This post overlaps with a fuller one: Heater Vent Blowing Cold Air? Start With the Fan Setting.
A heater blowing cold air almost always comes down to one of four things: the thermostat fan is set to ON so the blower runs between heating cycles, a clogged filter is making the furnace overheat and shut its burners off, the pilot or igniter is not lighting, or the ducts in the attic are losing the heat before it reaches the room.
The causes, in order of likelihood
- Fan on ON instead of AUTO. Room-temperature air moving through a vent feels cold. Switch to AUTO.
- Warm-up delay. A short burst of cool air at the start of a cycle is normal.
- Dirty filter, high-limit trip. Heat, then cold, then heat again is the pattern. The furnace is protecting its heat exchanger.
- Pilot out or ignition fault. Standing pilots on older wall and floor furnaces in Pasadena, CA bungalows go out; newer igniters and flame sensors fail and the furnace locks out.
- Leaky attic ducts. A split seam or a fallen boot in a cold attic loses the heat on the way.
- Heat pump defrost. A few minutes of cool air while the outdoor coil defrosts is normal.
- Thermostat. Mode on COOL, a schedule, or dead batteries.
What to check, then when to call
Set the thermostat to HEAT and AUTO, replace the filter, open every register, and make sure the furnace switch and breaker are on (reset a breaker once, not twice). If the pilot is out on an older unit, relight it once following the instructions on the furnace's label. If it will not stay lit, or you smell gas, stop and call. Anything inside the cabinet, a furnace that lights and drops out, or a room that stays cold while the furnace runs fine is a job for a licensed tech.
Book a $69 diagnostic, credited toward the work, at Boost Services at 818-277-5056 or send us a message.
The full guide: Heater Vent Blowing Cold Air? Start With the Fan Setting
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.