The majority of the energy use in a natural gas furnace is still the core heating of air functionality. The electric ignition and fan aren’t most of the energy to my knowledge, although “decent chunk” is a fuzzy term.

Obviously the blower and controller don’t consume anywhere near the amount of energy that gas delivers, but it consumes more than most people think. The blower on a typical furnace consumes 75-400W per hour of operation, depending on whether or not it’s a single speed or variable speed type.

400 watts per hour isn’t a huge amount of power, a space heater consumes about 1,300 watts per hour, but it’s still a pretty decent chunk of electricity. A deep discharge marine battery (common for cheap backup power) could keep a larger blower running for maybe 3 hours tops, assuming an unreasonable 100% discharge and 100% conversion efficiency. So, not a lot, but still something that you’d need some pretty specialized backup gear to run.

I have an oil furnace, and when I was looking into it it sounded like setting up a battery backup that could allow me to keep the fans running through a power outage like this would probably be more expensive than replacing the oil furnace with an electric one.

If you have to use a battery, then that's probably true. But then again, once you have an electric furnace, the battery requirements are going to go up far more dramatically.

As an anecdote, my furnace blower pulls about 11 amps on 120V AC. So I can run it with a little 2000W inverter generator but won't have much spare capacity for anything else.

Natural gas fireplaces don't need electricity to work, though, and they are fairly common in some parts (maybe not Texas, but Ontario). But they're not very efficient, either.

Some do, some don’t. Depends on how they’re designed to handle ventilation and heat distribution. Quite a few that I’ve seen are fully sealed and can’t be started without power. A lot of these are really for looks and not heat.

For backup heat generation nothing beats an EPA certified wood burning stove, especially the free standing ones.

I was surprised to learn when I moved to a cold climate that some gas water heaters can operate without electricity. Modern ones use a small piezoelectric igniter to light a pilot light.

Probably obvious in retrospect, but took a bit of brain rewiring to realize no-power != no-hot-water given I had mostly dealt with areas with electric water heaters with fairly small tanks up till then.

I’ve been wrapping my head around it too as that’s how our hot water heater works even though the gas-using furnace doesn’t work in a blackout.

I understand your surprise, but the difference is understandable when you think about how each system needs to operate. It's a matter of thermal mass, location, and complexity.

Both a hot water and a forced air furnace work by burning natural gas to heat something up until it's hot enough. In both cases the combustion is separated from whatever its supposed to heat by a chunk of steel, and the gas is simply burned in open air without any extra compression. Ventilation is usually done via convection, which works without power. So far, so good; both cases just need a small amount of power to open a valve and kick off combustion. The similarities largely end there though.

In the case of a hot water heater, the flame is heating a big slab of metal under 40+ gallons of water. It's very hard for this slab to overheat and get damaged, as water is both thermally dense and a great conductor. There's also no need to move the heat around; the flame is literally right underneath the thing that needs to be heated. This proximity also makes the control logic extremely simple; you turn on the flame when the tank is too cold and turn it back off when it isn't. This consumes a tiny amount of power, and you can use the heat generated by the pilot light to keep such a circuit alive.

A furnace on the other hand has an issue. While burning gas in a furnace is easy, getting that heat back up to where you need it isn't. Air is a pretty terrible thermal conductor, and without some cooling that combustion chamber is going to overheat and get damaged. You need to move a lot of air past the combustion chamber to get the heat away before that happens, which means you need a powerful fan to get that air moving. These fans consume a lot more power than you can reasonably generate locally, about 75-400W per hour, with an extra spike needed to get the fan moving in the first place. You also have the issue that it's not possible to measure the outcome of the furnace at the furnace; which means you're going to need relatively complex remote sensing and control devices, which only adds to the system's total power needs.

Yes, they do need need electricity. Source: I live in Ontario, Canada, and am currently being kept warm by one.

Most (gas) furnaces are forced air, and how exactly would the fan run without electricity? How do you think the thermostat runs its thermistor (and probably LCD display) without a power?

The Technology Connections YT channel has a recent good overview, "Forced-air Furnaces: The What, Why, and How":

* https://www.youtube.com/watch?v=lBVvnDfW2Xo

As for efficiency, you can get units that are 99% efficient (AFUE):

* https://www.lennox.com/products/heating-cooling/furnaces/slp...

* https://en.wikipedia.org/wiki/Annual_fuel_utilization_effici...

I have two gas fireplaces, neither use power (nor have any available). Valves and safety is handled via millivolt thermopiles in the pilot light and main flame. Heating happens via convection through ducts that run from below to around/behind the fireplace itself, then out over the top. It passively moves a fair amount of air and can make the room uncomfortably warm if left on too long.

I have no idea if this is common or not, but I know of at least two counterexamples to your statement here in Michigan.

> Yes, they do need need electricity.

Some do, some don't. Mine has a pilot light and a thermocouple. I used it to keep warm while our power was out for three days this past weekend.

I wouldn't be surprised if pilot lights aren't illegal now. They're terribly inefficient, more so if your furnace rarely lights.

I think pilot lights are still pretty much standard on certain appliances, like water heaters and fireplaces. Probably deliberately so they can run without power. All my other gas appliances are electronic ignition, e.g. we have to use a lighter to start the range when the power is out.

In the case of gas fireplaces, all of the heat generated is used, so I would think it would be perfectly efficient. As long as you shut it off during the warm seasons, anyway.

What's the make/model? Presumably not forced air?

The house I live in has two gas fireplaces, both 20-30 years old. They work in the same manner described above, with no requirement for electricity. Each one also has a blower that is simply plugged into the wall (one runs continuously, the other is thermostatically controlled; both are very loud). The blowers are optional, and the house gets adequately heated even when they are off.

Not sure about make & model, I'd have to find the paperwork from when the house was built 8 years ago. It's radiant, not forced air. That's an option I could add but it would require electricity for that part.

GP said fireplace, you're talking about furnaces. Plenty of fireplaces don't need power, but I've never seen a forced air furnace without power, and I don't think it's possible.

Gas fireplaces generally do not require electricity and many have no fans. Exhaust is passive, and thermostats use a thermopile that generates a small current from the heat of the pilot light. The pilot light is started manually with a piezoelectric starter, or a lighter.

Gas fireplaces are slightly less efficient than high-efficiency furnaces because some heat goes out the exhaust.