Which where most people live here averages to less then 1 day per year so not a big deal.
Way bigger issue here is some fall storm destroying the power lines and being without electricity for multiple days when it already is cold enough they one needs heating.
However, in some really cold places like Chicago, Minneapolis, etc - the days where current heat pumps are inefficient might be enough to make it cheaper to always run gas.
OP is proposing to have both systems and only run the gas furnace on extreme days - which would lead to a ~30% reduction in running costs.
I suspect the CapEx of having two heaters wouldn't make sense, though.
It'd be better to just have a hear pump that can run efficiently at colder temps.
Depends on how your utility charges, but I feel like many/most sell you the gas roughly “at cost” and then some fixed monthly delivery/connection charge that covers their regulated rate of return on their network equity.
Oil or propane supplemental heat might be cheaper than Natgas supplemental heat. My money is on wood pellet stoves for supp heat for those that don’t require full automation. Or maybe even dirty coal…
And that cost will only go up as people cut the gas cord.
Depends on how your utility bills out it’s infrastructure: some charge minimal monthly connection fees, others a lot.
I honestly wouldn’t want to own a residential-focussed gas distribution company unless someone revolutionizes stirling engines or micro cogen systems and people start cutting their electric cord.
So even if per unit of energy gas is way cheaper, heat pumps can still come out ahead.
It takes 7kw at 17ºF to provide 48k BTU. There are 0.293W/hr per BTU so 7,000W = 25k BTU "in" and 48k BTU "out" or a COP of 1.9. At -15ºF, it's using 5.9KW but can only produce 28,500. So a COP of ~1.4. Still better than electric resistive heating but not by much. At 40ºF, the COP is more like 3.3 which is in line with the very efficient numbers.
Fortunately my 99% design temp is ~8ºF and my area only sees 20hrs below zero per year, so this will work just fine for me.
Heat pumps might make sense when you have a well-insulated house, like the kinds in Northern Europe, to trap heat and reduce the power required to heat the room. American houses in most places are far too drafty.
If you need significant heating or cooling in a home, the first thing you need is to deal with is insulation, otherwise nothing you do is going to work well.
The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.
The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature, and you will need to update the heating system as traditional methods of heating (fireplace, stove, simple furnaces) depend on external air circulation.
> The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.
The WHO recommends a general minimum household temperature of 64°F for all populations for health reasons, with a higher minimum for sensitive groups including children and the elderly; maximums are more regionally variant because acclimitization (which takes years) plays more of a factor in high-heat health risks, but even in extreme regions seems to top out at about 90°F for the general population.
Lots of the US spends lots of time significantly out of at least one end of that range (and, especially given that the top gets lower in colder regions, lots of the US spends lots of time significantly out on both ends of the scale.)
> The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature
If you have a house that is insulated well and also lacks doors and windows that can be opened, sure.
But... that presents other problems, too.
I'm not sure if such well made devices are still available for purchase however.
Rural areas rely on propane delivery, which is roughly 2x the cost of the piped natural gas (per unit of energy).
At lower temperatures the modern heat pumps cannot attain this COP, which is why they are working on heat pumps that work at lower temperatures.
The reason why natural gas is about 1/3 of the cost of electriciy is because most natural gas power plants run at thermal efficiency of about 30%.
If everyone is suddenly using electric heat when it’s -20 in an area there might be load issues.
This is not to say we shouldn't be concerned, but denying yourself the most energy efficient technologies available (EVs, heat pumps, etc) because you're afraid of power outages 5-10 years from now seems like overkill.
So you can't just take a decades old system with oil/gas using finned radiators, just replace the boiler, and have it supply enough heat on the coldest day ("design day"). Rather you'd at least need to add some additional emitters, greatly increasing the scope of the project for a professional installer.
What I haven't been able to find an answer to is that everybody says hydronic heat pumps need low delta T of 5-10 degree F (implying high flow rate for given heat transfer). But I would think the real constraint would be just on their leaving water temperature, and a heat pump (load side) that took in 100F and put out 120F (at say 5GPM) would be happier and more efficient than one that took in 110F and put out 120F (at 10GPM). But I've yet to find anything that confirms this.
I don’t see why this would matter at all. Maybe the heat exchanger would need to be sized differently for a different flow rate, but in general a lower entering water temperature on the hot side seems preferable.
That's why designers are often specifying lower delta-Ts for low-temperature emitters: to allow the flow temperatures to remain as low as possible [for efficiency] at a given average water temperature [for effective heating].
Maybe it never comes up in practice because heat pump systems inevitably need some kind of buffer tank. If you're designing a system from scratch then you design for lower delta-T in the emitters to keep the max water temperature down. And if you're using existing emitters then you just live with the inefficiency due to higher max water temp, but still keep the flow from the heat pump to buffer tank high regardless (to keep the max water temp from being even higher).