What to do when your furnace runs but will not heat
When a furnace is not heating, the cause is often simple: thermostat settings, a tripped breaker, a dirty filter, a closed gas valve or a blocked vent or drain. Check those in order, using only safe steps, and call a licensed technician if the furnace still will not start or you smell gas.
What should you check first when the furnace is not heating?
Begin at the wall. Furnace problems often start with the thermostat, which costs nothing to check.
- Set the thermostat to heat and the temperature a few degrees above the room reading. Make sure the fan is on auto, not just on.
- Replace the thermostat batteries if it uses them, and check that a schedule or hold setting is not overriding your choice.
- Wait a few minutes. Some furnaces have a delay before the burner and blower start.
- Check that the furnace is on. Most have a switch that looks like a light switch on or near the unit.
- Look at the electrical panel and reset a tripped breaker once. If it trips again, leave it off and call a technician.
- Check that the furnace door or panel is closed. Many furnaces have a door switch that cuts power when it is open.
If the furnace now starts and heat returns, make a note of what fixed it. If it did not, move to the air and fuel side.
Could a dirty filter be the problem?
Yes, often. A clogged filter restricts airflow, so the furnace heats up fast, the safety limit shuts the burner down, and the blower keeps running with cool air. The result is a furnace that runs but does not heat, or heats only for short bursts. Pull the filter and hold it up to the light. If you cannot see through it, replace it with the same size. Check that vents and return grilles in the house are open and not blocked by furniture, rugs or curtains.
Filters are one of the cheapest forms of maintenance, and our heating maintenance checklist shows how to fit them into a seasonal routine. A weak but running blower, a different fault, is covered in the guide to blower motor problems.
What about the gas supply, vents and drain?
A gas furnace needs fuel, air in and exhaust out. Check that the gas shut-off valve near the furnace is open: the handle lines up with the pipe when it is open.
High-efficiency furnaces send exhaust and bring in air through plastic pipes that usually exit through a wall. In winter, snow, ice or a bird nest can block them, and the furnace will shut itself off. Clear snow or debris from the outside openings by hand, without poking anything into the pipes. These furnaces also produce condensate that drains away, and a clogged drain line or full pump can trip a safety switch. If water is pooling near the furnace, turn it off and call a technician.
Older furnaces may have a standing pilot. Relight it only if you are comfortable following the instructions on the unit's label. Otherwise leave it to a professional.
When should you stop and call a technician?
Some situations are not for homeowners. If you smell gas, or a carbon monoxide alarm sounds, leave the house immediately and call the gas utility or emergency services from outside. Do not turn lights or switches on or off. Call a licensed technician if you see soot, scorch marks or a yellow flame, if the furnace keeps cycling off, if the breaker trips repeatedly, if there are burning smells, rattling or banging, or if the furnace is still cold after the checks above.
A technician will check the igniter, flame sensor, gas valve, pressure switches, limit switch, control board and heat exchanger, parts that need tools and training. The winter heating emergency guide explains how to stay warm while you wait. Ask for a written diagnosis and a quote before any large repair, and if the furnace is old, consider weighing the cost against replacement with the repair or replace guide.
How can you prevent a no-heat failure?
Most no-heat calls come from neglected basics. Change the filter on schedule, keep the area around the furnace clear, keep outdoor vent pipes free of snow, and book a professional inspection each autumn, as described in the annual tune-up guide. Keep a carbon monoxide alarm working near bedrooms. If the furnace is near the end of its life, plan the replacement before the cold, using the furnace replacement guide.
Quick answers
Why is my furnace running but not blowing hot air?
Common causes are a dirty filter, a thermostat setting, a blocked vent or a safety limit shutting off the burner. A technician can find the cause if the simple checks do not.
Can I reset my furnace myself?
You can reset a tripped breaker once and, if your furnace has a reset button, press it once as the unit's label directs. If it fails again, call a technician.
What if my furnace blows cold air?
Check the thermostat fan setting, the filter and that the gas valve is open. If it still blows cold, the burner may be locking out for safety, so call a technician.
Is a furnace that will not heat dangerous?
It can be if there is a gas smell or a carbon monoxide alarm. In that case leave the house and call the utility or emergency services.