Furnace blowing cold air? Here's what to check first.

Start with the thermostat fan setting

This is the one that embarrasses people, and it is genuinely the most common cause. If your thermostat fan is set to ON instead of AUTO, the blower runs constantly — including between heating cycles, when there is no flame. You feel room-temperature air coming from the vents and assume the furnace has failed.

Set the fan to AUTO. If the air turns warm within a few minutes, that was it.

Check the filter

A clogged filter chokes airflow across the heat exchanger. The furnace overheats, a safety limit switch shuts the burners down, and the blower keeps running to cool things off — so you get cold air from the vents while the unit cycles on and off.

Pull the filter out and hold it up to a light. If you cannot see light through it, replace it. If the furnace has been short-cycling for a while, give it an hour after the change before judging whether it fixed anything.

Make sure it is actually getting power and fuel

There is usually a switch that looks like a light switch on or near the furnace. It gets flipped by accident more often than you would think, especially in basements. Check it, check the breaker, and check that the gas valve is open.

If your thermostat is battery powered, weak batteries can leave the display on while failing to call for heat. Fresh batteries cost a dollar and rule it out.

If the burners light and then go out

Watch the furnace through its inspection window. If you see it ignite and then shut down a few seconds later, that usually points to a dirty flame sensor — a thin metal rod that confirms a flame is present. When it is coated, the furnace cannot confirm ignition and shuts the gas off as a safety measure.

Many high-efficiency furnaces also shut down when the condensate drain backs up. If there is water pooling around the base of the unit, that is worth mentioning when you call.

Both are routine repairs for a technician. Neither is something to improvise with on a gas appliance.

When to stop and call someone

If the furnace is short-cycling, making a booming or scraping noise, or you have gone through the checks above with no change, it needs a technician. Repeated overheating damages the heat exchanger, and a cracked heat exchanger is a carbon monoxide problem, not a comfort problem.