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Comment by gwittel | Hacker News Reader

Modern heat pumps easily keep efficiency below freezing. Ours goes to 22F before there’s any efficiency drop. A different model goes below 0F (we just didn’t need it).

But yes, many makers push the crappier old tech models that bail around 50F.

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>Modern heat pumps easily keep efficiency below freezing. Ours goes to 22F before there’s any efficiency drop. A different model goes below 0F (we just didn’t need it).

No they don't. Their efficiency is linearly correlated with the ambient temperature. They don't "keep efficiency" nor is there a concept of "efficiency drop". It's just a straight line down.

https://docs.energytransitionmodel.com/main/heat-pumps/

You’re right. My wording was imprecise. I should have said the efficiency drop off is such that for practical purposes it doesn’t matter - until you hit a crossing point. You won’t need supplemental heat until then. Newer systems have pushed those points lower.

This wording is inaccurate. It isn't about "modern" heat pumps, its about if another $2-3,000 of electronics and clever injection systems are worth letting the refrigeration cycle work at the extremes. For a lot of the U.S. there is just no need to have a system that is a lot less repairable just to get that efficiency below 50F. The complexity differences between a variable speed system that can work over a 100F degree range and a standard single stage system with just a reversing valve are pretty big. The cost is basically never regained in savings in large parts of the southern U.S. so you just don't see them unless they are overseas models made with cheap labor; meant to be thrown away when they break.

Lots of cold weather heat pumps can be had for <$3k total, it’s not $2-3k of additional electronics.

And beyond cost savings on power (especially not needing to burn $30+ of electricity per day because temps dipped below freezing and resistive heat was needed), they just work a whole lot better, and the variable speed fans make them a whole lot quieter, inside and out.

They’re also simple to install yourself, so you don’t need to pay contractors the absurd fees they’re demanding currently.

Mine are from 2011, R410a and on the rare occasion that it gets to 22F here in Austin, they are painfully ineffective!

I installed a cold weather variant a couple years ago, it blasted out heat during the -5 degree cold snaps of last winter, maintained its full capacity, and did it surprisingly efficiently. But probably not worth it to go cold climate in Austin :-)

Which system did you install? Who installed? DIY or a contractor?

DIY, Midea 18k BTU, Hyper -22°F, 23.5 SEER2, 15.5 HSPF. Its air exchanger outside is a good bit bigger than other 18ks, I suspect that's part of how they hit the amazing efficiency at low temps.

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