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Homeowner guide · AC and heating repair

Furnace blowing cold air? What to check first

A furnace blows cold air most often because the thermostat fan is set to On instead of Auto, the filter is so dirty the furnace overheats and shuts its burners off, or the burners won't light. Check the thermostat and filter yourself. Ignition, gas valve and flame sensor problems are jobs for an HVAC pro. If you smell gas, leave the house and call your gas utility from outside.

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Quick answer

Start here

  • 1

    Thermostat on Heat, fan on Auto

  • 2

    Filter clean

  • 3

    Furnace switch and breaker on

  • 4

    No gas smell

Plain-English, no jargon

1. Check the thermostat

With the fan set to On, the blower runs all the time, so it pushes room-temperature air between heating cycles. Set it to Auto. Make sure the mode is Heat and the set temperature is above the room temperature. Replace weak batteries.

2. Give it a minute

Many furnaces blow a little cool air for a moment while the heat exchanger warms up. If you have a heat pump, it may blow cooler air for a few minutes while it defrosts in cold weather. Both are normal.

3. Change the filter

A clogged filter traps heat inside the furnace. A safety switch then shuts the burners off while the blower keeps running, so you feel cool air. A new filter often fixes it.

4. Leave the burners to a pro

If the burners won't light or keep shutting off, the cause may be the igniter, the flame sensor, the gas supply or a blocked drain on a high-efficiency furnace. These involve gas and electrical parts. Call an HVAC technician.

FAQ

Common questions

Why does my furnace blow cold air at first?

Many furnaces start the blower shortly after the burners light, so the first air can be cool. It should warm up within a minute or two.

Can a dirty filter make a furnace blow cold air?

Yes. It can make the furnace overheat, which trips a safety switch that turns the burners off while the fan keeps blowing.

Is it safe to keep using a furnace that's acting up?

If you smell gas, hear the CO alarm or see scorching, shut it off and get help. Otherwise, have it checked soon rather than running it while it misbehaves.

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Furnace Blowing Cold Air? What to Check First | Job Hound