A couple of years ago I had a 'failure mode' problem with my heating system. A winter storm knocked out power in my area. My heating system was a natural-gas-fired boiler that produced steam, which thanks to gravity would rise up through my radiators, condense to water, and flow back to the boiler. The power outage didn't stop the gas, so the only thing that kept my furnace from running was the electric thermostat that sent on/off signals to the electric device that controlled the gas valve and sparked an ignitor.

The thermostat itself had a battery backup, so it was trying to turn the heat on as my house cooled off. But the controller and ignitor had no backup. I couldn't even use a match to light the thing manually, because there was no way to open the gas valve.

I understand the safety implications of adding a manual valve and spark ignitor to a furnace, but letting my pipes freeze (and me too) also has safety implications. Besides, there's probably already a thermo-mechanical safety mechanism that would slam the valve shut if the furnace gets too hot.

Luckily, the power came back on in a few days, before my house got colder than light-jacket conditions. No frozen pipes.

Unfortunately the ones that were immune to this employed Mercury switches, which I don't believe are sold any more. We had an older gas heating stove which had this and worked even when the electricity was out (particularly good in the winter when heating with electricity). Eventually that part failed and being an older model, that part was discontinued by the manufacturer. This forced us to get a "new" stove which now uses an electrical system to control it, and which doesn't have a battery backup integrated. Technically one could add a UPS, but since it also uses an electrical fan it would probably empty that ups within a few hours.

I have an electric heatpump for heating and hot water. The first thing I did when moving in to the house was install a reasonably efficient, manual wood burning stove, with 2 m3 of wood in the basement. I have had the heat pump fail in the middle of the winter, but the wood burning avoided any freezing disaster.

Of course, that assumes you're at home. I added a wood stove (in addition to a fireplace that I already had) partly as a backup. But I also travel a fair bit.

I don't know if these are available for boilers, but many natural-gas-fired water heater controllers are powered by a peltier device, which is heated by a small pilot flame.

Any reason you didn't have a backup generator? I would think if you were at risk of frozen pipes without electricity, it would be a wise investment.

And so a problem caused by having a more complicated than necessary system is mitigated by adding in another very complex system for redundancy.

Now this can work, indeed as a engineer in SaaS, it's my job to make it work. But see how to make it work you have to hire engineers to nurse the all those systems along.

That's the situation for pretty much everyone in northern states. A generator that's wired in with propane tanks is a pretty significant expense and I'm still not sure how reliable it all is if no one is at home.

If the control line for your furnace is just a 24V loop (or a dry loop), a good hint is that you can keep a traditional thermostat in parallel with your 'smart" one - so that if either closes the loop, the furnace fires.

Then you can aim for "comfortable" with your "smart" one and "safe" with your real one.

That's probably true. And should also be part of the "smart" thermostat's design.

A nest failure in brutal New Orleans heat while I am out for a few days might spell death for one of my cats. I don't feel like Google takes their products seriously enough to trust them with the life of my pets.

This is a good point, but if I had pets, I don't think I would trust any air condition system at all to keep them alive for a few days.

But then I don't really know what it's like to own pets, and I don't know how housing in New Orleans works either.