Furnace Running But Not Heating?
Answer a few questions and Fixly Guide will help you understand what may be happening, how urgent it is, and whether this is a repair or a replacement conversation.
Stop and Get Help If You See This
Treat it as urgent if you see:
- A gas smell anywhere in the home
- Soot, scorching, or a rumbling sound from the furnace
- A carbon monoxide alarm sounding
- Repeated burner short-cycling with a burning odor
- Visible flames outside the burner compartment
Gas odors, soot, scorching, or a carbon monoxide alarm mean you leave the home first and call from outside. Never bypass a safety switch to keep a furnace running.
Quick answer
Start with the cheap, safe things: thermostat mode and batteries, a fresh filter, the furnace switch, the breaker, and the blower door. If the burner still will not light — or lights and shuts off within a minute or two — you are into ignition, flame sensing, condensate, or gas supply territory, and that is a licensed HVAC call. This page is educational information, not a diagnosis of your equipment.
Possible causes
Thermostat settings or batteries
A thermostat set to "fan on" instead of "heat", or with dead batteries, mimics a dead furnace.
Clogged air filter
A restricted filter overheats the heat exchanger and trips the high-limit switch, so the burner short-cycles.
Ignition or flame sensor failure
A dirty flame sensor or failed igniter is the most common no-heat call on modern gas furnaces.
Blocked condensate drain
High-efficiency furnaces shut down on a full condensate trap or clogged drain line.
Gas supply or valve problem
A closed valve, tripped meter, or failing gas valve stops combustion entirely.
Blower motor or capacitor
The burner may fire while the blower fails to move heated air into the ducts.
Cracked heat exchanger
A safety concern, not just a comfort one — a cracked exchanger usually means replacement.
Duct leaks or closed dampers
Heat produced but lost in the attic or crawlspace leaves rooms cold with the system running.
How this project is usually classified
General guidance for this problem type — not a diagnosis of your home.
- Typical urgency
- Emergency — act now
- Safety risk
- High safety risk
- DIY suitability
- Pro recommended
- Cost level
- Moderate cost
- Repair option
- Igniter, sensor, or control replacement after a combustion safety check.
- Replacement option
- Immediate replacement when the heat exchanger is cracked.
- Typical timeline
- Emergency in freezing weather; most repairs same day
- Contractor type
- hvac
What you can safely check yourself
- Confirm the thermostat is set to heat and the setpoint is above room temperature.
- Replace the air filter if it is grey, matted, or older than 90 days.
- Check that the furnace switch (it looks like a light switch) and the breaker are on.
- Look at the furnace door switch — a loose blower door prevents startup.
- Note whether the blower runs, the burner lights, or nothing happens at all.
Before the pro arrives
- • Know the furnace age, fuel type, and brand from the label inside the door.
- • Write down any error codes blinking on the control board.
- • Note whether the problem is constant or intermittent.
- • Have service records for prior repairs ready.
Questions to ask
- • What exactly failed, and what caused it to fail?
- • Is the heat exchanger intact — how did you verify that?
- • Given the age of this unit, is a repair the right call?
- • What does the repair warranty cover, and for how long?
- • Will replacement require duct or venting changes?
Common questions
Why is my furnace blowing cold air?
Most often the thermostat fan is set to "on" instead of "auto", so the blower runs between burner cycles. If the burner never lights at all, a dirty flame sensor, failed igniter, blocked condensate drain, or gas supply issue is more likely.
Can a dirty filter really stop a furnace?
Yes. A clogged filter restricts airflow, the heat exchanger overheats, and the high-limit switch shuts the burner down. The furnace short-cycles and the house never warms up.
When should a furnace be replaced instead of repaired?
Generally when the unit is past about 15 years, the repair approaches a third of replacement cost, or the heat exchanger is cracked. A cracked exchanger is a combustion safety issue, not a comfort one.
What should I do if I smell gas?
Leave the home immediately, do not flip switches, and call your gas utility or 911 from outside. Do not troubleshoot the furnace yourself.
Keep reading
Other common home problems
Start with the symptom and get likely causes, safety guidance, and a next step.
Fixly Guide provides educational guidance only. Nothing here is a professional diagnosis, an inspection, or a guarantee of contractor availability or pricing.