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.
Best Answer
I don't think you can beat the comfort that cast iron will give you when it is really cold out, like you experience in winter. With forced air heating, because the heat is carried on the air, these systems it will always feel breezy and drafty. With the cast iron radiators you not only get great convection heating but get all that radiation as well. Forced air cannot begin to compete in the comfort or price of hot water. Here is clue; In Europe where energy cost three times as much as it does here in the states you cannot find a home that is not heated with circulating hot water. Also when you get north of North Dakota into Canada where air conditioning in not nearly so much in demand, 90% of homes are heated with a boiler and circulating hot water. In Montana, in the Dakotas, the upper mid-west and the north-east, circulating hot water and steam are use far, far more more that forced air. They do this for two reasons; one is comfort the other is cost.