Most of the time a furnace blows cold air because the thermostat fan is set to ON instead of AUTO, the filter is clogged, or the furnace has shut its burners off for safety. Start with the thermostat and filter. If the air is still cold after that, the problem is usually inside the furnace (ignition, flame sensor, gas supply or a safety switch) and needs a technician.
Check these first (takes 5 minutes)
- Thermostat fan setting. If the fan is set to ON, the blower runs all the time, even between heating cycles, so you feel room-temperature air. Switch it to AUTO and wait a few minutes.
- Thermostat mode and setpoint. Make sure it says HEAT and the setpoint is at least 3 degrees above the room temperature. Replace weak batteries if the screen is dim or blank.
- Air filter. Pull the filter out and hold it up to a light. If you can't see light through it, replace it. A clogged filter can make the furnace overheat and shut the burners off while the fan keeps running.
- Furnace power switch and breaker. The switch looks like a light switch on or near the furnace. Make sure it is on and the breaker is not tripped.
- Condensate drain (high-efficiency furnaces). If your furnace has white plastic vent pipes, look at the small drain hose and any condensate pump. Standing water or a full pump can trip a float switch that stops heating.
7 common causes of cold air from a furnace
- Fan set to ON: normal behavior, not a breakdown.
- Dirty filter: the furnace overheats, the high-limit switch shuts off the burners, and the blower keeps pushing air to cool it down.
- Dirty flame sensor: the burners light for a few seconds, then shut off because the furnace can't confirm a flame.
- Ignition problem: a worn hot-surface igniter or pilot problem means the gas never lights.
- Gas supply issue: a closed gas valve or a supply problem leaves nothing to burn.
- Blocked drain or vent: on 90%+ furnaces, a clogged condensate line or blocked intake/exhaust pipe (snow, ice, nests) trips a safety switch.
- Leaky ductwork: if only some rooms get cold air, warm air may be escaping into the attic, crawlspace or walls before it reaches you.
Is it normal for a furnace to blow cold air at first?
Yes, for a short time. When a heating cycle starts, the burners light first and the blower waits until the heat exchanger warms up. Most furnaces start the fan within about a minute. At the end of a cycle, the blower also keeps running briefly to pull leftover heat out, and that air gets cooler near the end. Cold air that lasts more than a few minutes is not normal.
Also keep in mind that heat from a furnace often feels cooler than you expect. Supply air is usually warm, not hot, and it can feel cool on wet skin or near a drafty window. If you have a thermometer, hold it in front of a supply register for a minute; air that is well above room temperature means the furnace is heating.
What not to do
Do not remove the burner compartment door, try to light the burners by hand, or tape down or bypass any safety switch. Those switches shut the furnace off for a reason, often overheating or a venting problem. If you smell gas, leave the house first and call 911 or Nicor Gas at 888-642-6748 from outside.
When to call a pro
Call for service if the thermostat and filter checks don't fix it, the furnace keeps lighting and shutting off, you see an error code blinking on the control board, or you have water around the furnace. Yogi Heating & Cooling handles furnace repair across Schaumburg and the northwest suburbs. Call (814) 431-1709 or book a visit online. If you have no heat on a very cold night, see our emergency service page.
A yearly fall tune-up catches many of these problems early, including dirty flame sensors and clogged drains. Learn more about our maintenance plan.
More questions
Why does my furnace blow cold air and then warm air?
A short blast of cool air at the start or end of a cycle is normal. If it switches between warm and cold every few minutes, the burners are likely shutting off early, which points to a dirty filter, flame sensor or a safety switch.
Can a dirty filter really make my furnace blow cold air?
Yes. A clogged filter restricts airflow, the furnace overheats, and a safety switch turns the burners off while the blower keeps running. Replacing the filter often fixes it.
Should I reset my furnace?
Turning the power switch off for 30 seconds and back on is fine once. If the furnace keeps locking out, stop resetting it and call a technician, because repeated resets can hide a real safety problem.
Why is only one room getting cold air?
Check that the register in that room is open and not blocked by furniture. If it is open, the duct to that room may be disconnected or leaking, which a technician can inspect.
Related
- Furnace and heating repair
- No heat? 8 things to check before you call
- Why does my furnace keep turning on and off (short cycling)?
- Maintenance plan