Electric heat pumps (Mini-Splits) vs Natural gas boilerspost by Steven Byrnes (steve2152) · 2021-05-30T15:51:53.580Z · LW · GW · 9 comments
I’m kinda out of the loop on this stuff, but it seems like suddenly out of nowhere everyone around me is converting natural gas heat into electric heat pumps—specifically a thing called a “mini-split” which can be both a heat pump in the winter and an air conditioner in the summer. It consists of one or more indoor units connected to one or more outdoor units by a small-ish (well, small compared to an air duct) tube of flowing liquid refrigerant (which transfers heat between the indoor and outdoor unit).
An electric heater converts 1W of electricity into 1W of heat. A heat pump converts 1W of electricity into >1W of heat, because it pulls heat from the outside air—it’s basically an air conditioner pointed backwards. The ratio of heat to electricity is called Coefficient of Performance (COP).
The COP for heat pumps gets worse (albeit still >1) as the outdoor temperature goes down, basically because the outdoor air is colder so it’s harder to extract heat from it.
I live in a pretty cold climate; I figure the weighted-average outdoor temperature (weighted by how much heating I'm using) is 30°F (~0°C). A quick search suggests maybe I can expect COP of 3-3.5. The highest figure I found anywhere for that outdoor temperature was 5. I could be wrong here. (Update: in this comment [LW(p) · GW(p)] I used a different method based on the "HSPF" spec and concluded that I should definitely expect weighted-average-COP below 3.5 where I live, unless I messed up the calculation, which is entirely possible.)
Meanwhile I also need to account for my gas boiler not successfully transferring all the heat from "burning gas" to "warm interior" (as opposed to losing heat out the chimney or whatever). I think 80% is a reasonable guess for our not-particularly-well-tuned boiler. It should go up a bit, maybe towards 90%, when I get it tuned up or replaced at some point in the future, which I need to do anyway, it's super old and the plumber says it could catastrophically break any day.
I looked up my marginal electricity price and marginal gas price from my most recent utility bills, and did the unit conversions to a common unit of gigajoules (GJ). (I'm using marginal price because it's not feasible for me to rid of gas altogether, for other reasons.) It turns out that 1 GJ of electricity costs me almost 10× as much as 1 GJ of natural gas! (I fully expected the electricity to be more expensive but didn't know what ratio to expect.)
The COP>1 and boiler inefficiency help close that 10× gap, but they don't close it all the way, not even close. The existing natural gas boiler is still looking way cheaper to operate than an electric heat pump for me:
It's looking like a mini-split would amount to many hundreds of dollars per year of higher operating costs compared to my status-quo natural gas boiler.
Some additional considerations:
- Supposedly during summer the mini-split is a more efficient air conditioner than a normal (window) air conditioner. But I spend way more on heating than air conditioning (as assessed by the fact that my electricity bills are barely higher in the summer than the winter, whereas my gas bills have a blindingly obvious seasonality). So it doesn’t make a noticeable difference in the overall calculus.
- I live in Massachusetts, USA. The warmer the environment you live in, the more attractive a mini-split would be compared to a natural gas boiler. That's because of both higher COP at higher outdoor temperature, and greater weight on the previous bullet point (air conditioning efficiency).
- I’m not sure whether my most recent electric & gas bills reflect a typical price ratio. Maybe this is an unusual period (or I’m in an unusual area), with anomalously cheap gas and/or expensive electricity? (Update: A commenter points out that I live in an area with more expensive electricity than most of the USA—link.)
- There are some nice things about mini-splits, like they’re quiet, they allow air-conditioning without blocking windows, they don’t involve pipes that can freeze and burst, etc. However, none of these things is worth "many hundreds of dollars a year" to me.
- This table could even be overly flattering to mini-splits, because in cold climates like mine, there’s extra energy spent running the mini-split system not accounted for in COP, particularly there's an electric heater that keeps the outdoor unit from freezing over. Not sure how important that is.
- I guess mini-splits could be more "green", depending on how "green" my electricity provider is (I forget). But I think that I wanted to spend many hundreds of dollars a year being more "green", I doubt that this would be a cost-effective way to do so.
- Installation and maintenance costs are, I think, not radically different for mini-splits vs status quo in my particular situation, at least not compared to operating costs. My electricity provider has a rebate for mini-split installation, but I don't think that's enough to change the overall picture.
- Update: A commenter adds that there's a cost savings from being able to set different temperatures in different rooms, especially if you have, say, a rarely-used guest room that you can allow to get very cold. Mini-splits let you do that, although there are other ways too, like baseboard radiators with adjustable air flow vents. Anyway this isn't a consideration for me personally; I'm already set up with as fine-grained temperature zones as I could possibly want to use.
So, switching to mini-splits seems like a bad idea in my particular situation. I don't know why everyone else is doing it. Maybe I'm missing something. I guess to be thorough I ought to call a local mini-split installer, explain my thinking, and let them try to talk me into it. And ask my friends and neighbors... ¯\_(ツ)_/¯
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