Your furnace is running but not heating
Safety first, then the four things it usually turns out to be.
Is this an emergency?
You’re probably seeing
- The blower runs but the air from the vents is cool or cold
- The furnace starts, runs for a minute or two, then shuts down before the house warms
- You hear the igniter click and the gas valve open, but no flame catches
- The furnace does nothing at all when the thermostat calls for heat
- The house is colder in some rooms than others while the furnace runs constantly
What it probably is
The thermostat is not actually calling for heat
Most common, and free to rule outFan set to ON rather than AUTO blows room-temperature air between burns, which feels exactly like a furnace that will not heat. Dead thermostat batteries produce the same thing.
A clogged filter tripping the high-limit switch
Very common, and the filter is yours to changeThe furnace runs a few minutes, gets hot, then shuts off before the house warms up - and repeats. Starved of return air, it overheats and protects itself.
A dirty flame sensor
Very common on furnaces over five years oldThe burners light, you see flame for a few seconds, then everything shuts down. A sooted sensor cannot prove the flame exists, so the control board closes the gas valve.
A failed igniter
Common - a technician job, but a routine oneYou hear the sequence start and the inducer fan run, but no glow and no flame. On a hot-surface igniter this is a wear part with a finite life.
A blocked flue or condensate line
Seasonal - common after storms or a long idle summerThe furnace locks out and will not restart, often with a flashing error code. High-efficiency furnaces refuse to fire when the condensate drain backs up.
A cracked heat exchanger
Least common - and the one that ends the diagnosis immediatelySoot around the burners, a flame that moves or rolls when the blower starts, or a CO alarm. This is the dangerous one.
What to do before anyone arrives
Rule out a carbon monoxide problem before anything else
If your CO alarm has sounded, if you smell gas, or if people in the house have headaches, nausea or dizziness that improve when they leave, get everyone outside and call from there. CPSC notes that CO symptoms mimic flu, which is why a household that feels unwell during a heating fault deserves the benefit of the doubt.
Do not go back in to investigate, open windows, or switch anything off. Leave first. Call the gas utility or the fire service from outside.
Check the thermostat is on HEAT, the fan on AUTO, and the setpoint above room temperature
Raise it five degrees and listen. If it is a battery thermostat with a blank or faint screen, change the batteries before anything else - this is a genuinely common cause of "the furnace is dead".
Pull the filter and hold it to a light
If light does not pass through it, replace it. A blocked filter is the most common reason a furnace lights, overheats and shuts down a few minutes later. Note the size on the cardboard edge.
Check the furnace door switch and the service switch
The furnace will not run with the front panel off - a safety interlock cuts it. Make sure the panel is seated properly. Also check the light-switch-looking service switch on or near the unit, which gets turned off by mistake more often than anyone admits.
Check the breaker, and look for a condensate backup
Reset a tripped breaker once. On a high-efficiency furnace with a PVC flue, look for a full condensate trap or a blocked drain line - many will refuse to fire until it clears.
If the breaker trips again, leave it off and call someone.
Read the diagnostic LED, then stop
Most furnaces flash an error code through a small window in the front panel. Count the flashes and photograph the legend printed inside the door. Telling a technician the code on the phone saves diagnostic time and money.
This is the end of the homeowner sequence. Flame sensors, igniters, gas valves and heat exchangers all sit behind the burner compartment, on a live gas appliance. Do not go further.
What it costs to fix
Ranges below are for the repair itself. For full replacement pricing see the hvac cost guides.
| What it is | What the part costs | What decides your bill |
|---|---|---|
| Thermostat batteries | A couple of dollars | Nothing - no visit needed |
| New air filter | A few dollars | Nothing - this one is yours |
| Flame sensor clean | No part at all in most cases | Entirely the visit and the labour |
| Hot-surface igniter | A modest part | Mostly labour and truck time, plus access |
| Blocked condensate clear | No part in most cases | The visit, unless the blockage has damaged the board |
| Cracked heat exchanger | A major component | This is the repair-or-replace decision, not a repair quote |
U.S. Bureau of Labor Statistics, Occupational Outlook Handbook (May 2025 wage data).
Read this as the shape of the bill, not a quote. No primary source publishes a national installed price for these repairs, and the aggregator figures that do are undated, so we do not print one. What you can anchor on: the national median wage for HVAC mechanics and installers is $29.33 an hour (BLS, May 2025), billed on top of a diagnostic call. A flame-sensor clean therefore costs almost nothing in parts and is still a legitimate paid visit. If a quote is far outside that shape, ask what portion is diagnosis, labour and parts.
Can I fix this myself?
A gas furnace burns fuel inside your house and vents the products outside. If you smell gas, hear hissing, see soot around the burners, or a CO alarm sounds, leave the building and call from outside - do not investigate first. CPSC records more than 200 unintentional non-fire carbon monoxide deaths a year from consumer products, with more than half falling in the four cold months of November to February.
Reasonable to DIY
- Changing thermostat batteries and checking the mode and setpoint
- Replacing the air filter
- Making sure the front panel and service switch are on
- Resetting a tripped breaker once
- Reading and reporting the diagnostic flash code
Call a pro
- Anything inside the burner compartment, including the flame sensor
- Igniter, gas valve, pressure switch and control board work
- Flue and venting inspection
- Any heat exchanger inspection or diagnosis
- Any work at all after a CO alarm has sounded
The first four checks on this page are genuinely yours and resolve a large share of no-heat calls in under ten minutes - a filter and a set of thermostat batteries cost less than a service call. Past that point you are working on a live combustion appliance, and the failure modes that are left are the ones that vent carbon monoxide into the house when they go wrong. Get the flash code, then hand it over.
Is this covered by insurance?
Whether a claim is worth filing depends on the cause, your deductible and how the damage happened - not on the repair price alone.
Sources
Sources last checked
- Carbon Monoxide Information Center - Home Heating EquipmentU.S. Consumer Product Safety CommissionRetrieved 2026-09-14
- Carbon Monoxide - research and statisticsU.S. Consumer Product Safety CommissionRetrieved 2026-09-14
- Heating, Air Conditioning, and Refrigeration Mechanics and Installers - median pay, May 2025U.S. Bureau of Labor Statistics, Occupational Outlook HandbookRetrieved 2026-09-14
Sources for the figures and the legal requirements cited on this page.
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