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Comment by virtue3 | Hacker News Reader

And their efficiency plummets starting at 40F and below.

The efficiency of modern cold-climate heat pumps plummets to 200-300% at 40F and perhaps 175% at 5F.

But 5f is still high.

This is actually terrible and supports their point. Heat pumps are not sized as if they were running at a mere 100% or less, they are sized for the btus they normally produce, and aim to run close to 100% of the time, meaning they will be too small and not be able to do the job when they drop below 200-300% efficiency, unless they are grossly oversized. That leads to short cycles which makes condensation and moisture in the summer and wear and tear all year and less electrical efficiency because starting a pump and blower costs more than running it.

Current inverter types that can run slow do allow them to be oversized but it's still not great. Those are more expensive and have more failure points in electronics too.

So at -20 and below they are basically very, very expensive resistive heaters. Good for medium climates, bad further north.

Also either need very expensive oversizing or backup heat sources to compensate reduced heat output, even if you get occasional cold spikes.

Unfortunately nothing in life is as simple as it first seems.

Here is a great Technology Connections video which lays out why the sizing issue might not be as bad as you're thinking. The gist is that many people are steered away from heat pumps (or sold very expensive oversized units) by outdated industry thinking which vastly overestimates load calculations.

I won't try to make his case here, but he is pretty convincing that a reasonably sized heat pump system can be sufficient even in very cold climates (by US standards).

https://youtu.be/DTsQjiPlksA

It's all theoretical. In real life, retrofitting heat pumps into exisinting homes is a bit different, as those homes are neither made for air heat sources (at least in my region), nor they are insullated well. You might say, dough, insulate it, but it is easier said than done, as a lot of those old homes relied on their leakage for ventilation, not to mention that is very expensive investment (much more expensive than the heat pump itself).

I would reccommend keeping backup heat source when installing heat pumps into older homes. A simple wood stove, used during cold spikes, could be enough.

My country has subsidies for heat pump installations into older homes and it had requirement to get rid of chimneys to get that. After the whole europe energy crisis thing, this requirement is gone now. Turns out having alternatives is a good thing :)

I live in Canada in an older home with terrible insulation.

Out two tonne heat pump works just fine even at -25C this past winter.

We still have a natural gas furnace but our gas usage is down 65% despite this winter being colder than last year’s.

Tell me again how heat pumps don’t work in colder climates?

This is why I opposed historical designations. Most old buildings are obsolete and need to be destroyed. There are only a tiny number of buildings that are really historic, sure save them, but the vast majority are just old and need to be knocked down for something better. If it isn't taught in history class then it isn't historic.

It is easy to see the loss of something old. However the opportunity cost from not building new is something that is very hard to get people to understand.

This is not theoretical: my heat pump purchased in 2021 worked well for four straight days of below 0F. Kept our temp at 68 inside. We live in an old home from 1917 with poor insulation between the brick exterior walls and drywall.

How often does it get to -20f in New York City?

Ahh damn, I forgot unit, I meant -20C, so -4F

You have to go quite far north for -20F to become frequent.

If it EVER gets to -20 that is what matters. You need to prepare for the worst case weather, not the common ones.

Fortunately resistive heating is just about the cheapest possible heating in capital costs and trivial to retrofit as a backup.

It will still work, just not as efficently as in normal temperatures. So you're using 50kWh for the 3 days a year it's that cold and 10kWh for the other 362 - better than using 20kW every day.

Only if there is enough power generation

And a good 24,000 BTU rated heat pump will produce just under 20,000 BTU at -13 F, about 18,000 BTU at -22 F (-30 C).

1990s called, they want their efficiency back ;)

Sure, you can still find good weather heat pump that crap their pants near or under freezing. However, the majority sold here in Europe still stay well of 200% down to -15C, some even -25C.

75% of rated capacity at -22 F (-30 C), over here.

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