Mini-split vs gas boiler for heating

Question

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.

Answer 1

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.

Answer 2

The thing is: heat pumps move heat around rather than creating it, so they are more than 100% efficient. They collect heat from the outside air and move it inside, and depending on the available heat outside, the output inside can be several times greater than if that same energy were to be used to generate heat (as in a resistive space heater).

With your numbers, and assuming your boiler is 80% efficient, you only need the heat pump to be about 170% efficient to be a net win — that is, a COP of 1.7 or greater. This is easy for a heat pump to achieve, even in cold weather.

In this chat discussion Shalvenay pointed me to a chart for Mitsubishi heat pumps, and the MSZ-GL12NA-U1 has a COP of 3.84 when the outdoor temperature is 47° F and 3.13 when it's down to 17°. Assuming your prices are as you say, this is a definite win for the heat pump.

But are you sure you're factoring in the full electricity costs? I know it's a year later, but looking at my Eversource bill, I see $0.11397 for Generation Service Charge, but there are bunch of per-kWh Delivery charges as well, coming out to about $0.223 total per kWh — or $6.54/therm. That makes the math look somewhat different — you'd need a COP of 3.4 or greater for the heat pump to be a win over gas priced at $1.50/therm. According to the data sheet, your units should still hit that when it's relatively warm out, but won't as it gets cooler.

There is also another factor to consider. Burning natural gas is relatively clean as fossil fuels go, but still releases CO₂ — about 12 lbs per therm burned. In Massachusetts, about .9 lbs are released per kWh of electricity generated — about 25.7 lbs per therm. If we again go with 80% efficiency for the boiler, that means a COP of 1.7 or better is a win for carbon emissions, even if not for your pocketbook.