furnace troubleshooting
Furnace is blowing cold air
What it probably is, what you can safely check yourself, and the point where it becomes our job.
It is still running, so you have a few days. It is not going to fix itself, and the part that failed rarely fails alone.
What is most likely going on
Either the burner is lighting and then dropping out while the blower keeps running, or the fan is simply set to on and moving unheated air. Listen for a brief burner light followed by a shutdown, and check the thermostat fan setting.
Likely causes, most common first
- 1
Thermostat fan set to “on” instead of “auto”
With the fan in the on position, the blower runs continuously and moves room-temperature air between burner cycles. It feels like cold air blowing and it is simply a setting. Some people run it deliberately for filtration and air mixing, so it is worth knowing which you have chosen.
- 2
Burner dropping out on a dirty flame sensor
A furnace that lights, runs for a handful of seconds, then shuts the gas off while the blower continues will feel exactly like a cold-air furnace. The board is not seeing flame proof and is doing the safe thing. It is a common and inexpensive service item.
- 3
Overheating on the high limit
A blocked filter, closed registers or a failing blower cause heat to build in the heat exchanger until the limit switch cuts the burner, leaving the blower running to purge. The cycle repeats every few minutes. Repeated limit trips are hard on a heat exchanger, so this one should not be left alone.
- 4
Blocked flue or condensate causing a pressure switch trip
Mid-cycle draft faults shut the burner while the blower finishes its off-delay. Blocked sidewall terminations, a plugged condensate trap on a high-efficiency furnace, or a frozen drain in an unheated crawlspace are common local causes during a cold snap.
- 5
Fan control or board fault
A stuck fan relay or a failing control board can run the blower without a heating call. This is diagnosed with a meter at the board rather than by observation from the hallway, and it should be sorted before the pattern is blamed on the thermostat.
What you can safely check yourself
Work down the list before you book anything. Plenty of calls end here, and we would rather you keep the money.
- Set the thermostat fan to Auto rather than On and see whether cold air stops between cycles.
- Replace the filter, and open all supply registers and clear return grilles of furniture.
- Listen through one full cycle and note whether you hear the burner light and then shut off within seconds.
- Confirm the furnace panel door is closed and the door switch is engaged.
- Check the outdoor vent terminations for snow, ice or nesting debris.
What is deliberately not on that list
Nothing above involves refrigerant, gas piping, or opening a live electrical panel. Refrigerant work is federally regulated and needs recovery equipment. Gas work done wrong kills people. An energised panel turns a $300 repair into a hospital visit. Electrical work on the circuit itself is done by a licensed electrical contractor. If a step you have read elsewhere involves any of those three, stop and call instead.
When to call us
Call us if the burner lights and drops out repeatedly, or if the blower runs cold with the fan in Auto. Repeated limit and flame-proof failures should be diagnosed rather than lived with through a winter. Reach us at (360) 592-3824 during regular hours of Monday–Friday 8:00 AM–5:30 PM.
Monday–Friday, 8:00 AM – 5:30 PM · Emergency calls answered 24/7
Tell us what it is doing
Describe the symptom and we will tell you what it takes to fix, what it is likely to cost, and whether it is worth fixing at all.
Monday–Friday, 8:00 AM – 5:30 PM · Emergency calls answered 24/7