It occurs to me that it shouldn't take much power to operate the relays or electronics if a gas or oil heating system. That has me wondering about the practicality of putting that one device on a transfer switch so it could be run from generator or batteries.

How insane an I?

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    Gas or oil usually runs off a 15 amp circuit, so most small generators should power them well. Could go a bit smaller but a 2000w should be fine just for the heat, probably a light and radio/tv also.
    – crip659
    Commented Dec 22, 2022 at 19:57
  • Do you have a local storage of fossil fuels or do you rely on them being piped to you home?
    – gerrit
    Commented Dec 23, 2022 at 8:02
  • Do you live in a part of the world where power outages lasting more than 1 hour per year are likely to happen? I don't remember the last time I experienced a power outage, but not in the last 20 years.
    – gerrit
    Commented Dec 23, 2022 at 8:09
  • 3
    The electric power is needed for the circulation fan . Commented Dec 23, 2022 at 15:38
  • @gerrit I live in a wealthy town in the United States, and we've had maybe half a dozen power outages this year, two lasting most of a day. Recently we had a 24-hour outage of all internet providers as well as cell phone service. I see you are in Germany. These situations might be more common than you expect.
    – Reid
    Commented Dec 23, 2022 at 15:39

6 Answers 6


Completely not insane at all. However if you say that in the Rust Belt, you will be arrested for ThoughtCrime. They're not allowed to have truly electricity-free furnaces such as Williams or Empire wall or floor furnaces, which are common in the sunbelt. That's by tradition, not any sort of law. It might have something to do with the need to heat bathrooms to avoid pipe freeze (since someone might leave the door closed, they need to put a duct in there).

The load in a gas or oil heating system is the circulating fans or pumps. Temporarily jury-rig it to be cord-and-plug, and stick a Kill-a-Watt on it. You'll see.

Because they are presumed to be connected to grid power, absolutely nothing has been done to make them energy-efficient or battery-wise.

The Code issue is that furnaces are not allowed to be cord-and-plug, unless you can find an exception in NEC 400.7.

However you can certainly do a critical loads subpanel, and they also make kit that fits inside a 4x4 junction box that provides that for a single circuit, complete with generator inlet. Just don't snake extension cords through cracked doors or they may find your family dead from carbon monoxide poisoning. That happens a lot.

  • 1
    Can have it on a sub panel with an generator isolator and inlet should be code?
    – crip659
    Commented Dec 22, 2022 at 20:03
  • 4
    @crip659 yeah, that'll work. Critical loads subpanel. Commented Dec 22, 2022 at 20:04
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    Aren't those common in the sunbelt because they are inefficient, and you need a lot less heat in the sunbelt than in the rust belt?
    – KMJ
    Commented Dec 22, 2022 at 21:49
  • @KMJ Well, your average run-of-the-mill "bang-bang" forced air furnace ubiquitous in the snowbelt is 70-80%. Our wall furnace is 78% so it's at par, maybe a bit better since air circulation is free lol. It's hard to beat 80% if you're using convection to move exhaust gases. If you want to scratch in the dirt to buy yourself another 10-15%, that takes electricity and bunches of it, which then complicates running on battery. This is why coal and gas power plants can't blackstart. My feeling is if you'll be that dependent, go heat pump. Use a Williams furnace as emergency heat. Commented Dec 22, 2022 at 23:43
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    That just depresses me. 90% furnaces have been pretty straightforward tech for decades (add an induction fan) and the different in energy use over their lifetime is massive.
    – KMJ
    Commented Dec 23, 2022 at 4:08

It's not sane.

Because, for a little additional time and money you can get a LOT more. Like, twice the cost and 1.5 times the effort gets you 50 times the functionality.

Yes, you COULD set up an inlet, and transfer switch, and a small portable generator or even a battery generator just for your gas furnace and it would work very nicely. If that's your only criteria, it's sane.

The thing is, the time and effort and money to set up backup power is roughly in the order of:

  • A: A 1kW generator with extension cords run through a window to power your fridge and charge your phones: About $300, no effort to install, half an hour to fire up when needed. This has the danger of CO poisoning.
  • B: A 5kW to 10kW portable generator, inlet, manual transfer switch powering your gas furnace, refrigerators, sump pumps, some lights, a couple of TVs, one small window air conditioner, and lots of other things: About $1500 and a couple of days if you do the work yourself, or $3000 if you get someone to do it, and an hour to fire up when needed.
  • C: A most-of-house generator that starts and switches automatically and uses utility gas: About $20,000 to $50,000 depending on how big your house is and what you put on it.

What you are proposing is closer to (A) in functionality but somewhere between (A) and (B) in cost. In my opinion it's not entirely sane because you may as well do (B).

I note also that:

  • with (B) you are increasing value/saleability of your home. You can say "equipped for generator power" and your buyers will observe that. In the USA you can add the whole installation to the cost basis of your home.
  • with your furnace-only solution you cannot advertise "generator-ready" and your buyer's inspector will see a home-made transfer switch in a box mounted next to the furnace. Even if code-legal, there will be a neutral or negative mention of that in the report. Never mind your cost basis, you may have to dismantle it before selling!
  • 1
    And (B) is exactly what I did. Except the cost was a bit more because I needed a full panel replacement/heavy up because I had two old fuse boxes and one small breaker panel and needed to get it all replaced (and some other work done) in order to add the inlet, interlock, etc. And from what I've seen (C) is actually not that bad - more in the $10k to $20k range - but still a lot more than (B) (which is why I did (B)). Commented Dec 23, 2022 at 3:00
  • Hmmm. I am not fully convinced that I want to make that $10k+ investment right now. But I'm also not going to risk CO poisoning, and I honestly don't care about the fridge, just about the place being livable in. So it's either what you're calling halfway, or don't bother.... I'm not significantly worried, just wanting to understand the dynamics and options.
    – keshlam
    Commented Dec 23, 2022 at 5:24
  • D: good enough insulation such that it won't get cold indoors if power and heating are out for 12 hours? E: a big battery that can power the home for 12–24 hours, hopefully enough until power is back (if OP is not living in Ukraine)?
    – gerrit
    Commented Dec 23, 2022 at 8:03
  • 2
    @keshlam I hope it's clear, I'm not proposing you spend $10K+ ! I'm proposing you spend $1500 on a solution that will run most of the important things in your house including the furnace but much more, and take you 3 days effort altogether including research, shopping, and installation instead of spending $600 and 2 days on a similar solution that will only run your furnace. ... 1/2
    – jay613
    Commented Dec 23, 2022 at 14:41
  • 2
    ... 2/2 or if you won't do the work yourself, about equal amounts of effort, and $3000 vs $1500. Also thinking $$$ , with the $3000 solution when you sell your house it looks like and you can advertise "prepared for generator power". There's a nice transfer switch on the wall. With the furnace-only solution it looks like you have a science experiment and even if it's code-legal, a buyer's inspector will call attention to it, probably negative. I edited the answer to note this.
    – jay613
    Commented Dec 23, 2022 at 14:51

Not insane at all. When we had an extended power outage due to a windstorm a few years ago, I'd take a UPS to work with me, plug it in all day to charge, then take it back home to power our gas stove and gas on-demand water heater. It'd be the same concept with a generator. The electrical requirements of those devices were extremely small because it was just running the circuits to tell the gas burners what to do.

Your main issue with an oil or gas-fired furnace would be the airflow fan. However, those are usually sized so that the entire machine can plug into a 15A or 20A 120V circuit, so as long as your generator or power bank was decently sized you'd be fine.


Actually it is a good idea especially if you live in one of the colder climates where it goes sub freezing for a portion of winter. Expect more power failures in the near future, we are seeing them now. I did it the easy way,I used a UPS in my previous home. I used deep discharge batteries and mounted them external. You need to be sure you size it big enough, the heating system draws quite a bit of power when first coming on. Besure to include all of the motors at 150% (that is the number I used) or greater. It was totally automatic. The only maintenance was to check the batteries and top them off with water. I later put in a whole house generator and did not change anything. Now when the power goes off there is no delay the furnace keeps on going. My complete heating system was on one 15A circuit making it easy.


With an automatic transfer switch, commonly used with a "whole house" generator, you have to make sure it can handle everything being switched, all at once, every time. That gets a bit tricky unless you have either automatic load shed devices, large electric loads that simply default to off (e.g., oven, clothes dryer) or have relatively little electric stuff or use a critical loads subpanel. Or a really big generator - which many of neighbors got after a huge number of outages several years ago - as much as I'd like to, I can't justify spending the money on it.

However, at the lower end, you are generally in manual transfer territory. Which means when the power goes out, nothing happens automatically. You manually:

  • Turn off all breakers
  • Move the interlock
  • Turn on the generator inlet breaker
  • Plug in and turn on the generator
  • Turn on critical loads

The bad part of a manual system is that it doesn't happen automatically. But you can save a lot of money, provided you use a smaller generator. The key is to determine which loads are critical (always turn them on), which are optional (turn them on when you need them but watch total usage) and which are don't you dare turn these on when the generator is running. Label them all on your breaker panel so you don't have to think about it when the power goes out.

With that setup and either a big set of batteries (but doesn't need to be Tesla Powerwall sized) or a decent size generator (anywhere from 2,000W to 8,000W depending on your needs), you can typically power:

  • Forced air gas or oil furnace/air handler
  • Lights in many rooms
  • Telephone/internet equipment
  • Computers (especially laptops)
  • Refrigerators/freezers

For the typical home, those are the critical loads in the winter - stay warm, see what you're doing, surf the web (or watch videos to pass the time, whatever), and keep your food safe. If you have a gas cooktop you can even cook.

After that come other loads. With my setup (around 7,000W) I should be able to run the microwave on other appliances selectively - turning on those breakers when I need them. Can't run the air conditioner (well, maybe if I don't use much else) or oven or clothes dryer.

This is also a reason to keep a natural gas furnace even if you switch your primary heating to a heat pump.

  • 1
    All makes sense. Lights would be a lot less load than in the part, given LED bulbs. That almost made me question upgrading to 200A service...
    – keshlam
    Commented Dec 22, 2022 at 21:36
  • 2
    Must have a way to watch cat videos on youtube.
    – crip659
    Commented Dec 22, 2022 at 23:31

I've pretty much done that:

I have solar panels and a hybrid inverter with batteries. It has a backup output to power critical loads in case of a blackout.

Compared to a generator, solar is silent and will also shrink your utility bills. So unlike a generator that you use only in a blackout, you use it all the time. However, it is intermittent and dependent on weather: if you want a guaranteed output to run your critical loads, then you must install many more solar panels than you actually need. Even then it is not guaranteed: if the panels are covered in snow there is no power, if you get a week of really bad weather, you can expect a 10kWp installation to produce 1-2 kWh per day which is quite useless. So the viability of the solar option as a backup really depends on your climate and should be studied carefully before writing a big check.

Right now the critical loads are: two freezers, plus the heating circulators and associated electronics.

The inverter will automatically power critical loads, but the heating changeover is manual. Normally I use a heat pump, but that is not usable in a blackout situation. I also have an old wood furnace that was already there when we bought the house, so we left it in. In case of blackout, I simply turn off the heat pump to prevent it from restarting automatically when the grid comes up, then load the wood furnace and fire it up. The electronics that control the circulators and underfloor hydronic heating then work on power from the inverter, hot water from the wood furnace goes into the underfloor heating, and I still get heat.

If you intend to do that, you should measure the total power consumption of your critical devices: freezer, heater including fans and circulator, and add some margin. The simplest way is to put each device in turn on a wattmeter socket and check the maximum reading. For example a freezer will, once in a while, use a lot more power to defrost so if you just look at the wattmeter for a minute you'll never know. The inverter should be able to provide the maximum power, otherwise it'll shut down. For motors, make sure to measure the startup current and check if it is much higher than the running current.

Then check your loads: most likely you will have some AC asynchronous motors (circulator pumps, fans, freezer, etc) and these run much better on good sinewave power. Newer inverter-based freezers and circulator pumps should be much less picky.

This is important because if you get a cheapo generator that outputs dirty power that only barely looks like a sinewave, your asynchronous motors will make strange noises or perhaps work terribly or not at all. Electronics may also crash if the generator has poor regulation.

So you should definitely test everything. If you use an inverter, it should be pure sine wave, because asynchronous motors strongly dislike square wave current from "modified sinewave" inverters.

You could also use an inverter generator.

However the power use from all your critical loads is likely to be much lower than the generator output, which means it'll waste a lot of gas to just keep running. This might be OK but it's also very noisy. So you could use a generator to charge inverter batteries, or one of these big Lithium power banks, and then use that to power your heating while not running the generator all the time.

In case of a large blackout, it is not guaranteed you will get gas. If you have a large heating oil tank this is not a problem.

Gasoline generators are annoying, because gasoline is annoying to store. It evaporates, when it's old it no longer works, and basically a gasoline generator will probably not work when you need it unless you take the time to do proper maintenance, rotating gasoline stocks so it's always fresh, etc. That's a problem with emergency equipment that rarely gets used: you forget about it and then when you need it, you find it's quite dead. So before buying you should look into that. Propane generators do not suffer from these issues, and I hear diesel is also more convenient.

To make sure everything will run properly in case of a blackout, I ran the critical loads on the solar backup for a week, it's much easier to debug the system when there's no emergency.

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