> Uh, you still need a furnace (though it could be electric) if you live somewhere that ever really gets cold, right?

For now - and only in part of the country. Most of the newest models can output 100% of their rating down to something like -5ºF -- they're easy enough to oversize as well, so if your 99% heating load is e.g. 48,000BTU, a 60,000BTU heat pump that's only outputting 80% of rated BTUs due to the extreme cold can still cover the full design load.

Here's the spec sheet for the newer Mitsubishi hyper heat models - 87º output at -4ºF and 76% output at -13ºF -- very few places in the states ever get that cold: https://static.appliancesconnection.com/attachments/D5bf5709...

I’d say -15°F or lower is something you need to design for at least into the light-blue area on this map:

http://4.bp.blogspot.com/-tdK_AMaZ9pg/Vb51WrvPDhI/AAAAAAAAD5...

Which is quite a lot of the country.

Light blue is a typical Winter low point in the -10°F to 0°F, which means you will see -15°F or lower often enough to worry about it.

Fortunately, they have county-by-county data (and hourly data if you so desire) that spells out the design criteria for heating systems;

https://www.energystar.gov/ia/partners/bldrs_lenders_raters/...

Though heat pumps are unique in that they produce less heat as the colder it gets - A few hours of -15º every few years shouldn't be the primary consideration in spec'ing a system that still produces 75% of its heat in that worse case.

I live in that blue area and ran through all of the math and considerations recently - I pulled the hourly temperature data for 6 years. Of the 94,000 data points in that period, a total of 26 hours were below -5º: https://imgur.com/a/P7A3kan

Meh. I live in Wisconsin. It's fine.

> Our modeling finds that even if Focus incentivizes 800,000 heat pumps with electric resistance backup (10 times the number of heat pumps as it did furnaces in the past four years), the state will still be able to meet its electricity demand with currently operating power plants, even on the coldest days. Depending on the efficiency of the heat pump, in-state winter generation capacity would still exceed peak demand by 1,400–4,300 MW on the coldest day.

https://rmi.org/three-questions-wisconsinites-are-asking-abo...

The dataset is typical winter low for 1984-2014. Earth is warming thanks to climate change, and the lowest lows have notably warmed. See, e.g., https://xkcd.com/1321/. In my 12 winters of living in the light blue zone, I think I've spent like a grand total of 36 hours or so in sub-0°F temperatures, and that was pretty much the one period mentioned in the xkcd comic.

You can simply add backup resistive heat to cover the occasional 4AM drop to -15F. It’s more expensive to operate, but as long as this is rare it doesn’t significantly change your bill.

We have a set of those hyper heat mini splits to supplement a hydronic system which wasn’t expanded to several additions. They’re generally great but during the Midwest’s recent extremely cold snap down to a week or two of negative temperatures they were pretty disappointing and could not keep up. We ended up having to close off a few parts of the house to keep the temperature reasonable in the rest. I think that trusting heat to a heat pump system is not yet feasible in much of the Midwest and northeast.

Yeah, we're remodeling right now - our design specs say heat pumps should be able to cover 100% of our load but we also have a few rarely used gas fireplaces and some space heaters in case that doesn't work out. Though like someone else mentioned, the bigger change will be reacting to power outages. A small camping generator can provide enough energy for to keep a gas furnace blower running whereas you need a very large generator to operate the heat pump.

i think this explains the oversizing. 76% seems pretty good, but a lot of places do get to negative temperatures once in a while, maybe once every year or two, and that’s when you really want the heat to work.

I like the redundancy of natural gas. So far, in less than 40 years, I have been kept warm for multiple days on 3 separate occasions by having access to gas while the electricity did not work. Also, I was able to keep cooking.

One of the states in the article is Oregon too, where I have family that just a few weeks ago lost electricity for 4 days, but were able to use an electric generator to keep the air handler going and gas to heat the the house and cook.

I fear heat pump only heat will fail exactly when I most need it not to.

Yeah if my heat were 100% electric I’d have to install a wood heating-stove to feel like I wasn’t being irresponsible. Or maybe get a couple portable kerosene heaters.

I think a sensible option, if you live in a place where electrical grids goes out for a couple of days fairly routinely, would be a transfer switch and a generator. If sized well enough you could use it to run a resistance heater (the cheap portable type) to keep one or two rooms warm in an extreme scenario.

Pipes freeze and burst, whole house has to stay warm. It’s not just about comfort or safety.

If you warm up a few rooms to comfortable then it's hard to imagine the rest of the house being at freezing temperature. It's going to be some sort of gradient. But sure, this is a concern/consideration.

My ex-wife had a two-day power outage last month (it was only a few hours at my apartment) during a cold snap. She has gas heat, but the problem is that the heat gets circulated by fans¹ which are powered by—electricity.

She had to get a hotel room for the night because she wasn’t comfortable sleeping with the gas fireplace on.

⸻

1. I would guess that thermostats also powered by electricity not working would add to the complication.

Of course, but gas utility supply means a much smaller generator is needed just to operate the fans and thermostat and much less on-hand fuel is needed to operate the generator.

Natural gas is just a very convenient and very dense source of energy when you need it most.

In the entirety of my life, most of which has been in the Chicago area, I have never seen a home with a generator for the fans and thermostat of a home. For that matter, the only home generator I ever saw was one my grandfather bought which he only used once to see if it worked.

They're pretty common where I live (SE Michigan) because the electrical grid is quite a bit less reliable than the gas distribution network. To the point where 5-10% of customers in the service area lose power in any given big storm.

We have an (almost[1]) all electric house. A year ago we lost power for six days. Last spring we had a generator installed. Over the summer we lost grid power for five days but the generator worked flawlessly the entire time.

I don't like having gas for a number of reasons and if the grid was more reliable we would never have bothered, but, for us, it's just so much more reliable.

[1]: We have two HVAC systems that service different sides of our duplex-ish house. One side is a ground source heat pump, the other is a 95% efficient gas furnace.

It sounds like your house is poorly insulated too ?

Temperature rating is irrelevant if the outdoor unit turns into a solid block of ice before it gets that low.

Oddly enough, it tends to snow when it's cold. Even rain can a problem since the nature of a heap pump means the unit is cooler than the surroundings.

So often for a few nights of the year, the alleged "heat pump" actually just falls back to electric heating.

The outdoor unit goes through heating cycles though to prevent that. I'm in Vancouver, BC, where it doesn't get that cold but my unit had no problem when it was snowing and -15C or so outside. It did have to work pretty hard though. We don't have electrical heating backup for the heat pump but we do have a gas fireplace as backup (so I know the heat coming from the vents is 100% heat pump, not an electric heater in line).

I have a pair of heat pumps in a dual zone setup, each of them has a backup electric furnace as part of its indoor air handler. Newer ones can keep running without that even into quite cold areas.

The thermostat just sees the resistive heaters as another phase, so I have three phases, and if it doesn't see a temp rise within a certain time of calling for phase 1/2, then it goes to phase 3. Mine also has support for an external temperature probe, that can skip 1/2 if it is already too cold.

I also have other ways to make heat if I have a prolonged electrical outage, but outside of maintenance, I've not used that.

From my limited experience, I'd say yes - you still need a furnace.

I converted my garage into my office by adding a mini split AC/heat pump. The garage walls, door, and attic are insulated - but it's still a garage, which means that it's not nearly as well insulated as a regular room in a house.

The unit is 18k BTUs, which is quite oversized for the size of the garage. I made this choice because the heat pump BTUs are significantly less than cooling BTUs. This is definitely obvious, experientally - the unit can make my garage uncomfortably cold even in the peak of summer.

But even though this unit is supposedly rated for operation in temps down to about -20F, when winter gets really cold, it struggles to keep the space warm. Once the temps are below about 20F or so, I usually add a space heater to the mix, which, combined with warm clothes, makes working out here at least tolerable.

Technology Connections seems pretty happy with his. I think he lives in Chicago, admittedly not the coldest place, but it does snow every winter in that region.

He said 2.9 coefficient of efficiency average over a heating season for him - https://youtu.be/7J52mDjZzto?t=1522

Heatpumps have resistance heating backup for the exceptional periods when it is too cold to make use of the compression system. Most people live somewhere a heatpump would work well.

Problem is, if everyone has resistance backup and it gets really cold, I doubt if the grid could keep up.

This also assumes a resilient grid, which may not be the case for large areas of the rural US.

Oh, I’ve not see that kind, only the sort where the “e heat” feature on the thermostat is tied to a separate furnace.

Yes. I live in Utah and have a heat pump and gas furnace. I am told the heat pump is really efficient at medium-cold temperatures, but not so much on really cold days.

I believe heatpump technology has improved over time. Also, "less efficient but still functional" is an adequate option for really cold days.

Less efficient but still functional means my home was 50 degree inside instead of 67 when it was -1 for two weeks here.

On older homes, with much worse insulation, this would immediately be a problem.

Heat pumps are great, but they absolutely need some kind of emergency heat back up.

> Less efficient but still functional means my home was 50 degree inside instead of 67 when it was -1 for two weeks here.

No, that's just not functional. "Less efficient" means consuming more joules of electricity but still providing the required function.

> Heat pumps are great, but they absolutely need some kind of emergency heat back up.

They include it. You were sold an inadequate pump for your situation.

The best mental model for a heat pump is that they can maintain a certain temperature differential, say 80F. If it gets to be 0F outside and you want it to be 70? You don’t need to switch to a fully separate heating system, you just need to warm up either the incoming or outgoing air an additional 10 degrees via a small resistance heater. The super wide range heat pump systems will do that automatically, and the principle really applies to any differential you could want.

The wide range ones are basically two heat pumps back to back using different working fluids. They should only be used in applications where they are really required as there are obviously impacts to the COP.

> The best mental model for a heat pump is that they can maintain a certain temperature differential, say 80F. If it gets to be 0F outside and you want it to be 70? You don’t need to switch to a fully separate heating system, you just need to warm up either the incoming or outgoing air an additional 10 degrees via a small resistance heater.

Are your 70 and 80 switched? What you describe doesn't sound like it needs any addition.

What's really cold? There are heat pumps that are advertised as working at -15F. On the days when it gets that cold, your heat pump will need to switch to electric heat. So on those days, your heat pump is no better than an electric furnace. On all other days that the heater is running, the heat pump is a far more efficient option.

Note: this is only true for air-source heat pumps. Ground source-heat pumps aren't affected by ambient air temperature, but they're also much more expensive and require suitable ground. Also, I hear air-source heat pumps have made significant advances lately in how well they handle sub -15 °C temperatures.

Heat pumps are still more efficient, but you’d need damn good insulation (or a really big heat pump) in super cold weather. Furnaces scale-up pretty well on the other hand.

If your locale gets life threateningly cold though, I’d feel more comfortable with a furnace because of the fewer moving parts. Burn gas, get heat, dead simple.

Funny enough in the short term I've lived in my house, I already had a winter where the furnace quit.

It's a high-efficiency one with a control board (Nobody can convince me FCS isn't Fire Control System) and a separate draft motor.

One of the vacuum sensors went out and the furnace couldn't prove it was safe to run, so it would turn on the draft motor, suspect a clog, and then shut it back off.

An easy fix but not as simple as lighting a Bunsen burner. And I haven't seen the electrical cord for it, I'm not sure how I would hook it to a generator if I lost power. The water heater oddly enough is battery-powered, so I guess I could just fill the tub with hot water.

I live in Maryland and the first townhouse I owned, in the late 80s to early 90s, had a heat pump. It included an auxiliary electric heater for days that it got really cold. I don't remember it coming on all that often. (There was a light on the thermostat to indicate when it was on.) My current townhouse has a gas furnace. I plan to move soon into a single level home, more appropriate for an old greybeard, and I'll have to evaluate then if I want to switch to a heat pump, if the house doesn't have one already.

Cutting edge heat pumps can now work down into the sub zero (F) temperatures. Manufacturers have really been pushing hard on this in recent years. But these units still are more expensive than your run of the mill ones.

No, you need better insulation.

(And maybe a rocket stove combined with a heat storage hypocaust.)

https://solar.lowtechmagazine.com/2017/03/heat-storage-hypoc...

Depends on the heat pump, and the quality of your house. Hopefully houses in northern US states are of better quality than houses in California or West Virginia/South Ohio, so a furnface might not be needed.

Because for the scandinavians reading the thread and with the "It works in my country", US house build quality, in my experience, is even worse than UK houses build quality (and that's a pretty low bar).

They make ones that handle the cold quite well, but in my mind dual-fuel is where it's at, especially if you're doing a retrofit. Unfortunately there does seem to be a desire (even with the rebate systems) to kill the old furnaces.

The capabilities work fine for much of the country that doesn't get exceedingly freezing temperatures. It only gets down to about 26F on the coldest of cold nights here once or twice per year.