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How To Wire Your Gas Furnace To A Generator

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, thus requiring a Electrician or you have to temporary rewire the breaker to the generator for temporary power and then rewire everything once the power comes back.

But this isn’t the case, NEC (National Electric Code) simply requires that appliances must be on their own dedicated circuit. By simply rewiring your furnace you can have the ability to use your furnace by powering it with a portable generator.

When the power is out you simply disconnect the furnace from the outlet and connect it to a extension cord. You run this cord outside to your generator, start the generator and once it is sufficiently running you plug the furnace in and use it like you normally would, go to the thermostat and crank up the heat. In this way you can use your generator and fuel more wisely and only as needed.

videoHow To Wire Your Gas Furnace To A Generator

 

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

 

Materials;

The cost for the materials for this particular job was less than $20, and all can be found at most hardware or big box stores.

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

Tools;

The tools can be much more basic than this, the drill is a little overkill but if your installing the Outlet box onto your furnace with sheet metal screws than the drill will be needed.

  • Drill
  • Hammer
  • Screwdriver (Multi-Tip)
  • Side Cutters
  • Wire Strippers
  • Multi-Meter
  • Voltage Stick (Not Required)

How To Do It;

As is the case every and anytime your 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 still get shocked, it’s rare but like driving a car sooner or later your going to be in an accident, hopefully it will be a manageable one.

Turn Off your breaker or light switch that controls your furnace. Keep in mind that a high number of Breaker panels are mislabeled. Open your furnace and find where your power is coming in from, most likely one of the sides. Trace it down to the junction box or in my case simply the circuit board.

Pull off any covers and then test for voltage. I like to use the Fluke Voltage Stick first, But you first must make sure it works on a live circuit and then you can test it out. After you have verified the power is off, carefully remove the wire nuts and test with a Voltage or Multi-Meter. Once you are darn sure there is no power then you are safe to work on it.

Write down or label your wires if they are not the normal 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, at this time you can either wire in the POWER CORD or install the outlet box.

In the video I install the OUTLET BOX first. 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 at this point. 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 your facing the outlet this will be the Right side this screw is normally Brass/Gold in color, the White or Neutral wires up the larger bladed side this screw is usually silver in color, and 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 that is wired you must carefully push the receptacle back into the outlet box and screw it in, put on the COVER PLATE and were ready to move onto the final part.

Installing the POWER CORD, Next part 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 through the Power Cord and secure it down.

Now you must rewire the power cord to the connections on the furnace, if your using wire nuts be sure to Double Check that the wires are secured by tugging on each individually and then taping the connection with some electrical tape. You can further secure with tie straps if needed.

Make sure to put back on any cover plates, double check that everything is wired properly and secured. At this point you can turn the power back on and check the outlet with your meter (Not Shown In Video) to make sure everything is wired properly. Now you can plug in your furnace and check operations.

ALL DONE

Now that your done you can test it out with or without your Generator, I recommend to do a test run on a Dark Rainy Night so you know what it’s going to be like when your going to have to use it.

There are many variations of this including different types of plugs and wiring methods, I have done this many times and have had several pass electrical inspections. As stated in My Disclaimer always check with your local code or city regulations as to what is required.

If your interested in getting the 2011 NEC Code Book and your going to get it online help me by buying it through my link, Thanks.

 

26 comments

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  1. Greenbeans

    Struggling with a week long power outage due to Hurricane Sandy. This was very helpful. Thank you! Wish you posted more videos.

    1. Rick

      Thanks, I will be doing more soon..been kind of crazy last few months…

  2. mike

    Rick,
    This is an awesome tutorial, and found it amazingly informative. During Sandy, we were out of power and managed to get our hands on a generator. Luckily, the power came back on before the cold temps and snow storm arrived.
    But, this got me thinking to prepare for next time.
    I have a hot water baseboard heating system. While the converting the electric connection to a plug and outlet shouldn’t be a problem, I wondered if the thermostat needs any attention (and any special power considerations)?

    Thanks again for your info and great work,
    Mike

    1. Rick

      Hi Mike,

      On the Baseboard heating, their are several variations…some use no power at all for the thermostat and most will use 24 VAC converted from 120VAC…if you power everything else for the water heater circuit then you should be just fine.

      Glad you survived and enjoyed the videos.

  3. john blazich

    hi rick,
    i tried your video on wiring the furnace to a portable generator.i’m sure i messed up at some point because i no very little about electricity.
    first i shut off the breaker to the furnace !!!
    my power from the breaker box goes through an on/off switch mounted on the furnace case so i removed the cover on the front of the furnace and saw that after the switch it was black wires [125a] and white [neutral].the ground was attached to the conduit going into the switch on the outside.i then attached my 14 guage jumper [balck to black-white to white] and ground wire to a screw on the inside of the case.i didn’t have the generator set up and running so i thought i would go into the next room and just plug into a wall outlet.well needless to say all that did was put the breaker off in that room.i tried another outlet and got the same result.
    did i do something wrong in the wiring or maybe you just can’t plug into another wall outlet for some reason?
    thanks for any info you can pass along.

    p.s. i really enjoyed the video it made me feel that i my be able to do this myself and save some money.

  4. Rick

    Hi John,

    First I’m glad you gave it a try but…whoah be careful…sounds like you jumpered it correctly…I would double check to make sure the jumper connection was really tight and not touching the case of the switch. For best results and to not risk shorting out, try to remove the wires from the switch and use wire nuts to your jumper cable…That would rule out any issues with that.

  5. Andrew

    Hello Rick,

    Great video. What should I do when I have 2 wires coming into my furnace? I do not have the 3rd (ground) wire. Do I cap off the ground wire on the other end of the plug? Then connect the 2 wires into the 3 prong receptacle?

    1. Rick

      Most likely the conduit is being used as the ground from your breaker panel to the furnace, I would just ground the wire in the plug to the furnace itself at a grounding screw or other tapped screw w/no paint in the furnace, this will provide your ground.

  6. Tristan

    Hey Rick,

    I never even decided to offer this as a service to my customers. By using this tutorial and a few other helpful resources, I was able to hook it up properly. Many of my customers have complained about us not offering generator services, I thank you for this post.

    Sincerely,
    Tristan.

    1. Rick

      Hi Tristan,

      I’m happy you were able to make something work, It’s good knowing people like yourself and your customers are getting prepared the more who are just make it all the much easier when the power goes out. I’ve gotten several emails from people who have survived a cold storm by following my examples. Thanks for taking the time to write.

      Rick

  7. Andrew

    Thanks for offering this very good site. I have completed the wiring as described in the video and I tested the furnace on the generator and the furnace started and ran well for about 2 minutes and stopped. I then tried the furnace on house power and it ran well. I did not ground the generator-did that matter or is it something else?

    1. Rick

      Hi Andrew,

      Did the furnace fire up and was running smoothly then shut off? Did you have any error lights, many of the control boards have leds that will flash a code so you can determine why it failed. I would try again, and be sure your generator is up and running for awhile before plugging in.

      1. Andrew

        No the furnace did not fire-I think that the hot surface ignitor did not function and the furnace went to lock down. Does the ignitor have a separate circuit? Or is it because the ignitor needed more power than the Coleman 2500 (1800 running watts) provided?

        I stated in the first post that the furnace ran about 2 minutes but it was closer to one minute before stopping. And I did let the generator run about 10 minutes before plugging in the furnace.

        Thanks
        Andrew

  8. Emmett

    Thanks Rick for sharing this DIY. This is exactly what I have been searching for. My main concern if a power outage should occur is heating the house. I didn’t want to spend the cash on a transfer switch, so your solution is perfect. In fact, I ran out to Lowe’s tonight and purchased the required materials. I planned on doing it tomorrow, but I couldn’t wait and did it tonight. It was fairly painless, except for the tight space I was working in. Turned the thermostat up a couple degrees and the furnace fired right up. We are expecting some interesting winter weather here over the next couple days and I wanted to be prepared. I feel much better knowing I have the capability to connect the furnace to my generator. Thanks again!

  9. Andrew

    No the furnace did not fire and I did not see any warning lights. What I think happened was the ignitor did not work and the furnace shut down for safety . So do you know why the ignitor did not work? Was I getting enough power from the Coleman 2500 watt generator?

    Thanks

    Andrew
    12-8-2013

    1. Rick

      Hi Andrew, Not really sure. Some ignitors are finicky about having a good ground, was a common problem to have to redo ground wires and terminals but usually not so with residential furnaces. I would think the Coleman 2500 would be enough to run that, might be another issue with the neutrals. Are the neutral wires (usually white) tied together or are they switched? whatever they are try doing the opposite and see what happens. I looked up my generator list and most coleman’s have what’s called a floating neutral. If that’s the case with your’s then you don’t want to switch the neutrals.

      1. Andrew

        Thanks Rick-I will look at the neutrals

        Andrew

  10. adam

    If my furnace ground is grounded to itself and only 2 wires black and white running to power do I remove ground from furnace bolt and add to ground spot on 3 prong plug?

    1. Rick

      Yes, you should tie the ground on the plug to your furnace ground either at the same spot or another bare metal surface spot should be fine.

  11. sebastian

    Hi Rick, thank you for posting the video, I’m going to do that with my furnace. Very helpful.

  12. Jamie

    Hi Rick, your video is helpful. I have a different setup, in that the power comes into a box mounted on the outtake. The box consists of an on-off switch and a TL-15 fuse. Would I have to put up a second box with the outlet, in line after the switch box? Thanks for your thoughts.

    1. Rick

      Sounds like that’s your circuit protection, the fuse would blow if you had any issues when running on your grid power. For best use I think afterwards would be the best, as Generators have circuit breakers that would trip if there was a problem and then that wouldn’t cause any conflict with your fuse setup. If you have any other Q’s you can just email me direct rick@ricksdiy.com…. Thanks

  13. Marc Pollok

    I have a very old gas boiler furnace (Sears Homart 600, probably from the 1960′s). I want to connect it to a generator during power outages, which are frequent here. If the only thing I would power with the generator is the furnace, which includes circulator, how many watts would a generator need to supply to make this work? Thanks.

    1. Rick

      Hi Mark, the easiest way to see how many watts you need is if the unit has a nameplate/sticker that might contain the amperage/voltage max ratings, just multiply your amps x volts to get your watts, then add everything up, if that’s not there you will need to put a amp clamp on the power lead and measure the max current draw that way. If you can’t do that look at the size of the breaker, if it’s a 15 AMP 120V circuit then your talking 1800 or so watts.

  14. Leni

    Thank you so much for this info, I would like to take it a step further though. It can get very cold up here and I do not want to leave a window or door open for an extension cord. What are my options? I am thinking I would have to make a male/male extension cord that goes from the generator to an outdoor outlet, that outlet is wired to an indoor outlet, then i run an extension cord from the furnace to that outlet. Obviously these two outlets are just wired to each other and do not have power. correct? or crazy.

    Thanks
    Leni

    1. Rick

      Hi Leni, CRAZY..lol….that would work but is definitely illegal and against code and could electrocute, kill or burn down your house…just saying. The easiest thing would be to put in a hole in the side of your place and run a long enough heavy duty cord through and then seal around it, My brother had this for awhile, most of the cord was outside rolled around a hose reel, then later we put in a single circuit transfer switch in which case is your next most economical option, that’s what led me to start my own business; Heezy.com where we make single circuit transfer switches.

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