Can You Hook Up A Furnace To A Generator? (Explained)

Many people cannot survive without a furnace during the winter. They need a generator to keep the appliance running during the coldest months of the year. But can you trust a generator to sustain a furnace?

The appliance concerns laypeople because it uses so much power. Their biggest concern is an overload. The National Institute Of Standards And Technology (US Department Of Commerce) has highlighted carbon monoxide poisoning as the priority because the gas is invisible and odorless.

However, homeowners are smart enough to operate their generators in a well-ventilated location. Unfortunately, it doesn’t always occur to them to limit the loads they attach to the generator.

Even if the generator survives, the unit can harm your devices by producing voltage fluctuations. That doesn’t even account for the fires you can start if you buy wires with a low gauge.

Gas furnaces are less of a hassle. Many homeowners don’t realize they need generators to run these appliances when a power outage occurs. Yes, a gas furnace uses gas. But it still requires electricity to operate the following:

According to Bob Mims, a furnace is useless to you if all it does is produce heat. You need a blower fan to distribute that heat. The blower fan runs on electricity.
Modern furnaces use electricity to ignite the burner. Electric burners are safer because a circuit breaker will trip when an overload or short circuit occurs, preventing a defective furnace from lighting the burner.
Modern furnaces use circuit boards. Without a circuit board, the thermostat won’t work. Circuit boards run on electricity. The same goes for the relays.

In other words, a natural gas furnace doesn’t eliminate the need for backup power during an outage.

Understanding Furnace Power Requirements

As was noted above, you don’t want the furnace to overload the generator. You can avoid overloads by matching the generator’s rating to the furnace’s power requirements. But you can’t do that without:

1). Determining The Wattage And Voltage Requirements Of The Furnace

You will find a furnace’s wattage and voltage requirements on the nameplate. The rating will vary drastically depending on the furnace type. Electric furnaces are the most common, and they use 10,000W to 50,000W. But Energy Sage thinks 20,000W is the average in residential settings.

That sounds like a lot of power, equating to 60 – 80 amps (220V). However, it is worth noting that electric furnaces don’t work continuously. First, they tend to cycle on and off, cutting their electrical consumption to 13,000 watts or less each hour.

Secondly, your location will determine the duration of use. Some people use their furnaces one or two hours a day, which lowers the impact of the appliance on their utility bills. Regardless of your preference, you can’t deny that gas furnaces are the cheaper option.

Electric furnaces will cost you an average of $400 monthly to run. That figure falls drastically to $10 for a 1/2HP furnace that uses 735 watts. The components of a gas furnace that use electricity are light-duty.

You can confirm a gas furnace’s superiority by checking its nameplate for the watts and volts. If you can’t find the nameplate, use the manual or contact the manufacturer. Once you provide the serial number, they will tell you the watts and volts.

2). Calculating The Starting Wattage And Running Wattage Of The Furnace

A furnace has a starting and running wattage. The starting wattage is a problem because the appliance uses more current when it cycles on. The lights in your home may flicker as a result.

But that surge is temporary. It may last a fraction of a second. Once the furnace starts, it switches to the running wattage, which is the power it uses during regular operations. The starting wattage is usually two or three times higher than the running wattage.

This is why the lights flicker as the furnace starts, but the flickering eventually stops. That initial surge will subside, and the furnace’s electrical demands will stabilize. Honda has given the example of a 1/4HP furnace with a running wattage of 600W and a starting wattage of 1600W.

You don’t calculate the starting and running wattage. The furnace’s nameplate, manual, or manufacturer will tell you this information.

Generator Capacity And Sizing

Once you identify the furnace’s specs, use that information to locate a generator with a suitable rating. That means:

1). Determining The Power Output And Capacity Of The Generator

The capacity is a generator’s maximum power output. It shows you the volume of power a generator can produce. According to energy.gov, the manufacturer determines the capacity and prints the information on the nameplate.

From what the Economic Times has seen, manufacturers express the output in watts and kilowatts. Use the formula (watts = volts x amps) to turn kW into amps, assuming you need the amps. The generator size and category will influence its output, for instance:

Portable generators are the smallest and weakest option. Some people take them camping.
Inverter generators are stronger than their portable counterparts. They can run multiple appliances.
Whole-house generators will run the entire house.

2). Ensuring The Generator Has Sufficient Wattage To Handle The Furnace Load

Common sense will tell you to find a generator whose running wattage exceeds the furnace’s running wattage. However, modern generators have a starting wattage as well. They are designed to accommodate a furnace’s initial surge. The generator’s starting wattage should exceed the furnace’s starting wattage.
The generator’s wattage should be twice as high as the furnace’s wattage. This gives you room to breathe if a short circuit or surge occurs.

Selecting The Right Transfer Switch

The transfer switch is the most important component. You can select the proper transfer switch by:

1). Understanding The Role Of Transfer Switches In Connecting Generators

A transfer switch isolates the generator from the grid. It shifts between the generator and the grid, ensuring your home uses one power supply at a time. Without a transfer switch, starting a generator during a blackout will electrocute the electricians trying to restore the grid’s power.

2). Choosing Between Manual Transfer Switches And Automatic Transfer Switches

Both options play the same role. However, manual switches are cheaper because you manually shift from the grid’s power to the generator’s power when a blackout occurs. Benefits include the following:

They are cheap to manufacture.
Installation is quick.
You have more control when a blackout occurs.

An automatic switch typically works with whole-house generators. It automatically shifts from the grid to the generator and vice versa. Benefits include the following:

The switch is convenient.
It’s safer. Laypeople won’t interact with the generator’s components.
It limits downtime by restoring the power as soon as possible.

3). Ensuring The Transfer Switch Is Compatible With The Furnace And Generator

Identifying the correct transfer switch means taking the following into account:

Select a transfer switch whose specs (watts and volts) match the power it will accommodate.
The best transfer switches have UL certification.
Find a switch with the features you want, such as tie bars and a weather-resistant inlet box.

Electrical Wiring And Connections

A generator is only valuable for you if you can find a way to connect it to the furnace. Many accidents occur because laypeople wire a generator and furnace poorly. Consider the following:

1). Understanding The Electrical Connections Between The Generator And Furnace

Many furnaces are hardwired. But an electrician can rewire the appliance, allowing you to plug the furnace into the generator (or its extension cord) during an outage. In some locations, you need a permit for this sort of work.
If the furnace is hardwired, connect a whole-house generator to the house. The transfer switch will automatically shift to the generator when you lose power.
I recommend cutting the power at the main breaker and connecting the generator’s output terminal to the furnace’s input terminal.

2). Using Appropriate Wiring And Connectors For The Connection

Pull the furnace’s wires through a conduit.
Use 12WG – 14AWG gauge plugs (If you’ve decided to wire the furnace with a standard plug).
Use 14AWG wires and extension cords (or thicker, depending on the furnace’s rating).
Make sure you install a fuse next to the appliance’s terminal.
Cutting the furnace’s wires (hardwired) may void the warranty.

3). Ensuring Proper Grounding And Following Electrical Codes And Regulations

The local authorities may compel you to apply for a permit before adding a standard plug to the furnace.
Use a licensed electrician to make modifications.
You must install a transfer switch.
The furnace needs a dedicated circuit.
The wire’s gauge should match the furnace’s wattage.
De-energize the circuit before doing electrical work.
If the local regulations demand a hardwired furnace, don’t use the plug option.
Don’t forget to separate the generator’s grounding rod from the ground before connecting the unit to the furnace.
Connect the portable generator to your home’s ground. Don’t ground it separately.
Ground a gas furnace by hooking the grounding to the neutral wire (after finding the grounded outlet and placing a jumper over it) before connecting the ground wire to the gas line.

Installation Process Of Hooking Up Furnace To A Generator:

How do you intend to install the transfer switch? How will you connect and test it? Consider the following:

1). Installing The Transfer Switch And Conducting It To The Furnace And Generator

Cut the power at the main breaker
Match the furnace’s circuit to the pre-wired transfer switch’s circuit inlet.
Remove a knockout at the bottom and pull the wires from the transfer switch through.
Use a locknut and bushing to secure the flexible conduit.
Mount the transfer switch box on the wall using suitable connectors.
Remove the furnace’s circuit breaker and disconnect the hot wire.
Attach the red wire (that corresponds to the furnace circuit) to the disconnected breaker before reinstalling it.
Use a yellow wire connector to bind the black wire (from the transfer switch circuit) to the old feed wire.
Use two red leads for a 240V circuit.
Run a neutral line from the transfer switch to the neutral bus bar in the panel.
Run the ground wire from the transfer switch to the grounding bar in the panel.

2). Properly Mounting And Securing The Transfer Switch And Generator

Mount the transfer switch box on the wall with its edge eighteen inches from the panel’s center.
Use mounting screws to secure the box to the wall.

3). Testing The Connection And Ensuring Proper Functionality

Keep the power at the main breaker off.
Place the switches on the LINE setting.
Run a power cord from the generator to the switch box.
Turn the generator on and let it run for a minute.
Move the furnace circuit switch on the transfer switch box to GEN.
Check the onboard meters for balanced wattage levels.
Turn the switch to LINE and shut the generator down.

Safety Precautions:

Generators and furnaces are dangerous. You can avoid potentially fatal accidents by:

1). Ensuring Proper Ventilation And Safe Placement Of Generator

This goes without saying. Keep the generator outside the home. The exhaust pipe should face away from your house. Otherwise, you may succumb to carbon monoxide poisoning. Make sure the unit sits on a flat surface to minimize vibrations.

2). Installing Carbon Monoxide (CO) Detectors And Alarms

Just in case you position the generator poorly and carbon monoxide drifts into your home, the Texas Department of Insurance recommends installing an alarm that beeps when it detects the toxic gas.

3). Following Safety Guidelines For Working With Electricity And Generators

Don’t place the generator in an enclosed space. Keep it 20 feet from your home.
Give the generator four feet of clearance on all sides.
Use grounded extension cords
Replace damaged cables.
Apply GFCI technology if the NEC calls for it.
Don’t start or stop the generator without first disconnecting the load.
Don’t operate a wet generator.
Don’t overload the unit.
Don’t pull the cable to remove a plug from an outlet.
De-energize the circuit before performing electrical tasks.
Electrical cords are tripping hazards. Store them correctly.
Use childproof outlets.