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Comment by cosmosisjones | Hacker News Reader

so what about places like Duluth, MN where it can be -20F ambient, then -40F to -50F with a windchill for several weeks of the year?

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Windchill doesn't effect, and you can handle small dips below -15 F (like for a couple hours at night) because the house has some thermal mass. However, Duluth, MN is right at the edge of being too cold for this heat pump (99th percentile cold temperature in the coldest month is -15.16 F). If I was in Duluth and was considering a heat pump, I'd want to make sure I had an electric furnace or some space heaters as a backup. However, it would still be worth it to get the heat pump because you'd be able to heat your home much more efficiently/cheaply >99% of the time.

Even in coastal California, most ducted heat pumps have a built in electric or gas furnace.

I imagine anything sold in areas where it actually gets cold will as well.

Here at 7000' near Santa Fe, I know dozens of homes, many new construction, that are heat pump only to all effects and purposes (i.e. they may also have fireplaces but these are not part of the normal heating systems nor part of any plan for the 0F-5F winter nights we can get.

Heat pumps can handle things by themselves down to around -10F to -15F, if correctly sized and installed.

What brand?

Most of the low temperature ones I’ve seen include resistive heating. At some point the COP of the heat pump approaches 1.

Also, having heat to -15, then switching to no heat isn’t ideal.

Mitsubishi. Their "extreme" models do not switch to resistive heating, but deal with the critical problem at very low temps: partial freezing of the refridgerant. They divert some of the heat into fully melting the refridgerant. This reduces the COP but keeps them functioning.

It's not designed for those areas. The world is plenty large, they don't need to accommodate every single address in the country to have a viable or useful product.

I would be very interested in stats on the gradient of home energy use by average outdoor temperature. I imagine that "Places where it gets too cold for heat pumps" represent a large percentage of heat usage; That is, there's (made up numbers) 10% of the population living in those areas, but they represent 90% of heating energy use.

If every home that could use your heat pumps year-round installed one and used it exclusively, what's the upper bound on energy saved? If every home across the country installed one and used it primarily (whenever it was warm enough outside), what would be the energy savings? Not to disparage, I'm certain it's still significant, but I'm curious to quantify it.

This is quantified by "heating degree days" and I don't think your intuition is correct here.

https://upload.wikimedia.org/wikipedia/commons/d/da/United_S...

The difference between "Can use a heat pump" and "can't" is only maybe 10º or 15º. The coldest places are only ~20% colder than much more populated areas with millions of more residents.

Metro Boston has around 5 million people -- that's more than 3x as many as who live in North Dakota and South Dakota combined. Their climate is cold and they get around 6,000 HDD every winter compared to the frigid upper plains who average something like 8,000 or 8,500. So 1.3x more heat requirement per person but spread out across 1/3 as many people.

Not to mention all of e.g. Chicago, Denver, Des Moines, Cleveland, Detroit, New York, the rest of New England, etc. etc. where heat pumps do just fine.

Windchill is irrelevant here, it only indicates what the temperature feels like to warm blooded mammals.

I was tempted to give a similar answer, but I'm not so sure it's irrelevant. Yes, the windchill won't affect the energy efficiency of the heatpump. But it will affect the rate at which the house loses heat, and thus the BTU's the system needs to provide to maintain a temperature. If the heatpump has been exactly sized to maintain a house temperature of (say) 70F at the expected winter low, adding a strong wind on the exterior of the house seems like it might make for a chilly morning. The effect isn't as strong as it is for bare skin at body temp, and hence the exact numbers will be off, but it's still there, right?

windchill as a concept can be applied to all evaporative cooling systems.

Heat pumps are not an evaporative system.

You can't lower the temperature across a gradient lower than the fluid temperature, no matter how fast that fluid is moving. You can cool things down faster, but you can't go lower.

So if it's 10F, a 5F heat pump will work equally well at 0mph wind and 30mph.

You got it. A brief technical aside, heat pump performance is hurt by frosting, which happens at intermediate outdoor temps (~35-50F). Basically the outdoor coil is both below freezing, and below the dewpoint of the air. The frost forms a thermal insulating layer around the coil, and intermittently you have to run a defrost cycle to melt off the ice. This is a small hit to system efficiency.

Yes, I never claimed they were. I was responding to the idea that only warm-blooded mammals have the concept of wind-chill. Any system that takes advantage of evaporative cooling will have the concept of wind chill. A "swamp cooler" is one example.

Custom ground source heat pump my friend :) side benefit (energy cost) free hot water.

Ground source is great! (if you can afford the install).

Not every product is for everyone.

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