You should select a generator to run furnace based on its power requirement. You can run a furnace off a generator safely as long as you follow the necessary safety precautions. It is important to know the basics and purchase a suitable generator that can deliver the power. So, let’s find out what you need to consider when determining the size of the generator that you need to run your furnace.
How Many Watts Does It Take to Run a Gas Furnace?
The wattage is an important consideration when deciding the size of a generator. Most houses have furnaces that consume 3500 – 5000 W of energy. This is around 20 amps at 240 volts or 12 amps at 120 volts off the circuit. The most suitable way to determine the watts required by the furnace is to look up the user manual. You will find the model number and the input or output voltage of the furnace. Calculate the required wattage using the equation below.
Watts=Volts×Amps
Moreover, the size of the generator needed also varies depending on the type of furnace you have.
Gas Furnace
It all depends on the horsepower of your furnace’s fan. If it is a gas furnace with a quarter horsepower fan, then the furnace will need 1000 – 1600 starting watts. Once it starts, the furnace will need 600 W rated power to continue running. A gas furnace with a 1/2 horsepower fan will require more than 2000 W starting watts and around 900 W watts to keep running.
| Appliance | Running Watts | Starting Watts |
| Gas Furnace Fan Blower (1/8 HP) | 300 | 500 |
| Gas Furnace Fan Blower (1/6 HP) | 500 | 750 |
| Gas Furnace Fan Blower (1/4 HP) | 600 | 1000 |
| Gas Furnace Fan Blower (2/5 HP) | 700 | 1400 |
| Gas Furnace Fan Blower (3/5 HP) | 875 | 2350 |
Since home generators are usually around 5000 W, you can easily power up your gas furnace and other appliances during a blackout. So, if you only need to power up your furnace, you can even go for a smaller generator. Portable generators weigh less and can be maneuvered easily. They even generate lesser noise compared to larger generators. But you should go for a generator that has a little more wattage than what is needed for your furnace.
Electric Furnace
A generator with a capacity of 10,000 W is necessary to run an electric furnace. You can power up all your needs easily with a natural gas standby generator. In addition to using a furnace, you can reverse the direction of your ceiling fans to retain more heat inside the house during colder seasons. Here are some electric furnaces and their respective running wattages.
| Appliance | Running Watts |
| Central Electric Furnace (15 kW) | 15,350 |
| Central Electric Furnace (20 kW) | 20,490 |
| Central Electric Furnace (25 kW) | 25,670 |
Oil Furnace
Although oil furnaces may not use electricity as much as electric furnaces do, they still rely on electricity. This is similar to the gas furnaces that use natural gas as fuel. Both these furnaces need electricity to start. A normal oil-fired furnace requires more than 800 W to operate. So you need a generator over this value for both the burner and the blower to operate. The motor draws oil from the tank and pumps it through the igniter to be ignited. Then the air is warmed and forced into the room by the blower. Here are some oil furnaces and the respective power needed.
| Appliance | Running Watts | Starting Watts |
| Oil Furnace Fan Blower (1/8 HP) | 300 | 500 |
| Oil Furnace Fan Blower (1/6 HP) | 500 | 750 |
| Oil Furnace Fan Blower (1/4 HP) | 600 | 1000 |
| Oil Furnace Fan Blower (2/5 HP) | 700 | 1400 |
| Oil Furnace Fan Blower (3/5 HP) | 875 | 2350 |
What Size Generator Do I Need to Run a Furnace?
So, the next question is what size generator to run furnace? A 1000 W generator is not sufficient to run a furnace. You need to consider both the starting and running watts of a furnace for this. A furnace on a 20 Amp circuit will use two and a half times that power. Considering an oil-fired furnace, you will need a furnace starting watts of 2000 W and a surge of 3 – 4 Amps. Since the power output of the furnace depends on the voltage, you should select a model that can give the required voltage and current.
A 12 Amp furnace will run on a 5 HP generator. Diesel generators are usually cheaper than other natural gas generators. They even have a higher power output than gasoline generators. To run a gas furnace, you can even use a portable power station, which is an electrical appliance. It is easy to use and can be placed indoors since they do not emit fumes. They are also known as solar generators related to how they are charged. They are also highly reputed for low noise output.
The power requirement of a typical household furnace is around 300 – 500 W. This will be more or less depending on the size and the brand. The wattage required will be indicated on the furnace label and you need a generator that can handle twice the power as that.
Is It Safe to Run a Furnace Off a Generator?
Yes, it is safe to run a gas furnace using a generator if you follow the safety precautions. The first necessity is that you should keep the generator outside while it is running. This is to make sure that no toxic gases can get indoors to harm the residents. Some generators also come with a carbon monoxide detector that will automatically shut down the system if it senses harmful levels. So always keep your generator at a safe distance from your home in a well-ventilated area.
Also, ensure that the furnace running on the generator does not malfunction or turn off. This is to confirm that it is running on proper power levels. This will not be a problem if the generator is well above the horsepower of the furnace’s fan.
Keep the furnace and the generator safely connected using multiple outdoor-safe cords. It should be able to handle all the watts that need to power the furnace. In addition to the gas furnace, if you are using an extension cord to plug in other appliances, make sure that the extension cord is safe to use with multiple cords.
Just like the gas furnace, generators also use gas as fuel. Keep extra fuel away from the generator as a precautionary measure to prevent fires. If your generator is battery-powered, always keep your generator fully charged in a safe place, ready to use when needed.
If you are using a smaller generator and need to run other appliances along with it, then you should consider having two generators. You can run your furnace on one generator while the other generator will power the appliances. This will be a lot safer as you are not overloading your generator.
Can a Generator Damage a Furnace?
There is a risk of damaging the furnace if you power the furnace with a generator and run it for a longer period without necessary precautions. The power that fluctuates repeatedly, known as the “dirty power” can cause damage to the furnace.
You should not run too many appliances on the generator that powers up your furnace. It will cause surges unless the generator is high-end with surge protection. Some generators are less vulnerable as they are EMP-proof. If the generators do not have true sine wave outputs, they can be plugged into a true sine wave UPS. It will work without draining the battery and you can use a voltmeter to confirm this. When the furnace is running, the generator needs to produce at least 110 V AC.
How to Run Furnace Off Generator
The following steps will help you in determining how to run a furnace with a generator.
- If you are using a portable generator, place it outside away from the house in a ventilated area. Turn off the circuit breaker to keep the electricity away as you start the process to avoid electrocution.
- You need to have a transfer switch installed to proceed with the connection. If you do not have one, get it installed by a professional.
- Connect the furnace to the generator using outdoor-safe extension cords.
- Rewire the cords and plug the generator into the furnace and check that both the generator and the furnace work.
Frequently Asked Questions
What Is the Quietest Generator for a Furnace?
A gas furnace can run on a portable or stationary generator. But the noise generation is a major concern when running any appliance. Battery-powered generators are the quietest among all types but are quite expensive. A portable generator will be a quieter option to operate your furnace. But you need to make sure that it contains enough wattage to run the furnace. The YAMAHA EF2000iSv2 Gas Powered Portable Inverter is a suitable option to keep the environment calm and power your furnace.
Will a 3500 Watt Generator Run a Furnace?
You can run a small gas or oil furnace that requires around 1000 W to start and 600 – 800 rated watts with a 3500 W generator. However, you will not be able to run a lot of other appliances off this generator. It will also not be able to provide power for a long time like a larger generator. If you are not running a furnace, you can run certain other appliances on a 3500 W generator.
Will a 2000 Watt Generator Run a Gas Furnace?
A 2000 W generator is powerful enough to run your natural gas or oil furnace. This will be enough since the starting wattage of these furnaces is 1400 W with a running wattage of 700 W. If you are not running a furnace, a 2000 W generator can run multiple lights, a heater, and even a couple of fans. Learn more about the fuel and oil consumption of a portable generator.
Conclusion
You can pick the most suitable generator to run furnace easily based on the description above. Always keep in mind that you need a larger generator than the calculated power need of your furnace. Follow the normal precautions and test them before you run the generator. Always have the proper accessories including the high-quality power cords when connecting the furnace to the generator.