Calculating break-even point when mini-split costs same as running boiler
I have original steam boiler that is maybe 75% efficient (it is converted to gas 10-year old boiler). Recently, because we wanted AC, we installed air-source heat pumps (Mitsubishi MXZ-2B20NA with MSZ-GE06NA and MSZ-GE09NA head units). Basically one heat pump has one 6K and one 9K heads.
I am located in Boston. COP of Mitsu I calculated at 2.6 since HSPF is 8.9.
So what I am struggling to figure out is at which outdoor temperature it makes sense to switch from boiler, that costs $1.5 per therm at current New England prices, to mini-splits at current $0.243 per KWh.
It was suggested to me that it is somewhere around 40F outside, but my gut is temperature number might be lower.
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