Before you start
Work through the 11 checks in order. Each one is safe for a homeowner. Allow about 5 minutes after each change, since a furnace runs a startup sequence before warm air comes out.
Thermostat and power
1. Thermostat mode, setpoint, batteries and schedule
Set it to HEAT and at least 3 degrees above room temperature. A blank or faint screen usually means dead batteries. If a schedule or away mode is holding the house cooler, set a temporary hold.
If this fixed it: It was a setting or the batteries. Replace them yearly. Stop and call if the thermostat is calling for heat but nothing happens at the furnace within 5 minutes.
2. Circuit breaker
Find the breaker for the furnace, boiler, air handler or heat pump. A tripped breaker sits in the middle position. Push it fully to OFF, then back to ON. Heat pumps usually have two breakers, indoor and outdoor.
If this fixed it: Keep an eye on it, because breakers trip for a reason. Stop and call if it trips again. Don't reset it twice.
3. Furnace power switch
Near the furnace there's a service switch that looks like a light switch, sometimes with a red cover plate. Make sure nobody flipped it off by mistake.
If this fixed it: Label the switch so nobody turns it off again. Stop and call if it's ON, the breaker is fine and the furnace shows no sign of life.
4. Furnace door panel
The lower furnace door presses a safety switch. If the door is loose or crooked, the furnace won't run. Push the panel firmly into place, and never tape the switch down.
If this fixed it: Seat the door fully every time you change the filter. Stop and call if the door is seated and the furnace still won't start.
Filter, gas and ignition
5. Air filter
A clogged filter makes the furnace overheat. A limit switch then shuts the burners off while the blower keeps pushing cool air, or the furnace keeps starting and stopping. If light barely shines through the filter, replace it.
If this fixed it: Check the filter monthly in winter. Stop and call if the filter is clean and the furnace still shuts off after a few minutes.
6. Gas supply and other gas appliances
Do your other gas appliances, like the stove or water heater, still work? If none do, contact your gas utility. With propane, check the tank gauge. Never adjust a gas valve yourself.
If this fixed it: Once service is back, have appliances checked and relit by the utility, supplier or a pro. Stop and call if a propane tank ran empty, because the system needs a leak check before anything is relit. If you smell gas, leave first.
7. Status light and error code
Many furnaces have a small window in the lower door showing a blinking light on the control board. Count the blinks and match them to the code chart on the door label. Read the code, but don't open the burner area.
Also listen at startup: the inducer motor whirs, a hot surface ignitor glows or a spark clicks, then the burners light. Burners that go out within seconds often mean a dirty flame sensor.
If this fixed it: A one-time power reset can clear a lockout, but the fault likely remains. Mention the code when you call. Stop and call if the code comes back or a standing pilot is out. Leave relighting to a pro. See furnace repair.
High-efficiency furnaces: drains and vents
If your furnace vents through two white plastic (PVC) pipes instead of a metal flue, it's a high-efficiency (condensing) furnace, and these two checks apply.
8. Condensate line and trap
A condensing furnace makes water, which drains through a trap and hose to a floor drain or condensate pump. A clogged trap, frozen line or full pump trips a pressure switch, and the furnace won't fire. Look for water under the furnace, a full or unplugged pump, or a kinked hose.
If this fixed it: If the pump was just unplugged, watch for leaks over the next day. Stop and call if water is leaking, the pump is full or the line is frozen. Don't take the trap apart.
9. Outdoor vent and intake pipes
Follow the PVC pipes outside. Snow, ice, leaves or a nest can block them, and the furnace then shuts down to keep exhaust out of the house. If the pipe ends are at ground level, gently brush snow away by hand.
If this fixed it: Switch the furnace off and on once to restart it, and keep the pipes clear all winter. Stop and call if ice is inside the pipes, they keep getting blocked or they exit through the roof. Don't climb up there.
Heat pumps and boilers
10. Heat pump outdoor unit
Frost in cold weather is normal. During a defrost cycle the fan stops for a few minutes and you may see steam. A thick block of ice or a unit buried in snow is not normal. Clear snow off and around the unit with a broom. Don't chip at ice or pour water on it.
If this fixed it: Check the unit after each snowfall. Stop and call if ice doesn't clear within about an hour, the fan won't run or the house keeps getting colder. Meanwhile, EM HEAT (emergency heat) on the thermostat runs the backup heat. See heat pump repair.
11. Boiler pressure gauge
The boiler gauge shows water pressure. A cold system typically reads about 12 to 15 psi, rising as it heats up. Low pressure can lock the boiler out. High pressure can make the relief valve drip.
If this fixed it: Reading the gauge fixes nothing, but note the number for the pro. Stop and call if the cold reading is well below 12 psi, the needle is near 30 psi or the relief valve drips. Adding water, bleeding radiators and checking the expansion tank or circulator pump are jobs for a pro. See boiler repair.
Stop and call a pro if...
- You smell gas or a CO alarm sounds. Leave first, then call from outside.
- A breaker trips a second time.
- You smell burning or see smoke.
- Water is leaking, an error code keeps coming back or the pilot is out.
- Heat pump ice won't clear, or boiler pressure is out of range.
- You've done every check and there's still no heat.
- Vulnerable people are in a cold house, or pipes may freeze. Don't wait to finish the list.
When you call, you may be asked for your ZIP code so the call reaches an independent heating company that serves your area. Mention the checks you did and any error code. The pro diagnoses, quotes and schedules directly with you, with no obligation. House getting cold? See emergency heating repair.
