Most no-heat calls in January come down to something small. A good number of them are fixed before the truck leaves the base, on the phone, by one of the checks below. Run through them in order.
Start at the thermostat
Make sure it is set to Heat, not Cool or Off, and that the set temperature is above the room temperature. If the screen is blank or dim, change the batteries. A dead thermostat looks exactly like a dead furnace.
The switch that looks like a light switch
There is a plain switch on or beside the furnace. It gets knocked off by a laundry basket or a curious kid more often than you would think. Check it, then check the furnace breaker in the panel.
The filter
Pull the filter out. If it is grey and matted, the furnace has been starving for air. It overheats, shuts itself down to protect the heat exchanger, and restarts once it cools, over and over. Put a new one in, matching the size printed on the frame, and change it every one to three months through the heating season.
The pipes outside
A high-efficiency furnace vents through two white plastic pipes out a side wall. Snow, ice, leaves or a bird's nest blocking either one shuts the furnace down. Clear them gently by hand. If they are iced solid, do not chip at them.
Water where it shouldn't be
High-efficiency furnaces also make condensate, which drains through a small hose. If that hose is blocked or frozen, the furnace will not run. If you see water pooling under or beside the unit, note it and call.
Reset once
Turn the furnace switch off, wait thirty seconds, turn it on. If it lights and then quits, or a small light on the control board blinks a pattern, count the blinks. That code tells the technician a lot.
Reset once. A furnace that keeps shutting itself down is doing it for a reason, and each restart risks damage to parts that are expensive to replace.
Stop here
Anything behind the front panel, including the igniter, flame sensor, gas valve, pressure switch and heat exchanger, is licensed gas work. A cracked heat exchanger can leak carbon monoxide into the house, which is why we do not suggest poking around. If a carbon monoxide alarm sounds, get everyone outside and call 911 first.
While you wait
Close doors to rooms you are not using. Use a space heater in the room you are in, well away from anything that burns, and never with an extension cord. In a deep freeze, let a tap on an outside wall trickle so the pipes behind it do not freeze while the house cools.
Still cold after all that? Call. A licensed gas technician is dispatched around the clock.