Furnace blowing cold air or shutting off on a freezing night

Why a furnace blows cold air or shuts off

A furnace blowing cold air is often caused by a setting, a clogged filter or a safety shutoff. It is not always a broken part. The blower can keep running while the burners stay off, and that feels like cold air from the vents.

Below are the usual causes and the checks that are safe on a cold night, then the point where the work belongs to a licensed contractor.

What makes a furnace blow cold air?

Start with the thermostat. If the fan is set to "on" instead of "auto," the blower runs all the time, and the air can feel cool between heating cycles. Dead batteries or the wrong mode can also keep the burners from starting.

A clogged air filter is the next suspect. When airflow is choked, the furnace gets too hot and a safety switch shuts the burners off. The blower keeps moving air until things cool down, so the vents blow cool air. That is overheating, and it tends to repeat.

A dirty flame sensor looks different. The furnace lights, then the flame drops out after a few seconds because the sensor can't confirm it. The burners stop and the vents blow room-temperature air.

If the safe checks in the next section change nothing, furnace repair is the next step.

What can I safely check on a cold night?

These checks need no tools and don't open the furnace:

1. Set the thermostat to heat and the fan to auto. Raise the temperature a few degrees above the room, and replace the batteries if the display is dim.
2. Pull out the air filter. If it is gray and packed with dust, put in a clean one of the same size.
3. Make sure the vents and returns are open and not blocked by furniture or rugs.
4. Look at the breaker panel. If the furnace breaker tripped, reset it once. If it trips again, leave it off.

After these checks, let the furnace run a full heating cycle before you decide anything. A furnace that warms up and stays warm was likely a setting or a filter.

What should stay with a licensed contractor?

Some causes sit behind the furnace panel. Gas burners and valves, electrical parts and wiring, and sealed components are not homeowner work. A licensed contractor tests the safety switch, cleans or replaces a flame sensor, and checks the burners.

Don't remove panels or bypass a safety switch to get heat back. That switch is shutting the furnace down for a reason. To see what a visit usually covers, read what a contractor checks during a furnace repair visit.

What if I smell gas or a carbon monoxide alarm sounds?

Stop troubleshooting and follow these steps.

If you smell gas, leave the house, then call the gas utility from outside.

If a carbon monoxide alarm sounds, get everyone out and call emergency services.

Don't flip switches, light anything or go back in to check the furnace first.

How do freezing nights change the picture?

A furnace works hardest when the weather is coldest. The NOAA weather station near Spanish Fork averages 118 days a year with a low of 32°F or below, so a furnace on that kind of schedule may run long stretches through winter.

Extremes happen too. The coldest checked night in NOAA's daily record for that station fell to -20°F, on 1989-02-07. Long runs like that are when a clogged filter or a weak flame sensor shows up as repeated shutoffs.

The average December to February daily low at the station is 23.5°F. A furnace that quits on a night like that leaves a house cooling down, so the safe checks are the place to start.

When to call about a furnace that keeps quitting

Call when the checks are done and the furnace still blows cold air, lights and drops out, or trips the breaker again. Those patterns point to parts a homeowner should not touch.

Where a contractor is available, the line connects the call to that contractor. Fees are a separate matter. Each independent contractor confirms its own fees. For more on the service itself, see furnace repair in Spanish Fork.