It's cold, the furnace isn't running, and you'd like heat now rather than after a service call. A proper reset is a reasonable first step, and it genuinely fixes a fair number of no-heat situations. The trick is knowing how to do it safely — and recognizing when the furnace is locking out for a reason that a reset won't cure.
Before you reset anything, check the simple stuff. Make sure the thermostat is set to HEAT, set above room temperature, and has working batteries. Check the furnace's breaker in the panel. Check the service switch — it looks like an ordinary light switch on or near the furnace and gets bumped off more often than you'd think. Make sure the furnace door is fully latched (a safety switch kills power if it isn't), and look at the filter. A clogged filter is the most common reason a furnace overheats and shuts itself down.
How to reset a modern gas furnace. Most furnaces made in the last 25 years reset by cycling power. Turn the thermostat down or off. Switch the furnace off at the service switch or the breaker. Wait a full five minutes — this lets the control board clear its lockout and any residual gas dissipate. Turn the power back on, set the thermostat to call for heat, and watch the sequence: the inducer fan starts, the ignitor glows, the burners light, and the blower comes on a minute or so later.
If your furnace has a reset button. Some furnaces, and most oil burners, have a physical reset button — often red or yellow, located on the blower motor housing or the burner assembly. Press it once and hold for a second or two. That's it. Press it once only.
The one-reset rule. If the furnace runs normally after one reset, a power blip or a one-time hiccup was probably the cause. If it shuts down again, stop. Repeated resets are where people get into trouble: with oil burners, each press can spray more unburned fuel into the chamber; with gas furnaces, you're overriding a safety lockout that tripped for a reason — commonly a dirty flame sensor, a failing ignitor, an overheating heat exchanger, a blocked flue, or a pressure switch problem. None of those improve with repetition.
Read the blink code. Most furnaces have a small LED visible through a sight window on the door that blinks an error code, with the key printed on a label inside the door panel. Note the code before you cycle the power — a reset erases it, and it's the single most useful piece of information you can give a technician.
Skip the reset entirely and call if you smell gas (leave the house first and call your gas utility), a carbon monoxide alarm is sounding, you see soot, scorch marks, or water around the furnace, or the breaker trips again immediately.
If one reset didn't do it, Reinhardt Heating and Air will find out why — across Lincoln, Gaston, Mecklenburg, and Catawba counties, 7am to midnight, seven days a week, with up-front pricing before any work begins.