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Furnace blowing cold air: why it happens and what it costs
A furnace that moves air without heating it is usually lighting and then dropping out, or running its blower on the wrong setting. Both are common and neither is expensive to diagnose.
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Cold air from a running furnace usually means the burner is lighting then dropping out, commonly from a dirty flame sensor, or the fan is set to on rather than auto. Call (855) 321-3194 and we connect you with a local heating provider.
What you are seeing
Equipment symptoms
- The burner lights then goes out after a few seconds
- Air starts warm then turns cold mid-cycle
- The blower runs constantly between heating cycles
- Short heating cycles that never satisfy the thermostat
Distribution symptoms
- Cold air only from certain registers
- Ductwork running through an unheated attic or crawlspace
- A visibly disconnected duct run
- One floor noticeably colder than another
Do and do not
| Do | Do not |
|---|---|
| Check whether the fan is set to auto rather than on | Assume constant airflow means the furnace is broken |
| Time how long the warm air lasts before it turns cold | Keep raising the setpoint to force more heat |
| Replace a loaded filter | Close registers in unused rooms to redirect heat |
| Report whether the burner lights at all | Reset the furnace repeatedly between attempts |
| Leave the building and call 911 if you smell gas | Investigate a sounding carbon monoxide alarm from inside |
What causes it, ranked by how often
- Dirty flame sensor. The burner lights, the sensor fails to confirm within a few seconds, and the board shuts the gas off while the blower keeps running. Warm air for a moment, then cold.
- Fan set to on rather than auto. The blower runs continuously, including between heating cycles, so unheated air comes from the registers much of the time. Not a fault at all.
- Overheating and limit tripping. Restricted airflow trips the high limit, the gas shuts off, and the blower keeps running to cool the exchanger. Cycles short and never satisfy.
- Duct losses in unconditioned space. Heated air cools in an attic or crawlspace before reaching the register. The furnace is fine, the distribution is not.
- Pilot or ignition fault on older units. An older furnace with a standing pilot that has gone out will circulate air with no heat behind it.
- Oversized furnace short cycling. A furnace too large for the house satisfies the thermostat before the exchanger is properly warm, so much of the delivered air is lukewarm.
How a technician diagnoses it
- Watch a full heating cycle
Where in the cycle the heat disappears points directly at the cause.
- Check the thermostat fan setting
Ruled out in seconds, and it explains a surprising share of these calls.
- Measure the flame signal
A weak microamp reading confirms a sensor problem rather than a gas supply fault.
- Check static pressure and the limit
Establishes whether restricted airflow is causing the furnace to overheat and cut out.
When to shut the system off
- You smell gas. Leave the building and call the gas utility or 911 from outside.
- A carbon monoxide alarm sounds. Leave and call 911 rather than investigating.
- There is a burning smell beyond the first run of the season.
- The furnace is cycling on and off every couple of minutes.
Service pages
Related guides
Safety first
Call now, or call the utility first?
Call now
- No cool air during a heat advisory
- No heat with outdoor temperatures below freezing
- Burning smell from the vents
- Water pooling at the air handler
- Ice on the line set
- Breaker tripping on every cycle
Call (855) 321-3194 and give your ZIP code.
Shut it off and call the utility or 911 first
- You smell gas: leave the building, then call the gas utility or 911
- Smoke or sparking at the equipment
- A carbon monoxide alarm is sounding: leave and call 911
Reference: CPSC recalls and US Fire Administration heating fire safety.
Questions
Furnace blowing cold air questions
Why does my furnace blow warm then cold?
The burner is lighting and then dropping out, most often because a dirty flame sensor cannot confirm the flame. The board shuts the gas off for safety while the blower continues, so warm air is followed by cold.
Should the fan be on auto or on?
Auto for most homes. On that setting the blower runs only when there is heat to move. Set to on, the fan runs continuously and delivers unheated air between cycles, which reads as a fault but is not one.
Can ductwork cause cold air?
Yes. Ducts running through an unheated attic or crawlspace lose heat along the way, and a disconnected run can dump heated air into that space entirely. The furnace tests fine while rooms stay cold.
Is this dangerous?
Usually not in itself. It becomes dangerous if accompanied by a gas smell, a sounding carbon monoxide alarm or soot around the burners. Any of those means leaving the building before making a call.
Why are the heating cycles so short?
Either the furnace is overheating and tripping its limit switch, usually from restricted airflow, or it is oversized for the house and satisfies the thermostat before the heat exchanger is properly up to temperature.
What does this cost to fix?
A flame sensor clean runs a figure the provider confirms on site and airflow correction a figure the provider confirms on site. A limit switch is a figure the provider confirms on site. Duct repair runs considerably higher at a figure the provider confirms on site depending on access.
Written and fact-checked by the HVAC Emergency USA editorial team. Last updated . Editorial policy.