[looofooo0]

As said, at some point you will likely replace your furnace by a heat pump. Then all investment to low temperatures will pay off. A modern gas furnace is no backup for a heat pump, as it also needs electricity. I would recommend a low tech solution like a wood stove for backup. I have one in my cellar. In case of a long power outage I would likely just install this through some open window and do some redneck stuff to make it airtight.

[sokoloff]

Backup doesn't necessarily mean "in case of grid failure". A gas boiler or a simple electric boiler is a valid backup to a heat pump being inoperative, in the middle of defrost cycle, or otherwise just unable to provide enough BTU/hr into the building due to an extreme cold temperature.

Particularly in temperatures where the heat pump is at/under 2.0 COP for a few dozen hours per year, an electric boiler makes a great deal of sense. It's almost as clean as the heat pump in the extremely rare times when it's needed for supplementation due to extreme cold temps, can allow for a more suitably sized heat pump to cover the 99% of heating hours [meaning greater efficiency there], is extremely reliable due to its simplicity, is fully automatable with existing controls, doesn't require local gas supply network, and is a pretty good match for a heat pump.

[looofooo0]

Most heatpump do have an eletric boiler included. At least in Germany I can also plugin multiple 2kw electric heater, where one cost 15€.

[Retric]

Trusting in the power grid for both is obviously not redundant, but it’s cheaper to get independent power for a furnace than heat pumps. Actual redundancy means you want completely independent systems so you shouldn’t then use the same heat delivery loops/pumps/fans either.

Wood stoves require you to physically be home and are kind of a pain to use thus make a poor backup on their own, though a solid 2nd backup.

PS: Most areas are better off with solar thermal + heat pumps. Solar thermal however requires sunlight which not everyone gets in the depth of winter.

[looofooo0]

I dont see the point in making an emergency option convienient. Forget solar thermal, this is to much hussle for anything. PV plus battery more likly the solution you want.

[Retric]

Less about convenient than being to go on vacation in the winter.

Work out the amount of solar + batteries you need in the winter when the power goes out and your COP is less than 2 because it’s very cold and you need more BTU’s than normal because it’s very cold. Solar thermal is just significantly cheaper on a large home in a cold climate.

[looofooo0]

No, this does not work. Solar thermal will not work without power. Any battery solution monitoring your home temperatures are enough. Then you can travel home or ask your neighbor.