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Comment by archontes | Hacker News Reader

They also don't function well below 40°F/4°C

This is not factually correct. As sibling comment mentions, they work down to -15F. You might need backup heat in the coldest climates on the coldest days, but a high thermal performance (insulation and tight envelope) of structure is most important.

https://cleanheat.ny.gov/heat-pumps-cold-climates-do-they-wo...

Weird. Mine (Mitsubishi Hyperheat) have a COP above 1 down to -15F, and I've seen them work just fine at 5F (New Mexico, 6000', winter nights). Check your data.

COP "above 1" is nothing to brag about. I'd say that a COP of around 2 should be the minimum at which a heat pump is considered "working". Otherwise this propaganda seemingly based around the cost of resistive heating actually becomes relevant. (I've no idea of the exact current state of the art though).

COP == 1 is the point at which a heat pump becomes a resistive heater. As COP drops below 1, a heat pump becomes more and more like an actual resistive heater.

Nobody is bragging about COP == 1 ... it's just a reference point at which the benefit of a heat pump goes away and it becomes ... just an inefficient heater.

I just think this whole topic gets derailed in many ways by the lure of comparing things to resistive heating (eg the original article), and using parity with resistive heating as a reference point for practical heat pump source temperatures is an instance of this. The benefit from a heat pump has left long before COP = 1. I'd say a COP of 3ish (modern modcon boiler efficiency divided by common gas power plant efficiency) is a straightforward honest appraisal of where a heat pump starts to be less than ideal.

Of course I recognize low temperatures are merely one small portion of the time. Residential heat pump system design has generally taken advantage of this by having backup resistive coils. It would be much more efficient to fall back to a gas furnace, but for the capital cost. So an actual comparison necessarily takes into account expected temperatures over the day and the heating season, likely nighttime setback, thermal mass, etc. Which certainly isn't easy to button up into a simple comparison, but the point is resistive heating is a red herring!

You're drawing the wrong conclusion from the right numbers.

Sure, a heat pump with a COP of <2 is basically broken, but consider on how many hours of how many days it's actually going to perform at such a poor efficiency level.

Overall, your greenhouse gas emissions will be an order of magnitude below gas heating. Today, or at least until recently, it's about break-even at a COP of 3 iirc, but with solar and wind being the targets, it's going to be near zero by the time a heat pump bought today goes end-of-life.

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