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Comment by solardev | Hacker News Reader

If you're considering a heat pump, be sure to check the minimum operating temperature before you buy, and/or make sure you have a backup heat source.

I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F.

Once it went down to -5, the heat pump didn't just work less efficiently, it stopped working altogether. It was the only real heat source in the house. We tried turning on the kitchen oven for a while, but it couldn't keep up. We had to leave the apt for those days and crash with a friend.

Other than that, though, it's been wonderful! SO much nicer than other heat sources (furnaces, space heaters, etc.) It's comfortable, cheap, quiet, and has its own thermostat. We just leave it on and it does its thing and it's magical.

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I don’t think it’s normal for a heat pump to just stop working completely like that vs. just getting more and more inefficient. Wonder if perhaps it iced up and the defrost wasn’t working.

It's my experience that they do stop working completely at some temp. The defrost cycle creeps up to 100% and there goes the heat.

That's because the heat-exchanger is not efficient enough to prevent too much frost from building up. The answer there is a better (probably bigger) heat-exchanger.

Yep, correct. As many have pointed out in this thread though, HVAC is full of trade offs. I COULD get a bigger heat exchanger.. Although my furnace is already a 5 ton unit. The question is, in the Ozarks, where I am, are the 1-4 days a year where it's too cold to run the heat pump worth paying the extra money for the larger system?

I added an efficient wood stove instead. There's a good tax rebate that covers installation as well - I took advantage of that last year.

It's entirely possible? Mitsubishi makes a bunch of different models, and ours is a cheaper one. Our area rarely gets under 0, so it's understandable why the landlords chose this model (and it's also an older model, for an older apartment). They have other models rated down to -15 or lower.

But even if it's because the defroster stopped working, or it's just an auto-shutoff feature to keep the equipment safe... the end effect is the same: There is no more heat when you most need it, and you'll freeze. We were wrapped up in multiple layers that day and tried to warm the house however we could, but ended up just needing to stay with a friend cuz it was so cold.

In some areas, "two sources of home heating" is mandated by law, but not where I live now. It still seems like good advice, specially if you can do that with two separate fuel sources (like electricity vs natural gas vs propane vs heating oil), in case one infrastructure goes down, as they often do during the worst storms.

My heat pump is 12 years old with a CoP of only 3 so when it gets down to -15C, it's less efficient than a resistive heater so it shuts off and the display shows P3. It was designed that way so you can have a secondary heat source set a few degrees below the temp you normally want so that kicks in when the heat pump shuts off. My backup is the electric baseboards I used before I got the heat pump.

Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.

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