MZ-GL12NA. I do not know if they are true heat pumps or not, but I believe they are.
I have original cast iron radiators (at least I believe they are the original radiators).
I live in New England, so a particularly cold winter day might be between -10F and 0F; normal winter temperatures are closer to 15-25 for a low.