I have a steam boiler (natural gas fired) and I'm wondering if I can safely use a small UPS to backfeed power to the boiler, in case of a power outage.

I'm not sure how much total run time I'd get - we're talking a 1500VA UPS with conditioned (sine wave) power output. However, the boiler only has an automated damper, the control circuitry, and the pilot ignition system - no fans, no pumps, no power vents - so the draw should be very low and not even constant.

I'd like to find out/test, in any case, how much run time I'd get with this setup (for emergency purposes).

The boiler is fed from a non-dedicated 15A breaker in the main panel. The feed goes into a J-box containing a dual socket receptacle and a switch. I have confirmed the switch cuts power to the boiler, when thrown. Therefore, the switch acts as a kill switch to the boiler.

Therefore, I concluded that if I were to throw the kill switch, throw the breaker for good measure, and then back-feed the UPS into the receptacle, the risk of back-feeding into the grid would be nearly zero. And it's temporary, no modification of existing wiring is required.

I know that folks can't really say this is safe, I know it's against code.

Are there any considerations that I haven't listed here?

  • Use an ampprobe on the 15 amp circuit to determine current draw. Then use that info to determine UPS runtime. – Tyson Feb 13 '17 at 3:22
  • Don't have one, unfortunately. it's likely not a consistent load as well. – Jeremy Feb 13 '17 at 3:24
  • Not sure why the load would be inconsistent. You can ball-park feasibility pretty fast tho just with guesses on the load. – Tyson Feb 13 '17 at 3:27
  • @Tyson the boiler runs a servo motor to open and close the flue, probably a solenoid to control gas flow, and an ignition mechanism. But it's only using those during initial start up, I guess the solenoid has some current while the boiler is heating. Most of the time, I suspect it uses very little if any current. Since it's hardwired I can't plug a kill-a-watt inline, unfortunately. – Jeremy Feb 13 '17 at 3:41

When temporarily testing things...

Just take box covers off and wire-nut stuff.

My cheater is male on one end, bare wires on the other. I just go into the box, wire-nut up the load under test, plug the cheater into the Kill-a-Watt or whatever, and do my testing. Easy peasy.

I test hardwired loads that way all the time, it's no big.

Don't bottom your UPS

Use the Kill-a-Watt. Tempting though it may be, don't run a UPS down to dead, except to test the battery. It has a lead-acid battery, every time you run them dead takes a big chunk out of their life. Lithiums don't have that problem.

I agree it's likely the furnace doesn't take much power.

You might also have the conversation with your furnace manufacturer as to whether it can be run on 12V or 24V battery power directly. It's quite possible that most or all of it runs on 24VAC and doesn't care AC vs DC.

  • Yeah -- a male-to-bare wires cheater with nuts is the way to go here (vs male-to-male plug abominations) – ThreePhaseEel Feb 13 '17 at 12:38

You'll need a proper transfer device

You'll need some way of properly transferring power between AC and the inverted battery output to do this. "Remembering to turn off a breaker" doesn't count; while you can get mechanical interlock kits that make it so only one of the two interlocked breakers can ever be on, they aren't suitable for your application.

Furthermore, backfeeding into an outlet is dangerous because of the exposed live pins on the male connector -- there's a reason receptacle outlets are universally female and device plugs are universally male, with the pin gender reversed for inlets on equipment or wiring and their mating plugs.

A better solution would be to make the steam boiler a cord-and-plug connected piece of equipment. That way, you can unplug it from the mains and plug it into the UPS. I'd simply use the beefiest, heaviest-duty piece of cordage (such as 16/3 SOOW or SJOOW) I could get my hands on along with a nice, screw-assembly type plug on it and a strain relief gland or fitting at where it exits the boiler.

At that point, you could even go as far as just having the thing on a UPS full-time if you wished.

  • I appreciate the answer. I'm aware that a transfer device is the right way to do it, and the risks of back feeding. Converting the boiler to a plug is a great idea. But I'd only do it if the experiment is successful in that I get a meaningful run time from the UPS. So for the purposes of testing run time, is there anything else I should be aware of? If the solution works, I'll come up with a way of doing a proper transfer. – Jeremy Feb 13 '17 at 3:16
  • @Jeremy -- if you can uncouple the boiler wiring at some point and temporarily fit cord-and-plug there, that'd be the way to do it. – ThreePhaseEel Feb 13 '17 at 3:19
  • I could open the J box and uncouple the switch from the receptacle, then back feed into the receptacle (which is now only attached to the boiler via hard wire). I know that's not exactly what you said but aside from dealing with a male to male plug (which I can plug into the de-energized boiler first), is there any functional difference? – Jeremy Feb 13 '17 at 3:23
  • 3
    @Jeremy -- for a temporary test, I would uncouple the switch + line neutral from the receptacle, nut off the loose switch ends, and then attach the length o' cordage to the receptacle's open screws. It's ugly, but it's the best I can come up with that doesn't involve bad, bad things like male to male suicide cables – ThreePhaseEel Feb 13 '17 at 3:27
  • makes sense, thank you. I'm honestly not worried about the male to male cord though. The UPS has an on/off switch, so I can make a connection between two de-energized female ports (and verify they are de-energized with a multimeter first, as well). Still I understand the need to be cautious. – Jeremy Feb 13 '17 at 3:45

You shouldn't have any issue. I think you can 'cord-wire' your boiler replacing existing switch with a fixed outlet and putting a plug on boiler cord. So if needed youcan just unplug your boiler from the mains and run it from UPS.

A third, to me better, option is to hardwire the UPS before the boiler so in case of outage it automatically kiks in keeping boiler running, setup&forget.


If the output of this 1500VA UPS is 120volts A/C hook it up and see if it runs your boiler and for how long. Just make sure you hook it up as the other guys suggest to keep yourself safe. This would give you an indication of the feasibility of your idea. If you live in an area with numerous power outages and want an alternative that allows you to fire your boiler with no outside power, a Powerpile or pilot generator system could be installed. The pilot would power the system except for the flue damper, which you have to open manually; (not cheap but possible)


From someone who's actually done it, the answer is YES it can be done. My setup was to use a car jump-start battery pack that had an AC receptacle. I wire-nutted an AC cord to the boiler shutoff box, and it ran the boiler for days. If we have another power failure I would approach it differently though: I will wire a step-up convertor (12 to 24VDC) directly to the transformer on the outside of the boiler. The power conversion will be much more efficient. (not having to convert from 12VDC to 120VAC inside the battery, and then the boiler transformer convert to 24VDC!) Then I can use my other battery pack that does not have the valuable AC outlet, or my car battery if I want. That way I can use the AC-outlet battery pack for other things.

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