Furnace Blowing Cold Air? Here Is Why

Furnace blowing cold air

The two free things to check first, then the five that need a technician.

Short answer. If the air is cold but the furnace is running, check the thermostat fan setting first: on ON, the blower runs constantly and pushes unheated air between burn cycles, which feels like cold air from every vent. If it is already on AUTO, the next most likely cause is a clogged filter making the furnace overheat, which shuts the burners off while the blower keeps running to cool the heat exchanger.

Check the fan setting before anything else

Your thermostat has two separate controls that people mix up constantly. One sets the mode, HEAT or COOL. The other sets the fan, ON or AUTO.

On AUTO, the blower only runs while the furnace is actually burning gas, so the air coming out is warm. On ON, the blower runs continuously, including during the long gaps between burn cycles. During those gaps it is pushing room temperature or cooler air through your ducts, and out of a vent that feels distinctly cold.

Switch the fan to AUTO. If the cold air stops between cycles, nothing was ever broken. This single setting is behind a large share of cold air complaints, and it costs nothing to rule out.

If the fan is already on AUTO

Then the furnace is genuinely failing to make heat, or making it and then stopping. The cause is usually one of these.

A clogged air filter. This is the most common real fault behind cold air, and it produces a very specific pattern: the furnace lights, warms up, then the burners cut out while the blower keeps running, sometimes for a long time. That is the high limit switch doing its job. Restricted airflow lets the heat exchanger get too hot, so the furnace kills the burners but keeps the fan on to cool it down. Pull the filter and hold it to a light. If light does not pass through, replace it.
A dirty flame sensor. The furnace ignites, runs for a few seconds, then the flame drops out while the blower carries on. You may hear it try two or three times before locking out entirely. This is a technician job, but an inexpensive one.
The pilot light is out. Only applies to older units with a standing pilot rather than an electronic ignitor. If you have a wall or floor furnace, which much of the older East Bay housing stock does, this is the first thing to look at.
No gas reaching the furnace. Check whether your stove and water heater work. If nothing gas powered works, the problem is upstream and it is a utility issue.
A blocked condensate drain. On high efficiency furnaces, the ones venting through white PVC pipe, a clogged drain trips a float switch that prevents ignition. You will often see water near the base of the unit.
Duct losses. If the air is warm at the furnace but cool at the far vents, the ducts are leaking or uninsulated. In a lot of Bay Area homes the ductwork runs through an unconditioned crawlspace or attic, so a disconnected run can dump most of your heat under the house.

Match the pattern to the cause

What is happening	Most likely cause	Who fixes it
Cold air constantly, from every vent, even when idle	Fan set to ON instead of AUTO	You
Warms up, then goes cold, blower keeps running	Clogged filter tripping the high limit switch	You
Lights, cold within seconds, tries again	Dirty flame sensor	Pro
Never lights at all, older wall or floor unit	Pilot light out	You, then a pro
Warm at the furnace, cool at distant rooms only	Duct leakage or missing insulation	Pro
Water pooling near the base, no ignition	Condensate drain blocked, float switch tripped	Pro
Nothing gas powered in the house works	Gas supply, not the furnace	Utility

When to stop

If you smell gas, do not troubleshoot. Leave the house, then call PG&E at 1-800-743-5000 or 911 from outside. If a carbon monoxide alarm has sounded, get everyone out and call 911 before anything else.

Beyond that, a furnace that repeatedly overheats and shuts down is worth taking seriously even though it is not dramatic. Repeated overheating is what cracks a heat exchanger, and a cracked heat exchanger is a carbon monoxide problem rather than a comfort one. Replacing a filter is cheap. Ignoring the pattern that keeps tripping the limit switch is not.

FAQ

Common questions

Why is my furnace blowing cold air when the heat is on?

The most common reason is the thermostat fan being set to ON rather than AUTO, which runs the blower continuously and pushes unheated air through the vents between burn cycles. If the fan is already on AUTO, the usual cause is a clogged filter making the furnace overheat, so the burners shut off while the blower keeps running.

Why does my furnace blow warm air then cold air?

That pattern points at the high limit switch. The furnace heats up, airflow is too restricted to carry the heat away, the temperature inside rises past a safe point, and the control board shuts the burners off while keeping the blower running to cool things down. A clogged filter is the usual culprit, followed by closed or blocked vents.

Should the furnace blow cold air at the start of a cycle?

A few seconds of cooler air at the very start is normal on some systems, because the blower can begin slightly before the heat exchanger is fully warm. Cold air for more than about a minute, or cold air throughout the cycle, is not normal.

Can a dirty filter really stop my furnace heating?

Yes, and it is one of the most common service calls there is. The furnace needs a certain volume of air moving across the heat exchanger to carry heat away. A filter that has not been changed in a year restricts that enough to trip the safety switch, which shuts the burners off.

Why is my heater blowing cold air and not shutting off?

That combination almost always means the burners have been shut down by a safety switch while the blower continues running. The blower is deliberately kept on to cool the heat exchanger. Check the filter first, then closed vents and blocked returns.