Preparing Your NH Furnace for a Winter Power Outage

A portable generator can carry a furnace through an outage, but size matters. A typical residential furnace draws modest running power, roughly the equivalent of a few large appliances, but the blower motor needs a surge of extra current to start. A generator that can handle the running load but not the starting surge will stall or trip the moment the furnace calls for heat.

As a practical rule, most single furnaces run fine on a mid-size portable generator in the 5,000 to 7,500 watt range, with room left over for a refrigerator, some lights, and phone chargers. Add up what you plan to run at once, include the starting surge for the blower, and leave margin. When in doubt, a technician can tell you your furnace model’s electrical draw from the data plate during a [fall tune-up](/furnace-maintenance-tune-ups), which is useful information to have before you shop for a generator.

Fuel is the other half of sizing. A portable generator drinks gasoline, and stations lose power too, so their pumps stop. Keep enough stabilized fuel on hand for at least a day of intermittent running, stored safely away from the house. Our comparison of heating oil versus propane for New Hampshire cold snaps is a reminder that fuel planning wins cold snaps, and the same thinking applies to generator gas.

Related: furnace repair, heating oil vs propane for NH cold snaps

A house loses heat slowly, which gives you a working window. Interior pipes are usually safe for several hours in a well-insulated home, but pipes in exterior walls, crawl spaces, and unheated garages freeze first. Open cabinet doors under sinks on exterior walls so room air reaches the pipes, and let faucets drip, both hot and cold, to keep water moving.

If the outage stretches past a day and the indoor temperature keeps falling, the calculus changes. Know where your main water shutoff is before the storm, because shutting off the water and draining the lowest faucets is the surest protection for an extended outage. Keep the thermostat set as usual so the furnace restarts itself the moment power returns.

Do not use ovens, grills, or camp stoves for heat. They produce carbon monoxide and start fires, and they do not warm a house anyway. Blankets, layered clothing, and one room closed off and shared are safer than any improvised heater.

The best outage plan is built in October, not during one. Get the furnace serviced so it is starting the winter clean and reliable: burners, ignition, controls, and safety switches all tested. Ask the technician about your furnace’s electrical draw and write it on the service sticker, so generator shopping is a numbers game instead of a guess.

Test your detectors, locate the water shutoff, and make sure everyone in the house knows both. Keep flashlights, batteries, and a battery radio where you can find them in the dark. And if you buy a generator, run it once before you need it, so the first startup is not happening in a snowstorm at midnight.

New Hampshire will test your heat every winter. A furnace that is maintained, a generator that is sized and fueled, and a household that knows the safety rules turn an outage from an emergency into an inconvenience. For the maintenance half of that plan, book the fall visit and head into the season ready.