A furnace that will not start is stressful on a cold day, but the first things to check are quick and safe. Work through this list in order, and stop at any step that involves gas or opening electrical panels.
Safety first. If you smell gas, or a carbon monoxide alarm is sounding, do not work through a checklist. Leave the house, and call 911 or your gas utility from outside. Our guide to carbon monoxide safety and furnaces explains what to do.
Everything below stays on the outside of the equipment and uses controls meant for homeowners. Do not remove sealed panels, bypass safety switches or handle gas valves or wiring.
Know your equipment type before you begin. A gas furnace, an oil furnace, an electric furnace and the heating side of a heat pump behave differently when something goes wrong, and the label on the unit tells you which you have. If your home uses a heat pump, the idea of a pilot light or gas valve does not apply at all, though the thermostat, breaker and filter checks still do.
Confirm that the thermostat is set to heat and that the setpoint is above the current room temperature. Raise it a few degrees to be sure it is calling for heat. Replace the batteries if the thermostat uses them. With a smart thermostat, check the app or display for a schedule, hold or hold-off mode, and for a message about a lost connection to the furnace.
Listen for a click from the thermostat or the furnace when the call for heat begins. Nothing at all can mean the thermostat is not sending a signal, while a click followed by silence points elsewhere.
Furnaces usually have a switch that looks like an ordinary light switch on or near the unit, often on the wall or on a joist beside it. It can get flipped by accident during storage or cleaning. Make sure it is on.
Next, check that the blower access door is fully in place. Most furnaces have a safety switch that cuts power when the door is open or not seated. If you recently changed the filter and the door is slightly off, the furnace will not start.
Check the electrical panel for the furnace circuit. If the breaker is tripped, switch it off and then on once. If it holds and the furnace starts, note it. If it trips again right away, stop. A repeating trip is a sign of a fault, and resetting it repeatedly can create a hazard.
If you have more than one thermostat or zone, confirm that the zone you are checking is the one calling for heat. Zoned systems have dampers and panels that can keep the furnace idle when one zone is satisfied.
A badly clogged filter restricts airflow, and a furnace that cannot move enough air can overheat and shut itself off on a safety limit. Some furnaces will not complete a startup at all. Take out the filter and look at it. If it is gray with dust, replace it with the proper size.
Choosing a replacement is easier with a few facts, which we cover in how often to change your furnace filter. An overly restrictive filter can cause the same airflow problem as a dirty one.
Many modern furnaces have a small window showing a blinking light. The pattern corresponds to a diagnostic code listed on a label inside the furnace door or in the manual. A code can point toward a pressure switch, a flame sensor, a limit switch or an ignition problem. Write down the pattern and have it ready if you call for service. It saves time and helps the technician arrive prepared.
Resist the temptation to press the reset button repeatedly. A furnace that locks out after failing to ignite is telling you something, and repeated attempts do not fix the underlying cause.
If your furnace burns natural gas or propane, make sure other gas appliances such as the range or water heater are working, which tells you gas is reaching the house. Check whether the exterior intake and exhaust vents on a high-efficiency furnace are blocked by snow, ice, leaves or a nest. Clear obstructions from the outside only if you can do so safely.
Do not adjust the gas valve, relight pilot parts you are unfamiliar with or tamper with burners. If your unit has a pilot light, follow the instructions printed on that specific furnace, and stop if anything seems wrong.
Take a quick look at the area around the furnace as well. Boxes, laundry or stored items placed against it can restrict airflow or create a fire hazard, so keep the space around the unit clear. Do not store paint, solvents or other flammable products near fuel-burning equipment.
A final safety point: if the furnace room has a burning smell, visible smoke, soot, scorching or a water leak around the unit, switch the furnace off at the breaker if you can do so safely and request service, and call 911 if there is any sign of fire.
If the checklist does not get heat flowing, the cause is likely ignition, a sensor, a safety switch, a control board or the blower. Those repairs need a technician. A furnace that runs briefly and shuts off, or that blows cool air, has its own set of causes described on the furnace and heating repair page. If the equipment is aging and failing repeatedly, our article on how long an HVAC system lasts helps frame the repair-or-replace question. A yearly check, covered in what a tune-up includes, helps prevent many no-heat calls.
Tell the technician about anything unusual before the visit: a recent filter change, a power flicker, work done in the basement or utility room, or a smell you noticed. Context like this can point straight to a cause that would otherwise take time to find.
Common reasons include a tripped breaker, an off power switch, an open blower door, a clogged filter or a lockout from a failed ignition. Work through the checklist, then request service.
Once. If it trips again, stop and request service. A repeated trip signals an electrical fault.
It is usually a diagnostic code. Compare the pattern with the label inside the furnace door or the manual, and share it with the technician.
Only if the label on your furnace gives instructions and you smell no gas. Many newer furnaces use electronic ignition. If unsure or if gas is present, leave and call your gas utility.
Tell us what your system is doing and we will follow up with next steps.