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Comment by xyzzyz | Hacker News Reader

You will not get the COP of 3 during most of the winter in the interior US with an air source heat pump. Their efficiency go down significantly once the temperature is below freezing, due to required thawing cycles, and because the bigger delta between inside and outside temperatures reduces pumping efficiency in general (efficiency is best when outside is not much colder than inside).

Air source heat pumps are really good tech overall, especially because they also double up as AC in summer. However, in winter, in a head to head comparison, they only handily win in areas with mild climate, like eg. all west, or much of southern US. In the northwest or midwest, they are unlikely to beat efficient gas furnaces.

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Here's a heat pump that has a minimum COP of 2.93 at 5 F, and 9.42 (!!!!) at 47 F: https://ashp.neep.org/#!/product/56885/7/25000///0

Your point stands, but your 1,500 sqft home needs about 68,000 btus.

That heatpump would be for about a 300-400 sqft room.

Seems like a bigger one would only be more efficient.

Surprisingly, this doesn’t seem to be true for most of the split system ductless heat pumps (aka “reverse cycle air conditioners), I’ve seen.

The smallest ones are almost the most efficient ones in the range.

I’m not sure why this is, and whether it generalises to house-size ducted systems, but it’s pretty striking nonetheless because it’s the exact opposite of most heat related systems I can think of.

Might it be because the surrounding air only has so much heat in it?

Thus, smaller units, more spread out, have access to more heat?

I don’t know why - But your single air exchange heat pump driving a single head unit in the 9,000-12,000 range seems to be incredibly efficient. Once you go to a larger unit, and more heads, the efficiency drops.

It’s fascinating to even see the huge increase in switchable units - running your 4/5 ton unit at 4 tons is much more efficient. (Usually)

If you look at the numbers that's clearly not the minimum COP. I'm not sure what it is, I think it's the COP when operating at minimum power.

Look in the right most column for COP at max power

Cold weather heat pump technology is consistently getting better, but I also think people underestimate how many heating days are above freezing even in cold areas and how much of the US and other places is mild enough in the winter for heat pumps to be fairly efficient nearly every day during the winter (especially if you look at population rather than land area).

If you look at a place like Minneapolis, which is normally considered a pretty cold US metro area, you do have a few months where the average temp is below zero. But you also have several more months where the temperature is cold enough to turn on the heat but well above freezing, where a heat pump is dramatically more efficient. When picking a heating system, it makes sense to consider comparative efficiency over the entire year, not just performance during the worst conditions in the middle of winter. Even in a city like Minneapolis, it's not obvious that heat pumps are less competitive over the entire year, even if their performance in January probably isn't fantastic.

I also think the practical geographic area extends further than people think. New York City definitely isn't the south, but per some random Googling, it doesn't appear to have any months where the average temperature is below freezing. Even Boston only has 2 months that barely drop below freezing. Most of the US northeast (definitely the mid-atlantic) by population arguably also falls into the range where heat pumps make sense.

But then you need two systems - heatpump for most of the year, and natural gas for the cold days when you would die without it.

At least in Canada I’d need to know that my system can handle a week of -30 or -40C in my city. Some places are even colder, for longer at a time.

No, electricity is versatile, you can just make electricity into heat on that day when it's too cold for your heat pump. Resistive heating is not exciting > 100% efficiency technology, but it works and if you live somewhere that realistically gets too cold to run the heat pump that's what you add, not a whole gas burning system.

I don’t know why I didn’t think of that.

I guess the only remaining concern is that winter storms cause both cold weather and trees falling onto powerlines. The latter is rare though

Right, but (a) most modern gas fired heating actually requires mains electricity to function properly, so if you are blacked out your heating probably is disabled and... (b) even if the heating does work, a modern home is a pretty miserable place with no electricity, unless you know power will be back soon (hours or sooner) you should probably set out for a communal safe respite centre where everybody can work together to stay warm, feed and care for each other until the emergency is resolved.

There are people who genuinely live in the middle of nowhere, on an otherwise uninhabited island as caretaker, deep in the forest manning a fire watch tower, Antarctic research base, that sort o thing, who do need have more options, because if they run out of options they're dead. If you're in that situation you still don't necessarily want gas heating though, a fossil fuel powered generator might make a lot of sense, maybe even more than one, because again electricity is so versatile.

A snow storm plus power outage makes me want to bed down, not flee. The idea of leaving your house for a community center during a power outage is odd to me on several axes (pipes freezing, digging out your car, driving on half-plowed roads), especially in the hyper-individualist US (do these centers even exist?)

It's much simpler to obtain and fuel a small generator to power meta loads like thermostats/circulators/blowers than a large generator that supplies the thermal load. Fully committing to a heat pump as your only heating would likely involve a large gas generator fed by municipal gas or at least a large buried propane tank (for capacity and temperature).

The straightforward backup heat answer is wood. It's easy to obtain and store, and the primary drawback of it requiring labor and attention isn't a big deal during an outage. Although if you don't do it regularly and make sure your stove is in good order, lighting a fire during an emergency situation might not be the best idea.

So true, I'd much rather batten down the hatches, put my frozen food outside, and make sure none of the pipes burst.

I heard about a guy who had copper piping with water flow going around his chimney (on the inside) and then through the house.

That'd be a cool system if you could switch out the heat source, electric and an indoor-safe wood stove. Just get a CO monitor!

Actually, I just got a wood boiler for this past winter. This is basically a stove that lives outside, surrounded by a water jacket (~200 gallons) and insulation. It modulates the fire to keep the water from boiling off regardless of how much heat you're drawing off. You then run pipes to your house and tie it into your hydronic heating system. I feed it wood twice a day, and it keeps my house as comfortable as burning oil did (actually even more comfortable since I can heat the unfinished basement when I want, and not feel bad). And since it's outside, I don't deal with any of the mess or work of bringing heating-scale quantities of wood inside.

They do require power to run (automated dampers, blower, circulators), so I have a small backup generator as per my last comment. And they've gotten a bit expensive (especially now that the US federal tax credit has gone away), but my estimated payback period is around 6 years.

You might wonder why I'm posting in a thread about heat pumps when I just got a different major heating source, but I see heat pumps and wood as two very complementary heat sources. Down the line I'm planning to add a water-to-water or air-to-water heat pump for the shoulder seasons (and summer DHW), so that I don't have to tend a fire (or burn oil) when the heating load is low. One four or five ton unit should take care of my heat loss most of the time, and I won't have to oversize the system to handle the dead of winter cold snaps.

The only problem with that is if your electricity generation capability is correlated with the outside temperature. You don't want everyone turning on their resistive heat right when you have the minimum generation capacity.

I can’t reply to telaramex directly but backup electric resistance is exactly how my heat pump hot water works.

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