[quickthrowman]

Furnaces don’t come with batteries because a 12v 100Ah battery that costs $180 will run a 1/3rd HP (700w) fan for 2 hours. To sustain the fan for 3 days, you’d need 36 batteries. Lead-acid batteries have a limited lifespan. You would also need a transfer switch and enclosures and at that point you may as well buy a UPS or a natural gas generator to provide backup power.

One thing the NEC 2020 update allowed is bi-directional power between an EV and its charger, you can set up an EV to provide backup power your house, with a transfer switch and everything.

Radiant heating systems with boilers require electricity for the pump, even if they’re gas fired. Electric resistive heating needs electricity. Forced air heating needs electricity to run the fan. All heating systems require electric power, even if the heat is generated by burning natural gas.

[8note]

The real question is why gas furnaces don't also have their own generators built in

[quickthrowman]

I wasn’t aware, but these actually exist: https://en.m.wikipedia.org/wiki/Micro_combined_heat_and_powe...

[monadic3]

Why would you use car batteries? Seems like a terrible fit for the job of daily charge and discharge.

[Dylan16807]

What? A backup battery for a furnace doesn't do anything daily.