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wall furnaces can lose their pilot light and then you are still without heat

How much redundancy is enough?

Neighbors, friends or property managers you can at least stay in contact with.

I have only seen couplings

@arcticman said "Pretty sure the vented heaters and electric heaters cannot operate automatically if the power goes out even for a second. When power is restored they need to be manually activated.

Our power goes out often… while we're away if there's even a brief outage they would become useless as a backup."

The Williamson COZY heaters that are not connected to the house's electrical system will not have any problem with the electrical system that it is NOT CONNECTED TO. Well that is my experience anyway.

This model Floor Furnace will keep a good size home at 50° even at very low outdoor temperatures. It involves cutting a pretty big hole in the floor, but you can cover it with a carpet when you are home as long as you turn off the gas. When you want it as a backup, light the pilot, remove the carpet and set the thermostat to 50°

You can get ones that don't require any power, very common for off grid folks. There are also plenty of propane fireplaces that will run off a couple of AA batteries.

A wall furnace that operates on LP Gas that has a 750 MV Pilot generator needs no electrical connection. Cozy Wall Furnace This model is one of that type of furnace. Set the thermostat to 50° and set the oil fired boiler to to 55°. If the oil burner ever fails to light, the Cozy will take over. They have several models that can be installed in several different configurations.

Pretty much what @EdTheHeaterMan said… with an addition. There are other things which can go amiss with an unattended house in the winter. Such as damage to a window. A prolonged power outage (your fancy thermostat won't tell you about that). And so on. I highly recommend hiring someone reliable and local — NOTE HIRING someone — who is tasked with — and paid for — going by the place at a mutually agreed interval (perhaps not daily, but at least weekly and after any major storm) and check on it. They would have a key — and not just drive by, but actually go in and poke around…

You could also install a couple wall furnaces or console heaters or even a floor furnace with millivolt systems that is big enough to keep the place above freezing if the boiler fails.

what needs to be reset? most of the controls that lock out try a couple times before locking out, if they get to the point that they lock out, something isn't right.