I know you will answer “it depends” but I am looking for an order of magnitude. How much electric power does a gas furnace need to light (I have a spark thing) and operate the blower? I am thinking of buying some power back-up to run the heating in time of emergency and was thinking of how much oomph I would need.
Are we talking hundreds or thousands of watts?
And what happens to a gas furnace that is burning when the power is cut? When the blower goes off, does the furnace?
Approximately 746 watts for each Horse Power of the blower motor.
3/4 HP would be 560 watts
But start up current of the motor is higher.
Your furnace gas valve is operated with 24 volts supplied from a 120 transformer and if you loose power or 120 volts to the furnace the gas valve shuts down because it is a normally closed valve that requires 24 volts to open. The flame and the blower will shut down at the same time leaving your heat exchanger hot because it requires the blower to cool it back to 100 degrees once the flame goes off. I would state that if this happens on a one time biases there would be no damage to the heat exchanger.
Ian,
I assume you are working with an Electrical Contractor to help size and install this aux. generator? I am also thinking you will be operating more than just the Furnace in an emergency situation, with this aux. system?
I would guess that a newer furnace will probably require no more than 6-10amps to start/operate.
I agree with Charley, damage can be done to gas furnaces that are short cycled off during a power outage, especially on a frequent basis. It can also be a fire hazard if there are any defects in the furnace, that might be unknown.
Good Luck finding the right Electrical Contractor
Too much power, and too complicated. I’ll give up and use blankets instead. It was just an idea.