Quick answer
Yes, you can run a gas furnace on a generator. The furnace burns gas for heat and needs electricity only for its blower, inducer, igniter and control board: roughly 400 to 800 watts running, with a higher surge as the blower starts. A generator of about 2,000 watts covers most furnaces. Connect it through a transfer switch or listed furnace inlet installed under an ESA permit, never by backfeeding.
Can you run a gas furnace on a generator? Yes, and it is one of the lighter loads in the house, lighter than a microwave or a kettle. What trips people up is not the wattage. It is the connection, because a furnace is hard-wired with no plug, and the type of generator, because some portable units leave the furnace lighting and dropping out every few seconds.
How much power a gas furnace draws
A typical residential gas furnace runs on roughly 400 to 800 watts, almost all of it the blower motor. The inducer fan and control board add little. A hot-surface igniter pulls a few hundred watts, but only for the seconds before the burners light, and the blower has not started at that point, so the two loads do not stack.
- Running load: about 400 to 800 W for most furnaces; variable-speed (ECM) blowers sit at the low end.
- Start-up surge: a conventional PSC blower motor can briefly draw two to three times its running watts as it starts.
- Practical minimum: a generator rated around 2,000 running watts carries most furnaces on their own, with room for the thermostat, humidifier and condensate pump on the same circuit.
- To size it properly: multiply the amps on the furnace nameplate by 120 to get watts, then leave margin for the motor start.
On a rural lot in Caledon or Flamborough the furnace is rarely the deciding load. A private well pump usually runs on 240 volts and starts hard, so a generator sized only for the furnace leaves the house warm with no water.
The legal way to connect a hard-wired furnace in Ontario
A furnace has to be fed through a transfer device, because it is wired straight into a dedicated circuit and has no cord to plug in. There are two accepted routes. One is a transfer switch at the electrical panel that moves the furnace circuit, and usually a few others, from utility to generator. The other is a listed single-circuit furnace transfer switch with a built-in inlet, mounted beside the furnace, that lets a generator cord feed that one circuit.
Either one is electrical work. In Ontario it is installed by a Licensed Electrical Contractor who files a notification, the permit, with the Electrical Safety Authority (ESA). That part is an electrician's job, not a gas fitter's.
- No backfeeding. A cord with two male ends plugged into a dryer or wall outlet energizes the whole panel and pushes voltage back out to the street, where it can injure a Burlington Hydro, Alectra or Hydro One line worker.
- No cut-and-splice cords. Cutting the furnace's supply cable and adding a plug so it reaches a generator is not a wiring method the Ontario Electrical Safety Code accepts for a furnace.
- No generator in the garage, the basement or under a window. Run it outdoors, well away from doors, windows and the furnace's own air-intake pipe, and keep working carbon monoxide alarms in the house.
Why a furnace lights and then drops out on generator power
A furnace that lights, runs for a few seconds and shuts down on generator power usually has a grounding problem, not a fuel problem. The control board proves flame by passing a tiny current through the flame to the grounded burner, and that only works when neutral and ground are bonded somewhere in the circuit. Most portable inverter generators ship with a floating neutral, meaning the two are not bonded at the generator. Feed a furnace from one through a device that also switches the neutral and the board cannot see its own flame.
The fix depends on the transfer device, which is why the electrician needs to know which generator you own. A panel transfer switch that leaves the house neutral connected keeps the bond at the main panel, and a floating-neutral generator is the right match. A switched-neutral device needs the bond made at the generator. If the furnace still drops out on clean utility power, why a furnace ignites and then shuts off covers the non-generator causes.
Rough power is the other issue. Open-frame contractor generators can swing in voltage and frequency as loads cycle, which control boards and ECM blower motors tolerate poorly. An inverter generator produces cleaner power.
Natural-gas standby generators need their own gas line sizing
A permanently installed standby generator solves the connection problem, since it comes with an automatic transfer switch, and creates a gas one. At full load, a whole-home unit can burn more gas than the furnace and water heater combined. The line feeding it has to be sized under the CSA B149.1 gas code for its input, its distance from the meter and everything else on the system, and the Enbridge meter may need upgrading to deliver the volume.
An undersized line shows up on the worst night: the generator starts, gas pressure sags, and the furnace or the generator itself faults. A TSSA-licensed gas fitter sizes, installs and pressure-tests a generator gas line. Cost depends on pipe length, diameter and whether the meter changes, so it is quoted in writing after a site visit; what gas line installation costs explains the drivers.
Before the next outage: a short checklist
- Have the transfer switch or furnace inlet installed and inspected before winter, not during a storm.
- Test-run the furnace on the generator for a full heating cycle and watch that the burners stay lit.
- Know the restart behaviour: after power returns, a furnace may sit in a delay before firing. See why a furnace will not turn on after a power outage.
If the furnace runs on utility power and misbehaves on the generator, the furnace is usually fine and the power source needs attention. If you are adding a standby unit, the gas side is ours: ZK Mechanical installs and upsizes gas lines across Burlington, Oakville, Milton and Hamilton with TSSA-licensed technicians. Call (647) 801-1252 or request a written quote.