How to Run a Furnace Off of a Generator

How to Run a Furnace without a Generator

As winter descends, it becomes crucial to comprehend how to run a furnace off a generator, serving as the vital solution to growing concerns such as power outages, frozen pipes, and potential flooding in your home. A generator emerges as a lifesaving tool, enabling you to maintain the operation of your home’s HVAC system and furnace during cold-weather blackouts. In this guide, we’ll delve into the key considerations for utilizing both whole-house and portable generators to ensure your home remains warm and safe, with a focus on your HVAC system.

Running a furnace off a generator can be essential during power outages or in situations where the main power source is unavailable. However, it’s important to note that not all generators are suitable for powering furnaces, and safety precautions must be taken. Here’s a general guide on how to choose and install a generator to run a household furnace:

  1. Check Generator Capacity: Ensure that your generator has enough capacity to handle the power requirements of your furnace. Check the furnace’s manual or nameplate for information on its power consumption, usually measured in watts (W) or kilowatts (kW).
  2. Select the Right Generator: Choose a generator with sufficient wattage to handle both the starting (surge) and running (continuous) power requirements of your furnace. The starting power is typically higher than the running power, so make sure the generator can handle the peak load.
  3. Prepare the Generator: Place the generator outdoors in a well-ventilated area to prevent carbon monoxide buildup. Keep the generator dry and protected from the elements. Use a heavy-duty extension cord to connect the generator to the furnace.
  4. Turn Off the Main Power Supply: Turn off the main power supply to the furnace to prevent damage and ensure safety.
  5. Connect the Generator: Connect one end of the heavy-duty extension cord to the generator’s power outlet and the other end of the extension cord to the furnace’s power input.
  6. Start the Generator: Follow the manufacturer’s instructions for starting the generator. Allow the generator to stabilize before turning on the furnace.
  7. Turn On the Furnace: Once the generator is running smoothly, turn on the furnace using its control panel.
  8. Monitor and Maintain: Monitor the generator and furnace to ensure they are operating correctly. Be aware of fuel levels in the generator and refill as needed.
  9. Shutdown Sequence: When it’s time to power down, turn off the furnace first and then the generator and disconnect the extension cord from the generator and furnace.
  10. Safety Considerations: Never operate a generator indoors to prevent carbon monoxide poisoning. Ensure proper grounding of the generator. Follow all safety guidelines provided by the generator and furnace manufacturers.

Always refer to the user manuals for your specific furnace and generator models, as well as local safety regulations. If you’re uncertain or uncomfortable with the setup, consider consulting a professional electrician or technician for assistance. Incorrect connections or inadequate precautions can lead to equipment damage or pose safety risks.


  1. Gas Furnaces: Gas furnaces use a burner to generate heat for a heat exchanger. They are powered by natural gas and require electricity for components. Yearly inspections are popular because they prevent problems like carbon monoxide leaks and are easy to use.
  2. Electric Furnaces: Because they use electricity to heat elements, electric furnaces are non-flammable. They are smaller and easier to install without natural gas access, but they consume a lot of electricity, potentially leading to higher operating costs.
  3. Propane Furnaces: Propane furnaces, like natural gas models, use propane stored in a tank. Refills are required regularly. Regular maintenance ensures efficiency in areas where natural gas is inaccessible. Prices may vary depending on local propane prices.


Understanding the power requirements of different furnace types is crucial for selecting and installing the right generator size, especially when considering a hardwired generator setup. For gas furnaces, a hardwired generator should possess a minimum of 5,000 watts, while electric furnaces necessitate a generator with a minimum capacity of 15,000 watts. Opting for a slightly higher wattage is advisable to ensure the hardwired generator can effectively handle any startup surges that may occur during normal generator run operations.

Generator size chart for running a furnace:

Type of Furnace Average Wattage
Gas Furnace 600-1200 watts
Electric Furnace 10,000-50,000 watts
Propane Furnace 700-1500 watts


Whole-house generators, also known as standby generators, are durable, permanently installed systems that provide continuous power to your home during outages. They run on propane or natural gas and have capacities exceeding 22,000 watts, making them a reliable choice for long-term power supply.


Portable gas generators prove suitable for powering vital appliances such as furnaces, but the generator selected must have sufficient capacity, especially when considering HVAC systems and heaters. Electric furnaces may demand up to 25,000 watts for both starting and continuous operation. Despite lacking automatic functionality, portable gas generators can still be connected to furnaces, albeit with additional steps, ensuring a reliable power source for your HVAC system and heater.


The method for running a generator-powered furnace varies depending on the type of generator used. Because they can power the entire house, hardwired generators, which are permanently connected to the electrical panel, require little maintenance. For multiple-circuit power transfer, portable generators require the use of a transfer switch. A licensed electrician or skilled DIY HVAC expert can complete this installation, ensuring a smooth transition of power during outages.


Power fluctuations have the potential to damage the electrical components of a furnace, particularly the blower motor, resulting in decreased efficiency. To avoid any negative effects on your furnace, it is critical to select a generator that provides a stable power supply.


If you need assistance or have questions about whether your furnace is capable of working with a  generator, Green City Heating and Air Conditioning is here to assist in Kent, Auburn, and surrounding areas. Contact us at (206) 249-9772 and get a quote or schedule a service today.

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