Furnace Not Heating? Causes and What to Check First

Most no-heat calls come down to a handful of causes, and several of them cost nothing to check. Work through this in order — the free checks first — and stop at the line where it becomes technician work.

Stop now and call for help if: you smell gas, a CO alarm is sounding, you see soot or scorching around the furnace, the burner flame is yellow and unstable rather than crisp blue, or the venting looks damaged. Leave the house for a gas smell or a sounding CO alarm and call the gas utility or emergency services from outside. None of the checks below are worth taking a risk over.

Two very different failures

A dirty flame sensor causes a large share of "runs then quits" calls. The board lights the burner, fails to sense flame within a few seconds, and shuts the gas valve for safety. And the LED on the control board flashes a fault code — the legend is printed inside the access panel.

Could the thermostat be why my furnace is not heating?

Free, and a surprising share of calls end here.

• Set to HEAT, not COOL or OFF.
• Setpoint above room temperature — by a few degrees, so it definitely calls.
• Fan on AUTO, not ON. This is the classic "blowing cold air" cause: with the fan on ON it runs continuously whether or not the burner is firing, so you feel room-temperature air and assume the furnace has failed.
• Batteries. A thermostat with dying batteries behaves erratically or goes blank.
• Schedule and hold. A programmed setback or a vacation hold you forgot about.
• Recently replaced or reconfigured? Wiring or system-type settings on a new thermostat can be wrong. See heat pump thermostat settings for the analogous issue.

2. Power

Also free.

• The breaker for the furnace. Check for tripped, and reset once. If it trips again, stop — a repeatedly tripping breaker is a fault, not a nuisance.
• The furnace service switch. A wall switch near the unit that looks exactly like a light switch and gets turned off by accident constantly. Check it.
• The blower door / access panel switch. Most furnaces have an interlock that cuts power when the panel is off. If someone removed the panel and didn't seat it properly on reassembly, the furnace won't run at all.

Can a dirty filter stop a furnace?

The single most common cause of the "runs then shuts off" pattern.

A clogged filter restricts airflow across the heat exchanger. The exchanger overheats, and the high-limit switch shuts the burner down to protect it. The blower keeps running to cool things off, then the cycle repeats — which is why the symptom looks like short cycling.

• Pull the filter and look at it. If light doesn't pass through, replace it.
• Check the direction arrow on reinstall.
• Don't over-filter. A very high MERV rating in a system not designed for it restricts airflow and causes exactly this problem. See hvac air filters and MERV explained.

While you're there: open every register and make sure returns aren't blocked by furniture or rugs. Closing registers in unused rooms doesn't save energy and does restrict airflow.

4. Gas supply

• Is the gas on? If other gas appliances work (a range, a water heater), supply is fine. If nothing gas-fired works, contact the utility.
• The furnace gas valve should be in the ON position.
• Recently ran out of propane or fuel oil? Obvious once you check it, easy to overlook.

Do not attempt gas piping work, valve replacement, or leak testing yourself.

Can a blocked condensate drain shut off a furnace?

If your furnace is a high-efficiency condensing model — plastic vent pipe out a sidewall rather than a metal flue up a chimney — it produces condensate that has to drain.

A blocked drain backs water up, trips a float switch, and shuts the furnace down. It's an extremely common cause and it's often clearable.

• Check the drain line and pan for standing water.
• Check the condensate pump, if there is one — is it powered, is its float free, is the discharge line clear?
• Clear a blocked trap or line if you can reach it safely.

This one accounts for a lot of mid-winter no-heat calls on newer furnaces.

Why does my furnace ignite then shut off?

Older furnaces with a standing pilot: if the pilot is out, the furnace can't fire. Relighting instructions are usually printed on the unit. Follow them exactly, and if it won't stay lit or goes out repeatedly, stop — that's a thermocouple or gas valve issue.

Modern furnaces use electronic ignition — a hot surface igniter or spark. If the igniter has failed, the furnace will attempt to start and lock out. You may hear the inducer motor run and then everything stop.

A dirty flame sensor produces a distinctive symptom: the burner lights, runs for a few seconds, then shuts down. The sensor confirms flame is present; when it's fouled it can't, so the control shuts off gas as a safety.

All of these are inside the burner compartment. Diagnosis and repair here is technician work. You can note the symptom and the pattern of any diagnostic LED flash codes — most furnaces have one visible through a sight glass, and the code legend is on the inside of the access panel. Reporting the code saves the technician time and you money.

7. Safety switches

Furnaces have several devices that stop operation when something's wrong. If one is tripping, the switch is doing its job — the fault is upstream of it.

• High-limit switch — overheating, usually airflow-related
• Rollout switch — flame escaping the burner area. This is serious; it indicates a venting or heat exchanger problem
• Pressure switch — inadequate draft, often a blocked intake or exhaust pipe, a failing inducer motor, or a blocked condensate line
• Float switch — condensate backup

Check outside if you have sidewall venting: intake and exhaust terminations can be blocked by snow, ice, leaves or a nest. Clearing a snow-blocked termination fixes a genuine share of winter no-heat calls, and it's a good thing to check after any storm.

Do not repeatedly reset a furnace that keeps tripping a safety. Each trip is telling you something. A rollout switch in particular should never be reset without finding out why it tripped.

When should you call a technician about a furnace?

At this point, or earlier if any of the stop conditions apply. Give them:

• The symptom pattern — does it fire and stop, never fire, or fire without heat?
• Any diagnostic flash code
• What you've already checked — filter, drain, breaker, thermostat
• Any noises or smells
• When it started, and whether anything changed around then

That information routinely shortens the visit.

The problems that look like a furnace fault but aren't

• Some rooms cold, others fine. A distribution problem — duct leakage, poor balance, or an undersized run. See ductwork problems and cost.
• The house never quite reaches setpoint on the coldest days. Could be undersizing, but it's more often envelope losses. See how to air seal a house.
• Big temperature swings with the furnace cycling frequently. Often oversizing rather than a fault — see oversized HVAC problems.
• Rising fuel bills. Check duct leakage and envelope before blaming the furnace.

Preventing the next one

Most no-heat calls are preventable:

1. Change the filter on schedule. Set a reminder.
2. Annual service before the heating season, not during it.
3. Keep registers and returns clear.
4. Check sidewall vent terminations after snow and storms.
5. Flush the condensate drain on condensing units before winter.
6. Test CO alarms and replace them at their expiry date — see smoke and CO alarm placement.
7. Have a backup plan — see space heater electrical safety before relying on portable heat, and cost to run a space heater for what it costs.

If the furnace is past 15 years and failing repeatedly, it's time for the replace-versus-repair conversation — see how long does a furnace last.

The bottom line

Check the thermostat, the service switch, the breaker, the filter and (on condensing units) the condensate drain before anything else — those five cover a large share of no-heat calls and cost nothing. Note any diagnostic flash code. Stop at the burner compartment, and stop immediately for gas smell, a CO alarm, soot, or a yellow flame. And never keep resetting a furnace that keeps tripping a safety switch.

Compare heating options if it's time to replace: heat pump payback calculator.