How To Wire Your Gas Furnace To A Generator

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Contributed by Guest Blogger: Rick Brodersen
of Rick’s DIY (HVAC & Electrical DIY)

Wiring your gas furnace to a generator is a project that’s frequently either over-thought or not thought of at all. Some people think you need a high dollar transfer switch wired to your breaker panel, requiring an electrician, or you have to temporarily rewire the breaker to the generator and then rewire everything again once the power comes back on.

But this isn’t the case, NEC (National Electric Code) only requires that appliances be on their own dedicated circuit. So, by simply rewiring your furnace you have the ability to use your furnace with a portable generator (or other auxiliary power). Then when the power goes out, just disconnect the furnace from the outlet and connect it to your generator, and use it like you normally would.

video: How To Wire Your Gas Furnace To A Generator

Materials:
The cost of the materials was less than $20, and all of the items can be found at most hardware stores.

  • Outlet Box
  • Single Outlet Receptacle
  • Outlet Cover Plate
  • Power Supply Cord
  • Cable Connector
  • Misc Wire Nuts, Electrical Tape & Mounting Screws

Tools:

  • Hammer
  • Screwdriver (multi-tip)
  • Side Cutters
  • Wire Strippers
  • Multi-Meter
  • Voltage Stick (optional)
  • Drill (probably a little overkill but if you’re installing the outlet box onto your furnace with sheet metal screws it will be needed)

How To Do It:
Everytime you’re working around electricity you MUST check that the power is OFF and then CHECK AGAIN. You can never be too careful when it comes to this point. (Even after all these years I occasionally still get shocked.)

Turn Off the breaker that controls your furnace. (Keep in mind that a many breaker panels are mislabeled.) Open your furnace and find where your power is coming in from and follow it to the junction box (or in my case, the circuit board.)

Pull off any covers and then test for voltage. I like to use the Fluke Voltage Stick first (ensure it works on a live circuit and then you can test it). After you have verified the power is off, carefully remove the wire nuts and test with a voltmeter. Once you are darn sure there is no power then you are safe to work on it.

Write down, or label your wires, the BLACK (hot), WHITE (neutral) & GREEN (ground). (Some might be red, orange, brown, etc.) You must rewire it exactly as it was: Black to Black, White to White, and Green to Green/Ground. Pull the wires out to the outside of the furnace.

First, install the OUTLET BOX. I put the conduit with the wires coming from the breaker into the outlet box and then level it where I would like to mount it. Screw it to the wall or furnace with the appropriate screws.

Next, you need to wire in your Single Receptacle Outlet. You may need to cut back your wires and strip them. (This is hard to explain in text and is better understood in the video.) Wire your Black (or hot) to the smaller bladed side of the outlet (if you’re facing the outlet this will be the right side) this screw is normally brass/gold in color. The White (or neutral) wires up to the larger bladed side, this screw is usually silver in color. Then the Green (or ground) goes to the last terminal that is bonded with the metal framework of the receptacle, sometimes this screw will be green colored (but every manufacturer is different).

Once wired, carefully push the receptacle back into the outlet box and screw it in, put on the COVER PLATE and we’re ready to move onto the final part.

To install the POWER CORD we have to add in the CABLE CONNECTOR to where the old power cable came into the furnace. Once you have the cable connector in, you can pull the power cord through and secure it.

Next, rewire the power cord to the connections on the furnace. If you’re using wire nuts be sure to Double Check that the wires are secured by tugging on each individually and then taping the connection with electrical tape (you can further secure with tie straps if needed).

Make sure to replace any cover plates, double-check that everything is wired properly and secured. Turn the power back on and check the outlet with your meter (this is not shown in the video) to make sure everything is wired properly. Finally, plug in your furnace and check operations.

video: An Overview: How To Wire Your Gas Furnace To A Generator

ALL DONE
Now that you’re done, test it out with or without your generator. I recommend a test run on a dark, rainy night so you know what it’s going to be like when you’re actually going to have to use it.

Bio
Rick Brodersen is an HVAC/R Mechanic and has worked in the Seattle area for over 15 years, specializing in the commercial service industry. He has instructed at local trade schools and now continues teaching at RicksDIY.com. He is also an avid prepper, hiker, and fisherman who, along with his family, enjoys taking in all the Pacific Northwest has to offer.

Read Rick’s Disclaimer

NOTE: Comments have been closed for this post. Please feel free to read the many, many replies Rick has given, if you still need his help he has asked you contact him directly through his site at RicksDIY.com. 

(Wednesday: Book Review: The Doom and Bloom Survival Medicine Handbook)

119 thoughts on “How To Wire Your Gas Furnace To A Generator”

  1. One thing I would recommend is instead of a straight prong outlet/plug use the Twistlock version. This locks in the plug into the outlet, preventing the plug from falling out over time or something tripping over it. Of course the downside is that you will then need to also make a cable that converts the Twistlock to a straight prong, but the advantage is that no one will take that cord as it is only usable for your setup.

    • Yes, the twist lock is also recommended. There are many variations on this setup, Including the twist lock but as you said then you have to buy the converter plug and remember where it is… I will eventually post a follow up video with some of the variations on this, along with the furnace running on a generator.

  2. Very well written article. This post explain the steps of connect gas furnace to the generator very clearly. I am sure this information will definitely helps the people. Thanks for sharing this.

    • Thanks very much. I’ll definitely pass this on to Rick – he did a great job with it.

  4. Enjoyed your video, it had great detail and was easy to follow. The added tips were of value also. I’m wondering, will a high efficiency furnace with it’s electronic control board work with a generator? Was the furnace you worked on one with a control board? I’m thinking about the electricity put out by a generator compared to that of the power company. Thanks for your help.

  5. Yes The furnace on the video did have a control board. Just a little background; The control board is powered by low voltage as most electronics are, it gets it’s power via a transformer that converts the 120 Volts A/C to most likely 5-12 Volts D/C. Because of the transformer it helps condition the power. This makes it safe to use most any generator on the furnace.

    The main concern is that you don’t want to shut off the generator or run out of gas while the furnace is running this may cause a severe drop in voltage and cause the fuse or breaker to open on the furnace. But other than that it won’t damage anything, you would just have to replace the fuse or reset the breaker. I will have to do a video to show this as many people have concerns about that.

    • Thanks for your prompt reply. I always thought you needed an expensive generator with inverter to run furnaces with control boards. Does generator need to be physically grounded, as with rod and wire? I have a Briggs and Stratton generator and a Coleman Powermate with a Subaru ohc engine, and neither one mentions using a ground rod. Since they are portable generators is it OK to use them as is to power my furnace without a ground rod?

    • Just tried to run my high efficiency York furnace with a Honda 4000watt portable generator. Furnace would power on for about a minute (with blower running) and then power off. I grounded generator to the house grounding rod and same thing happens: furnace on for about a minute (note: LED on circuit board flashes 3(?) times … off for a bit … flash 3(?) times … rinse & repeat) and then turns off. Furnace only turns back on when I explicitly command it do so (i.e. toggle the on/off switch on the electrical box attached to the furnace). Any ideas as to why the furnace would not continue to run when powered by the Honda gennie?

  6. It is recommended to ground your generator but I doubt many do, even on construction sites I’ve seen them ungrounded with the electrical inspector walking around. If you have a location where you always run your generator I recommend to get a grounding rod, I use 1/2″ rebar myself as they sell these nice plastic caps you can place over the top so you don’t impale yourself. Plus much cheaper than a copper or brass rod.

    Drive the rod (At least 36″) into the ground with 2-4″ sticking up, place the orange cap on top to easily locate and protect. You can use an old set of jumper cables to connect the generator to the ground or install a copper ground line with screw tight connectors to easily remove. I set mine up with one end of a jumper cable permanently mounted to the generator…makes it quick and easy for anyone to use. Looks like I need to do an overview of my generator setup for everyone.

  7. I was thinking about cutting the incoming power and put in a male and female plug. That is before an On/Off switch to my furnace. I would place a retainer on the connection to make sure it remains connected, unless I wish to unplug it. When I wish to power the furnace off of the generator, I would turn off the circuit breaker and furnace power switch, unplug the male female connection, and plug the generator connection in, make sure the generator dedicated only to my furnace, is running well and full of gas, then flip the furnace on switch. I have a friend that installs furnaces, and says the current fluctuation from our power company is greater than anything I would experience from you generator, if I follow the basic plan.

  8. Yes that will certainly work but keep in mind that putting inline female/male cord ends are not considered “to code”. It will work and I have done this myself, including on my own furnace and families/friends. I don’t have any issues with it if it’s done correctly.

    As far as fluctuations in power from the power company, it is true in some areas you get high distortion of power, especially in heavy load times such as really cold winter days/nights and hot summer periods due to demand for heating & cooling. We see many problems with “Brown Outs” on the commercial side frying motors & components. In most cases fuses/breakers don’t trip or not in time.

    That’s one of the issues I have with people buying high dollar generators to run their computers. Your better off buying a cheaper generator or 2 or 3 and then buying a backup power/UPS and plugging your computer into that. Laptops are even safer as they already have a battery power/conditioner.

  9. Thanks, this was handy. Been having regular power outages in my area, need to get my furnace hooked up to a generator as soon as possible.

    • That comment made it sound like the other advice in this blog is bad. That’s not what I meant. I meant finally some common sense advice about connecting a furnace to a generator. Most other advice I have read insisted hiring an electrician and installing a transfer switch. I knew it could be done much simpler.

      • I figured (was hoping) that’s what you meant, and thank you. Rick helped me do it on my furnace and we have used it during a couple of power outages last year. Rick wrote a great article and he’s available for follow up questions through his site or here; he’s a good friend and a wealth of knowledge.

  11. Couldn’t you also locate the breaker that the furnace is wired to, turn it off (after turning off the main), remove the hot/black wire in the breaker, run a cord with male plug on one end and hot, neutral, ground on the other end. Connect the hot/black from this cord to the breaker, and the ground and neutral wires accordingly. Insert the 3 prong plug end into the generator, start the generator, flip the breaker for the furnace.

  12. No, You don’t want to do that. First off all switching the breaker off at that point wouldn’t do anything as you are powering it directly to the wire. If you were to flip the breaker you have now just energized one side of your panel and could potentially send power to the main line. Doing that violates all sorts of codes and laws and could lead to electrocution, fire or anything in between. If your house burns down or you electrocute a linemen your at fault.

    Better to be safe and keep out of the breaker panel. If you wanted to do this, do it properly and have a generator sub panel/transfer put in.

  13. I tried to run my Trane XL80 gas furnace off my Honda generator. At first it was a no-go. Problem is the circuit board in the furnace detects whether safety ground is hooked up otherwise furnace won’t fire up. The Honda generator has no connection between neutral and safety ground. Maybe that’s just a Honda thing? Work-around I came up with was to use a specific homemade extension cord for use ONLY for generator furnace power. The homemade cord has neutral and safety ground electrically tied together.

  14. Your ground in your electrical panel is already tied into your neutral, in fact the reason they added a grounded line in the first place was to give another path to ground in case of failure, in older homes you can still find outlets with only 2 connectors.

    That’s why i’ve stated before the importance of grounding your generator. Tying your neutral and common together is a common work around. I’ve done it plenty of times. I would gather that most portable generators don’t have a dedicated grounding circuit.

    • I think I’ve encountered this same problem – trying to power my gas furnace with my Honda eu2000 generator. I wired it up – the blower will come on but the furnace doesn’t ignite. I’d like to try it with the “special extension cord” mentioned in this post – can you please describe how to create it? Thanks!

  15. This is probably a dumb question but does it matter if it is a gas or oil furnace? Anything different with an oil furnace that has a tank less heater? Also, my furnace (Weil Mclain) is powered by a 15 amp on/off switch so when calculating the watts is it safe to assume the 80% rule? 80% of 15 amps is 12amps. 12 amps times 120 volts = 1440 watts. I am assuming thats starting or surge watts what do you think the running watts would be?

    • The Oil Furnaces usually are about the same, some have fuel pumps and the controls require a little bit more power but for the most part it you will require around the same power.

      The 15 Amp power to your furnace is the minimum required by law, so it could only run 5 amps or 13 amps…The only way to know what the starting and running watts are is by measuring with a clamp on meter. Not safe to assume what the starting or running Watts are, all manufacturers and all components are different.

  16. Best video on this topic on the net. Well done. I had no heat due to Sandy in NJ TILL TODAY. Took me a couple of hours but it came out great and my house is now warm. Didn’t realize that blower was only 120volts at .8 amps. Is that pretty much the main draw or does that circuit board require some wattage?

  17. I just tried to connect my generator before reading this article. I used the emergency switch directly at the burner cabinet. The burner powered up (blue flame, fan) and then shut down after 15 seconds … recycled a second time and shut down for good. The only step I didn’t follow was disconnecting the green ground from the switch and reconnecting it to the new outlet. Could that have been the problem?

  18. It’s always possible, some have a ground fault issue that may trip off the board, tie the ground into the neutral and see if that works. Could be other issues like the size of the generator or problems with the furnace? Good Luck…

  19. Great video. I tried 3 ways. I disconnected the main power and turned off all circuit breakers except for the furnace and a power outlet close to the furnace. I connected two male 3-prong wires and plugged into the open outlet and then to the generator. This supplied power to the thermostat and furnace, but the thermostat keeps resetting. This did not work.

    I took your idea, but instead of a plug box, I connected the wires in the furnace directly to the cord (same black cord you used). Then plugged to the generator, this did not work either. I hear a click sound when I unplugged the cords

    Then I connected the house wire back to the furnace wiring (kept the black cord as well intact) thinking the thermostat needs power to control the board…but this time the thermostat did not power up.

    Im running a 5000 rated generator with 6500 starting watts…and powering only a sump pump and charging phones on occasion. The generator is grounded to a metal under my deck.

  20. Hmm, I’m kind of stumped, It should be working at this point. If it is a issue with your thermostat you can try jumping on the heat. On your control board use a jumper wire or small piece of stripped wire and connect from R to W terminal and this will bypass the thermostat and at least rule that out. After jumping then plug it into the generator, it should continue running as long as you have the jumper in place, if not there is another issue going on.

    Hope that helps..

    • Thanks I will try that. Do you think the fuse or breaker on the control board be damage…somehow? If its damage, do I need a tool to test? Thanks again. Will post later and let you know how it goes.

  21. The most common fuse is a 3amp standard blade automotive type fuse or Carrier units will have a Mini-breaker that might trip…but unless you jump out high voltage on the board you can’t really damage anything…

    • This is a great post and RicksDIY’s detail instructions was on the money. I went back and checked all the connections and found a small tiny wire flailing…and then found where it should have been attached to. The male-male (socket to generator) will only work if it is close to the furnace. When working on the wires, make sure not gets disconnected. I was doing this in minimal light and did not see that I knocked the tiny wire loose. Now we are all warm and toasty….THANK YOU Rick!!!!

  22. More on Honda EU2000i and my Goodman furnace.
    It will not run it wiring conventionally. Regular house outlet: black(hot) to ground= 120v. White (neutral) to ground 0 volts. Black to white = 120v.

    Honda inverter gen
    Black to ground = 60v, white to ground=60v. Black to white= 120v.
    Code on for ace says polarity reversed. Reversing polarity gives same message.
    Honda says the EU2000 will not wok on all furnaces.

    Is the work-around tying neutral and ground together?

  23. Never seen that before, must be the way the inverter generators do it but that seems wrong. I would try tying them together and testing it out, worst thing would probably pop the breaker on the Honda…but do so at your own risk…

    Now that I think about try this;

    The other option is trying to ground the unit at the grounding lug on the front, tie that into your existing ground or a new ground rod and wire that to the generator…that may be your fix

    I have an older Honda generator and it puts out 130V AC…never seen the 60V to ground…other than when ground has been lost

  24. friend of mine tried this and says it worked successfully- first shut off the main breaker switch in the panel .
    cut one end of a extension cord (the female end) making it a male now having 2 males.
    plug one end into the generator and the other into outlets until he found the outlet that was on the same side that also gets power to the furnace (gas)
    He claims this method turned on the circulator pump and the thermostat that the electricity from the generator finds its way.
    He claims that this is safe an worked.
    is this true???

  25. NO NO NO….Not safe at all for many reasons, first you have a live male plug that could electrocute you, your wife, or your kids…Kids like to pull plugs from the wall…now you have a live DEADLY voltage on the male end coming from the generator….DO NOT DO THIS.

    If you need another reason… you are supplying 120V and roughly 13-15 Amps to one side of your breaker panel, which normally has 100+ AMPs… if you have other breakers turned on you could (probably will) overdraw and cause a short in the wiring possibly leading to burning down your house…and by the way you will be at fault..

    So not only could you kill someone but you could burn your house down. Having been electrocuted several times I don’t recommend it and I would personally never do this under any circumstances…way too risky.

  26. I was a little uneasy doing this myself so I had an electrician come and his suggestion was a good one. Instead of installing an outlet box he used the on/off switch on my oil furnace. Basically there is a plug that comes out of the side of the on/off switch box. He didn’t have to mess with the main power line or installing the outlet. So when the house has normal power the on/off switch is in the ON position. When the power goes out I need to switch it to the OFF position then hook up to the generator. to the plug coming out the on’off switch. When power comes back on I simply turn the generator off disconnect the extension cord and then turn the furnace switch back to the ON position. The benefits of this hook up is it cut the installation time in half and the furnace is not constantly plugged in its only plugged in emergency situations. Total installation took less than 30 minutes. Tested it on the generator and it works great. If anyone wants a picture send me your email

  27. Yes, That is another good safe option, you use a double pole double throw switch in place of a standard on/off switch. I’ve done it that way before but I did have a friend who had an issue when they sold their home through the inspection as doing it that way didn’t meet our local code as it does in the video I did.

  28. Thanks for the terrific video Rick – appreciate all the detailed advice!

    My setup has a long flex metal conduit power cable going into a 1-gang electrical box mounted on the side of the boiler (the box came with the boiler). Instead of moving the flex conduit to a separate 1-gang box/outlet off of the furnace, it would be easier and much neater if I just add another 1-gang box next to the one on the furnace and move the existing flex conduit power line to that and put the single outlet there. Then I would wire (a short) appliance cord to come out the original box and plug into the outlet on the new one. Basically same theory, with 2 electrical boxes side-by-side on the side of the furnace itself. Would that be up to code?

    Thanks!

  29. Hi Rick,
    I have a gas furnace/central A/C system. I noticed the outdoor unit runs when my furnace is running. Can I still use this set-up to run my furnace without doing anything with the outdoor unit? I currently have a 3500/4000 watt generator. I really enjoyed your video and thought it was very informative.

  30. Hi Don,

    Not sure, your outdoor or condensing unit shouldn’t run if you have a gas furnace unless in fact you also have a heatpump which uses the outdoor unit while running. You can still run the gas furnace directly off the generator and just bypass the condensing unit, If it’s a heatpump only then your generator isn’t large enough and you would need to rewire the 220V… best to have an HVAC mechanic take a look at it if you can; just to make sure what you have going on.

  31. I purchased a small 2800/3500 watt generator to get me through the power outage from Sandy. I wired directly to my furnace using 14 guage romex with a male plug to generator and existing junction box. With black to black & white to white & ground to furnace bx connector I got a blinking error code stating reverse polarity. Double & triple checked my wiring which was correct. Reversed furnace connections (black to white & white to black) and it fired right up and worked perfectly. The furnace is high effiency and about 2 years old. Any thoughts on this?

  32. Hi Mike, Not sure about the reverse polarity but it seems to come up quite often. Could be the brand of furnace or maybe the way the generator is wound..either way As long as it all works with no issues I would just run with it. Glad to hear you got it working.

  33. Rick,
    Great video. I pretty sure I know the answer to this, but you can never be to careful. Is it safe to mount the receptacle box to the side of the furnace? Thanks again for the video.

  35. I have a Weil-MacClain high eff.105 gas furnace. I wanted to hook it up to my portable Generac 3250 LP generator. I checked with two people from NJ NATURAL GAS CO. here in Ocean county, NJ and was informed by them that it can’t be done. The furnace requires what they both referred to a “TRUE POWER”. What is True Power and Is there any way this can be done without damaging the furnace.

  36. Hi Joe, I’m Familiar with Weil-Mclain Boilers but have never worked on any of their furnaces, after looking at their website looks like they offer a standard high efficiency furnace which appears to be just like nay other. The 105 your referring to must be the BTU’s? Or are you talking about their Ultra 105 Water Boiler?

    Regardless the term “True Power” they might be referring to is actually also known as “Watts”. More info can be found here if you feel like digging into it; http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

    But it’s not necessary, unless you have a furnace made by NASA or that runs on 400Hz you’ll be just fine running ANY generator capable of delivering 110-120VAC and 12-15 Amps ideally.

    The most critical part of your furnace is the electronics, it runs on DC voltage, There is usually a transformer that converts the AC to DC power, there are also other things like capacitors and voltage regulators that keep that power clean and stable…regardless of how bad your incoming power is.

    Most manufacturers have ranges of where the equipment will still function normally, for instance most 115V equipment can function just fine from 105-130VAC. I’ve seen voltage as low as 98 and as high as 133 with no issues, overtime if equipment is already being pushed to it’s limit this will cause it to fail.

    In commercial equipment we use 460V 3 Phase. I’ve seen ranges from 420-490…You wouldn’t believe how much your power fluctuates, you just never check and unless something goes out completely you won’t notice.

    Your biggest risk using a generator is running out of gas while your furnace is running, this could cause a problem…a fuse to blow etc. I wouldn’t worry about it, their is plenty of fail safes built into the furnace….remember these are the same equipment that get sent into 3rd world nations with very unreliable power…and I’ve never heard of any problems.

    Other than that I’m not sure why the guys you talked to said it wouldn’t work. I know if I lived anywhere near you I would love to setup it with you and prove their wrong. Generators wouldn’t be around if they couldn’t run electronics and motors and everything else etc…

    • I believe the “true power” being referred to is a term used by Generac to describe the electricity produced by their XP series of generators. From what I’ve been able to figure out you need clean electricity to run anything that has a circuit board such as high efficiency furnaces, or electronic washers & dryers and newer refrigerators & freezers.

      • Yea I looked at Generac True Power, which is a Trade Mark…they claim a 5% Max Total Harmonic Distortion.

        Like I said before, it’s not really necessary, all electronics run on DC voltage and Generators put out AC voltage. The AC is converted by the board, transformer or by another means. to DC voltage that is safe for the electronics. If you would like to learn more about this check out Solar1234.com and listen to some of the podcasts about generators and power…

  37. Rick: Thanks for all of the information.

    Back on Nov. 11, 2012, you replied to Don that ” your outdoor or condensing unit shouldn’t run if you have a gas furnace unless in fact you also have a heatpump which uses the outdoor unit while running. You can still run the gas furnace directly off the generator and just bypass the condensing unit”

    I have a newer Traine heat pump with a L.P. Gas “emergency” furnance that also works when temps drop below the high twenties. I would like to use the method in the video to run only my gas furnace. How do I go about bypassing the heat pump/condensing unit?

    Thanks

    Dan

  38. Hi Dan,

    There are a few ways this could be wired, this is what is called a duel fuel or hybrid unit. Most of the time it’s simply a thermodisc switch mounted somewhere outside usually in the electrical panel of the condensing unit. When the temp drops below a preset temp…around here I use 35 or 28 deg F thermodiscs.

    Then it shuts off the 1st stage heat through the thermostat, and switch over to emergency heat which would be the gas furnace. Sometimes they are wired without using the thermostat…in that case I would have to look at how it is wired to tell you how to bypass it… but being that it’s a newer Trane unit you should be able to just bypass it with the thermostat.

    On the thermostat you should be able to manually put it into Emergency heating mode…I know this is a long answer to what you needed but I felt i had to explain a little of what was going on..

    These setups are becoming more common and if you live in mild climate are highly recommended. I have installed several systems like this and I really like them with both the redundancy and efficiency you can get…couple that with an economizer for cooling and you have a very practical setup.

  39. Thank you. I do have an “emergency” setting on the thermostat that turns the lp gas unit and the heat pump off. I just could ‘t believe it was that simple.

  40. Rick,

    Great job explaining this. I’ve been wanting to do this for years and felt a little apprehensive, but you made it easy.

    Thanks very much!

  41. Thank you for this article! I have been thinking about getting my Coleman gas furnace wired to a generator in case of an emergency, but I didn’t know how. I think I might try it now!

  42. Thanks Everyone and Trace for all the positive feedback & comments, I have been humbled that I have been able to help so many people with this issue.

    I have some more videos that I’m currently editing where I show some other ways to wire your furnace to a generator, and I will be posting them on my site in the near future.

  43. Just finished the project this weekend. Took all of an hour. Easy as could be. Just took the main line that fed from the panel to the emergency on/off box and made that a single gang receptacle. Then took the line that ran from the emergency box to the boiler and made that into a three prong 14 gauge power cord.
    Now the boiler plugs into an outlet which replaces the on/off box. Next time power fails just unplug and plug in to my extension cord which runs to my generator and I’m good to go.
    No electrician, no fancy transfer panel. Cost was $14. I celebrated with a bourbon and a cigar. Just beautiful.
    Thanks so much for the confidence to do this.

  44. Thanks for the great blog post and video! I’ve been wanting to do this for a long time but didn’t want the expense of a transfer switch. This is so simple. But I’m wondering if it would void the warranty on the furnace? Manufacturers are pretty picky about any modifications made to their stuff. Any thoughts on that?
    Thanks!!

  45. Hi Jamie,

    I’m not completely sure about that one, On one hand all they require is for you to meet their power requirements which a generator does. The technician/company who comes out to work on your furnace is who submits warranty paperwork. I have done it too many times myself, and I would rather get the work covered by the manufacturer than not…

    Only when there is a problem over and over will the manufacturer send out a rep to check it out…has only happened one time to me and that was on a $200,000 unit that we had multiple failures with the compressors on. Never heard about any issues with residential equipment getting covered.

  46. An EXCELLENT video.i appreciated your comments about the dirty power not likely to cause a problem with the integrated circuit board. I’ve asked many hvac installers and never got a clear answer.I also found little help from both the generator mfg. and the furnace mfg.. thanks again.

  47. Pingback: Wiring A 2 Wire Start Generator To A Generac Switch | Electrics PDF

  48. This is a great video!! I currently have an old Lennox GH-6 and a Troy Bilt 5500/8250 peak generator and was thinking of updating to a new high efficiency elite series 295. One of the installers advised me that I cannot run it off of a generator because of the dirty power it produces. Now, I have done hours and hours of reading and people seem to point the finger at the variable frequency (generator not able to product constant 60Hz) especially when not under load.
    I know you have covered this in previous posts, but I was wondering if you have ever run into any problems since this video getting a newer high efficiency two-stage variable speed furnace working off a generator. And if so, if there was any resolution.
    I was thinking of hooking the generator up to a voltage regulator, but that doesn’t fix the potential frequency problem if there is one, the other option would be to use my home theater Monster Power line conditioner, but I don’t know if that will handle the draw of the furnace.
    I have also heard of people who had an issue with the generator operating the furnace with the new electronic thermostat, and when they replaced it with an old mercury switch it worked fine!
    Have you experienced and of these issues lately?

  49. I still to this day have not run into any problems with running a furnace high efficiency or not with a standard generator as long as the generator is sized properly and an adequate sized extension cord is used.

    I’ve even personally run a brand new high efficiency furnace off a Harbor Freight 800 watt generator and using a Honeywell Vision Pro touch screen thermostat.

    I think the problem is with proper grounding, using too small an extension cord and not letting the generator get warmed up.

    The problems with the frequency is not a serious issue, as even if your fluctuating 5-10% your well withing standard operating area, most equipment is also rated to run in Canada at 50 Hertz power, where ours is 60 Hz, I can tell you don’t change anything on the equipment…it’s all the same, maybe a different transformer here or there…

    • Thank you for the prompt response Rick! I don’t see why this would be a problem either. As far as extension cord sizing what would you recommend?

  50. I recommend the largest amperage, and the shortest length, needed for your generator to your furnace. At minimum 14AWG (13-15 Amps) for sustained use. I personally like to use 12AWG, if running anything over 50′ long. You can get away with a 16AWG, as long as it’s a short run and only used occasionally.

    Remember in wiring the smaller the number (the gauge) the larger the wire diameter, usually for extension cords you get what you pay for. The best prices I’ve seen for 50’+ heavy duty 12 gauge cords are at Costco. Otherwise be looking at spending $50-$100 for a good grade extension cord.

  51. I have a 5500 watt generator running my furnace through a 6 position transfer switch. Problem is, when running on GENERATOR, the blower fan in the furnace won’t run at full speed. It ignites, blower comes on, and it’ll run all day like that, and heat the house, but not at the same speed as if it were on the LINE power. Same with my 2 bathroom fans. When you switch from gen to line power, they slow down a bit. Can’t figure it out. This generator has run table saws, and well pumps without issue. I know it’s big enough to run the furnace. Any thoughts???

    • I think you may have an issue with your Generator, I couldn’t imagine that the transfer switch would cause the problem. I would see about maybe borrowing a friends generator to test it against yours and see if that shows you anything.

  52. Great Video. Thanks. My incoming power is connected to a switch, which then goes into some sort of control box, and when it does into the furnace there a modular plug. There are a lot of controls on the outside unlike the one inf the video. Any ideas?

  53. You need to just locate your power most likely coming into the switch then unto other areas or maybe powering other things like vent controls/fans/pumps etc. You will need to wire your plug at the switch or just before it. If you pull off the switch cover and there are only 2-3 wires, with the black terminated on the switch then you will just need to wire it there.

    • Not sure if this is the same issue, but my Weil-McLain runs into an Emergency gas shut off box that then runs into a conduit box with two transformers on the outside of it that have wires (red and white) connected to them that run to the different heating zone centers. From that conduit box I see two thick white wires. One runs into the House distribution board, the other I don’t know. Anyhow, I was wondering if the pigtail connection can be done outside the actual furnace, (say somewhere further along the line running into the distribution board) given the emergency shut off switch and the subsequent to another box with two transformers attached.

      • sorry, to be more precise, could I do the outlet-with-pigtail-setup at any point along the line that runs between the furnace and the control board, or does it have to be inside the furnace itself?

        • Thanks, Rick! I’ll give it a try, soon… We’re supposed to get more snow and power outages later this week.

          • I took apart the box to get at the line coming from the distribution board and there were four wires (red, black, white and green). The red, white and green all tie up with the line that comes from the emergency shut off line for the furnace.The black line goes to another set of wires that seemed to flow back to the distribution board. Question, can I divert that 4-wire line from the distribution board into the single receptacle outlet? If so, do I simply put the red and black wires into the hot side and tie everything else accordingly, white to white and green to ground?

          • Hi Craig,

            I’m just guessing here but usually if you have 4 wires, then the Red & the Black could be for 240VAC?, Black to white and the red to white will be 120VAC and then the ground, could be using the red for just another 15amp 120V circuit, so there is 2 circuits for power

    • Rick – Thanks for all your help on this! I had a similar setup here with the modular plug and a control box with a switch between the plug and control box. The control box had two lines going into it from the distribution board. One had three wires (black, white and ground) another had four (black, red, white and ground). The second one (with 4 wires) was split with just the red, white and ground connected to the switch that then led into the modular plug attached to the outside of the furnace. I just took those wires and connected them to a three wire set that I then connected to the outlet box. Then I attached the pigtail to the wires coming out of the switch from the furnace. Everything worked fine on start-up! Great work!

  54. Hi,
    How big an inverter do you need to run an average gas furnace, AND, since inverter generated AC does not travel well, how many kW will I need?
    Thank you so much for any help on this,
    Sonni Poulsen

    • Hi Sonni, It’s best to check your furnace nameplate, usually a sticker inside, It will show minimum amperage, but from my experience 800 watts is about the smallest I’ve seen run a furnace. Here is a sample from my website;

      “Most homes that use gas furnaces or oil burners for either heat or hot water require very little wattage, less than you think. You can look at the nameplate rating on your unit or motor, the amperage required is always listed. Then, it’s just simple math to determine your generator size:

      125 volts x amperage rating listed on your nameplate or motor = wattage required

      We add 125% for motor start up, this is the initial draw your generator will see.

      Example;

      We have a gas furnace that lists on the nameplate sticker (usually located on the inside panel) that it has a 1/3 hp motor and is rated at 4.2 amps add in any other motors like the combustion blower rated at 1.0 amp.

      125 volts x 5.2 amps = 650 watts of running power

      125% x 650 watts = 813 watts required for starting power. Add in the misc power needed for the control board and were probably around 850+/- watts maximum required.

      ** A simple 1000 watt generator would run the HEAT in this home **

      Keep in mind not all inverters are created equal and it’s best to err on the larger side and also be direct wired to a battery bank, no clamp on inverter will be big enough IMO.

  55. I have a Honda generator 3000 kW and I’m trying to run my gas furnace with an extension cord and I’m getting a warning from the furnace that is saying I don’t have a ground or reversed polarity. I was was told I need a transfer switch, is there a way to to this with out one? I have heard you can build an extension cord that will act the same as transfer switch with a common. How do I build this.

    • Hi, sorry I thought I answered this but looks like it didn’t go through.

      The main issue with the Honda EU and Yamaha inverter generators is that they use a un-bonded/floating neutral (no internal Ground-Neutral Bond) and can cause all sorts of issues with powering some equipment and powering RV’s.

      To alleviate the problem with unbonded neutrals specifically for the Honda Eu series we recommend either to create a NGB Edison plug aka; neutral/bond jumper, or to create the bond in the transfer switch on the Generator side so when you run the generator it will have the ground and neutral connected together.

      Here is a link to a NGB Picture http://ricksdiy.com/?attachment_id=353

      Another option that sometimes works is tying together all the neutrals in the furnace and/or the transfer switch, the generator will sense the ground from the main panel and work as normal.

    • Forgot to add that you would then simply plug this into an empty receptacle on your generator and your good to go, it’s quite popular fix in the RV world as some RV’s don’t have a NG bond when plugged into a portable Honda type generator.

      BTW, I will be adding a post up about this on RicksDIY.com to go into more detail.

  56. I have a furnace kill switch next to the door in my furnace room (code in Ontario). I don’t know the technical terms, but it’s fancy as it has the white cable (that came out of the BX) going in and out of the switch. This is different than the standard light switch next to it, which only has one white cable going in.

    Can I use that switch to my advantage? Or can I replace that switch with a plug like you made?

    If not, how do I find out if your solution with the outlet would be to code in Ontario?

    • Might need to call your local code enforcement office and see what they say as far as what’s required. I just helped someone that had this with a single 15 amp fuse in the box with the switch, we added the cord/outlet box after the switch/fuse box and it worked perfectly.

  57. My furnace is on a 20 amp circuit. I know that a 20 amp single receptacle outlet is required, but can I still use a 15 amp cord for the furnace if it draws less than 15 amps?

    • Hi Jacob, I would just check and be sure that the start up current is less than 15 amps, this can be checked on the nameplate sticker in or around the outside of the furnace. It might be that the furnace draws 12-13.5 amps at start up and manufacturers must not exceed 75-80% of the line current for start up purposes (can’t remember the exact number), so by code they just go up a notch and require 20 amp circuit, but in theory would still work on 15 amps.

      I’ll also be adding a video on how to do this in the next few days, as many people ask how to check this.

  58. Last winter I tried to connect my gas furnace to my generator. I could not get the furnace to run, What I was told from a HVAC contractor is that the generator will not put out ” a level power wave ” of power, so the gas furnace electronics will not allow the gas valve to stay open. He said i could use a show UPS to level the power levels. Also I did not have the generator grounded.

    Any idea’s?

    • Hi Bob,

      The problem could be that your Generator has what’s called a floating neutral, this might cause problems with running a furnace specifically the newer HE furnaces.

      But in general most generators should be able to power your furnace, the electronics are all powered by a transformer that uses DC power, the AC power comes in and is converted to a low voltage DC power as is the case for all circuit boards, the transformer also has capacitors that condition and keep the power level stable.

      I have installed hundreds of transfer switches and wired all sorts of Generators, from 800 watt ones to 30KW and have never had a circuit board fail due to the power from a normal functioning generator, now if that generator is old, runs out of gas or on the larger ones have a leg of power that fails then that could cause a brown out and trip a breaker, blow a fuse or short a transformer.

      If your Generator is old or has many hours on it then eventually the power created will become more and more out of phase and fluctuate and should be replaced, sadly most Consumer grade generators are only rated at 500 hrs and some like the Honda’s can go 2000-3000 hrs.

  59. Hi thanks for grat video. My furnace works using power from generator. Just wondering can I use the some idea to connect AC to generator . I have 20A circuit breaker on generator . Ac its 2 ton thanks again Jack

  60. Hi Jack, you’ll need to check your nameplate sticker on your AC unit, but most AC units run on 208-240V AC, so you would need to use a big enough Generator that put’s out a split phase power, these are usually at least 3500 watts or more.

    Also it probably requires the furnace blower unless your talking about a 2 ton AC window unit, then you’ll need to add that into the mix too. Add up the minimum required amperage (usually stated on the nameplate sticker) and see if you have enough power. I would bet a 5000 watt generator could probably run your 2 ton unit, but you’ll need to calculate everything out.

  61. Great video and instructions. Would it basically be the same process on a 240 volt well pump just using different parts?

    • Yea, same thing you would just need the 240 Volt plug twist lock plug equivalent on your generator, So check your generator and if it’s a L14-30P then get the male plug and the female receptacle and wire that up. When you need it simply unlock and plug into 240 extension cord into your Generator.

  63. Rick, this is great information.
    One question, though. I have a dual fuel system where a heat pump heats down to freezing, then the propane furnace kicks on below freezing. And I noticed that there are two thermostat wires coming out of my furnace, one going to the thermostat and the other going to the heat pump/AC unit outside the house.
    Since the outside unit is on its own power circuit, is it possible for there to be any power or other communication interference to the propane furnace from that? Or is the heat pump isolated, somehow, in the thermostat, itself?
    This may be opening up a whole new can of worms.
    Thanks, I’m looking forward to exploring your site, further.

    • Hi Jack, In a Duel Fuel setup there are 2 options for controlling the Heatpump and the furnace.

      1) Newer systems today use an outdoor temp sensor connected to the thermostat, when the thermostat senses it’s too cold outside usually set at 28-35 Deg F then it switches to the gas furnace, also the gas furnace is usually setup as the emergency heat on the thermostat.

      2) Usually older systems but not always; There is a thermo-disc or temp controller setup somewhere in the outdoor unit, these are a manually controlled temperature switch and has 2 paths for power, to the heatpump or to the gas furnace, again the gas furnace is usually setup as the emergency heat on the thermostat.

      With that said, if you verify that the gas furnace will work at any temp when set to the emergency heat setting(if you have that) then the easiest thing to do is keep it in emergency heat while powering with a generator. There isn’t anything that would get screwed up as far as the communication goes otherwise, if you tried powering it with the generator the indoor fan would come on but the heat pump unit would stay off is all, then eventually it would see that the temp isn’t rising and would eventually kick on the emergency heat which is your gas heat.

      • Rick,
        My thermostat allows me to set the cut on and off temps for both the heat pump and the furnace, so maybe that’s a solution, too. But I like yours better, just keep it in the emergency heat mode.
        Thanks again for all the great info on your site.

        Jack

  64. That is a great video and excellent idea. Many thanks.

    My furnace is in the basement and the control thermostat is on the upper level. In an emergency, when the furnace blower is powered by the generator (as shown in your video), how can the thermostat works? Is the electricity powering the thermostat comes from furnace? Thank you.

  65. Thanks for the info. Good to know, and really appreciated how you showed exactly what the amp draw was.

    I live in Minnesota. We lost power during a blizzard, when the temp was plunging. I didn’t have a back up…and luckily we got power back in about six hours.

    A mile away they took two days to get the power back. Temps were below zero, and water pipes don’t like that 😉

    I’m going to make sure I’m ready for the next one. Even in the coldest weather, a furnace runs only at most half time. At least around here, where we get 25 below.

    • Yea my father in law is near Leech Lake; we go out there usually every summer and winter for a little ice fishing and hunting. I setup him up with a single circuit 30 AMP transfer switch that will power his strip heat, but he usually heats with wood most of the winter. Pretty harsh weather there, first time I was there during the winter it averaged -15F and the day we left was -35F, never had to scrape the inside of a cars windows before, had to most of the way on our drive back to the cities.

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