Furnace blowing cold air
When your furnace is blowing cold air, the fix is often a thermostat setting or a clogged filter, not a broken furnace. A fan set to ON runs whether or not there's heat, and a dirty filter can make the furnace overheat and shut its burners off while the blower keeps going. Work through these checks in order. Leave anything involving the gas valve, pilot or igniter to a technician, and if you smell gas, leave the house and call from outside. A furnace blowing cold air is often working fine; it's the fan setting or a clogged filter.
Why a furnace blows cold air
A gas furnace has two jobs that run separately: the burners make heat, and the blower moves air through the house. When the thermostat calls for heat, the furnace lights the burners first and waits until the heat exchanger is warm before starting the blower. If the burners stop but the blower keeps running, you feel cool air from the vents.
Several safety parts can stop the burners on purpose. A limit switch shuts them off if the furnace gets too hot, a shuts the gas if it can't detect a flame, and on high-efficiency furnaces a pressure switch stops everything if the venting or is blocked.
Common causes of cold air
The blower runs between heat cycles.
The furnace overheats and the limit switch stops the burners.
The pressure switch shuts down a high-efficiency furnace.
The burners never light or won't stay lit.
| You notice | Likely cause | Start at |
|---|---|---|
| Cool air between heat cycles | Fan set to ON | Step 1 |
| No heat at all, blank screen | Thermostat setting or batteries | Step 2 |
| Cool only at the start | Normal warm-up | Step 3 |
| Warm, then cold, then warm | Overheating from a dirty filter | Step 4 |
| Burners won't light or go out | Flame sensor, igniter or gas valve | Step 5 |
| White plastic vent pipes, shuts down | Clogged condensate drain | Step 6 |
| Some vents cool, furnace hot | Leaky ducts | Step 7 |
Let's fix it
- Check the fan settingLook at the thermostat's fan setting. If it's on ON, the blower runs all the time, including between heating cycles, and that air will feel cool. Switch it to AUTO so the fan only runs when the furnace is actually heating.
- Check the mode, setpoint and batteriesMake sure the thermostat is set to HEAT, not COOL or OFF, and that the temperature is set a few degrees above the room temperature. If the display is dim or blank, replace the batteries. On a smart thermostat, check that a schedule or away mode isn't holding the temperature low.
- Give it a few minutesRight after the heat kicks on, the furnace needs a minute or two to light and warm up before the blower starts. Some cool air at the very start of a cycle is normal, and so is a little at the end as the blower clears leftover heat. If the air stays cold for more than five minutes, keep going.
- Replace a dirty filterPull out the furnace filter and hold it up to a light. If you can barely see through it, replace it. A clogged filter chokes airflow, the furnace overheats, and the limit switch shuts the burners off while the blower keeps running to cool things down. That cycle feels like warm air, then cold, then warm again.
- Look at the burners and error lightTurn the thermostat up and watch the burners through the small window, along with the blinking status light whose code chart is usually printed inside the panel door. If the burners light and then go out after a few seconds, a dirty flame sensor is a common cause; see the furnace short cycling guide for how to clean it. If they never light, or you see a code for the igniter, gas valve or pilot, call a technician. Don't take apart gas parts yourself.
- Clear the condensate drainHigh-efficiency furnaces, the kind with white plastic vent pipes, make water that drains out through a small tube. If that drain or its trap clogs, water backs up and a pressure switch stops the burners. Turn off power at the furnace switch, check the tube and trap for sludge or standing water, and clear it with a wet/dry vacuum at the end of the drain line. Restore power and try again.
- Check the ductsIf the furnace is clearly heating but air at some vents is still cool, look at the ducts you can reach in the attic, crawlspace or basement. A duct that's come apart or has big gaps can blow warm air into an unheated space and pull cold air in. Reconnect loose sections and seal gaps with foil tape or duct mastic, not cloth duct tape.
Watch it done
- Warm air comes from the vents within a few minutes of the heat starting
- The blower stops between heating cycles
- The house reaches the set temperature
If it's still blowing cold
If the burners light and go out again and again even after a new filter and a clean flame sensor, the furnace may be overheating from a blocked vent, closed supply registers or a weak blower. A technician can check the temperature rise and the limit switch.
If cold air only comes from one or two vents while the rest of the house is warm, it's more likely a duct or balancing issue. There's a separate guide for one room too hot or cold.
What a pro typically charges (a rough estimate; prices vary by area): An HVAC service call usually runs $100–$250. Replacing a flame sensor or igniter typically costs $150–$400, a pressure switch or limit switch $150–$450, and a gas valve $300–$800 installed.
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Furnace cold air questions
Why is my furnace blowing cold air?
Often the thermostat fan is set to ON, or a dirty filter made the furnace overheat and shut off its burners.
Should my thermostat fan be on AUTO or ON?
AUTO for most homes. ON runs the blower nonstop, which blows cool air between heat cycles.
Can a dirty filter cause a furnace to blow cold air?
Yes. It can overheat the furnace, which shuts off the burners while the blower keeps running.
Is it normal for a furnace to blow cold air at first?
A little, for a minute or two at the start of a cycle. Longer than that points to a problem.
Why does my furnace blow cold air between heating cycles?
Usually the thermostat fan is set to ON, so the blower keeps running after the burners stop. Set it to AUTO.