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Furnace Won’t Run on Generator? Top 3 Causes (+ Fixes)

When the power goes out in the middle of winter, you want to find a way to keep the furnace running to stay warm. Thankfully, you can hook your furnace up to a generator to continue enjoying heat throughout your home in the pitch-dark. However, if your furnace won’t run on your generator, you have a problem on your hands.

Your furnace won’t run on a generator because it can’t run on inverted power, there’s no bonding between the neutral and ground wires in the generator, or the wattage is too low. You can fix the generator by bonding the ground and neutral, but you can’t fix the problem if compatibility is an issue.

This post will help you understand why your furnace won’t run when plugged into a generator. I’ll also tell you what you can do to fix the problems.

The Furnace Can’t Run on Inverted Power

You’ll feel frustrated if you turn the furnace on only to realize that the generator isn’t supplying any power. Let’s see why this can happen. 

Compare your furnace power requirements to the features of the generator.

You may find that there are certain functions that the furnace relies on or something it’s not compatible with — the generator doesn’t meet the needs of the HVAC system. 

Many generators use inverted power to deliver efficient levels of electricity to your devices.

Unfortunately, this is also where you may face compatibility issues. The thing is, not all furnaces are compatible with inverted power. Some rely on standard power, and when the generator uses an inverter to convert power to AC, the furnace may not power on when connected to the generator. 

How To Fix

The fix for this issue depends on the type of generator you have. Look at the model number and brand name. Skim the manufacturer’s website or owner’s manual for information.

The manual should give you details on whether the generator uses an inverter. If the generator uses inverted power, check if this is the problem by looking at the furnace’s power requirements.

Sometimes, it may be possible to switch the inverter on or off. If your generator has an inverter switch, try switching it off. Now, try to start the furnace again to check if that solved your issue. 

If the generator doesn’t have an off switch for the inverter, you’ll probably have to get a different generator. 

It’d be a good idea to see if you can find another generator just for testing purposes. Perhaps a friend owns a generator that doesn’t use inverted power. Try their generator with your furnace to see if that fixes the problem. 

No Bonding Between Neutral and Ground

Another possible reason for a furnace not running on a generator may be related to the bonding between the neutral and ground lines. This issue is more common than you might think.

New generators, particularly portable ones, don’t have a connection between ground and neutral. This is a problem, as the furnace uses the bonded connection as a safety measure when it’s turned on. 

In this situation, no power will run to the furnace. However, other devices connected to the generator will run without any issues. 

How To Fix

While a common problem, the fix may not be as easy as you were hoping it would be. The issue lies within the generator — particularly in the electrical wiring found inside the device. 

If you want to bond neutral and ground lines, you’ll have to open up the generator. If you have experience working with electrical systems and wiring, you can do this yourself. If not, get an electrician to do it for you.

Here’s a brief rundown of what you have to do:

  1. Switch off the generator.
  2. Open the generator’s main panel.
  3. Identify the neutral and ground wires.
  4. Bond the two wires. How exactly you do this depends on the model in question.
  5. Put the cover back on.

Now, connect the generator to the furnace again and power it on. See if this process fixed the problem. If the furnace still won’t light up, the problem lies elsewhere in the system. You’ll need to investigate further to determine the issue. 

Incompatible Generator Wattage

Sometimes, the issue between the generator and furnace is fixable, allowing you to use your current generator without the need for a replacement. However, there are scenarios in which the generator doesn’t work with the furnace no matter what.

This is why you need to pay close attention to the whole spec sheet for your generator and furnace. That’s why you must ensure the two appliances are compatible before you commit to them.

How To Fix

Check the wattage output of the generator. Next, check the label on your furnace to see its wattage requirements. Do the numbers match up? If not, then this is where your problem lies. 

Note that the generator can have a higher wattage rating than what the furnace needs. Ideally, this should be the case in all situations. There are, however, some portable generators that provide just enough power for essential devices. In other words, the generator has a low wattage rating. 

Unfortunately, you can’t “fix” the generator to output more power to the furnace. Instead, you might have to consider replacing either the main furnace module or the generator. 

Before you do this, take note of one thing — some generators come with adjustable power. This is often described as an “Eco” mode. When the “Eco” mode is turned on, a lower wattage is provided to connected devices, which means the generator uses less fuel to generate power. 

If your generator has an eco mode, try turning it off, then see if it provides adequate power for the furnace to start. If the problem remains, look for a replacement. If the generator is still new, you might be lucky enough to get it exchanged for a larger model, with the condition that you pay any additional fees.


Generators are incredibly useful when the power is out, but they have limitations. While it may be tempting to hook the furnace up to the generator, you should ensure compatibility before investing. 

If you run into a problem where the furnace won’t run on a generator, get a professional to inspect your setup.


  • Vincent Steele

    Vincent is a freelance writer based in Santa Ana, California. When he isn't writing articles for Temperature Master, he can be found biking or hanging out with his cat, Shelly.

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