Why Is My Furnace Blowing Out Cold Air? 6 Causes and How to Get Heat Back
A furnace that blows cold air has either lost its heat source — the burners are not firing — or the heat it is producing is not reaching the rooms before the air cools. The blower is running, moving air through the ductwork, but that air is room temperature or cold. The furnace is in one of three states: the burners never lit (the furnace is blowing cold air from the start), the burners lit briefly and went out (the air started warm but turned cold), or the burners are lit but the air is cooling before it reaches the registers (a duct problem, not a furnace problem). Each state has a different cause.
The fastest diagnostic question is: was the air ever warm during this heating cycle? If the air was warm at the start of the cycle and turned cold while the blower kept running, the limit switch tripped and shut off the burners, or the flame sensor lost the flame. The blower continues to run to cool the heat exchanger or to clear unburned gas from the combustion chamber. If the air was never warm — the blower started, but the air was cold from the beginning — the burners never lit. The furnace went into its startup sequence, failed at the ignition stage, and the blower is running either because the thermostat fan is set to ON, or because the control board is running the blower as part of a lockout or cooldown cycle.
1. Thermostat Fan Set to ON Instead of AUTO
The most common cause of a furnace blowing cold air is the simplest: the thermostat fan setting is set to ON instead of AUTO. In the ON position, the blower runs continuously — 24 hours a day — regardless of whether the burners are firing. Between heating cycles, the blower circulates air that is not being heated. The air feels cold because it is cold: it is room-temperature air being pulled from the return ducts and blown back into the rooms without passing across a hot heat exchanger.
Check the thermostat. If the fan setting says ON, switch it to AUTO. The blower will now run only when the burners are firing and for a brief period after the burners shut off to extract the remaining heat from the heat exchanger. The cold air between cycles stops immediately. The furnace is not broken. The thermostat was set incorrectly. This is the most common furnace service call that does not require a service call.
Is the air actually cold, or does it just feel cold? Air moving at 2 to 4 miles per hour across your skin feels cooler than still air at the same temperature — this is the wind chill effect. Air from a supply register that is 85°F to 90°F during the first minute of a heating cycle (before the heat exchanger reaches full temperature) can feel cool against skin that is 92°F, even though it is warmer than the room. Wait 2 to 3 minutes after the blower starts before judging the air temperature. If the air is still cool after 3 minutes, the burners are not firing.
2. Furnace in Lockout Mode: Failed Ignition
According to the U.S. Department of Energy, furnaces should be inspected regularly because “furnace heat exchangers mix combustion gases with house air when they leak — an important safety reason to have them inspected” and “combustion gases leaking into the house can cause carbon monoxide poisoning” (energy.gov).
A furnace that attempts to light three or four times and fails goes into lockout mode: the control board shuts off the gas valve and runs the blower continuously to clear any unburned gas from the combustion chamber and the flue. The blower runs, the air is cold, and the furnace will not attempt to light again until the lockout is manually reset. Lockout is a safety feature. The furnace is protecting the house from a gas explosion by refusing to keep trying to light when something is preventing successful ignition.
The most common causes of lockout: a dirty flame sensor (the burner lights but the sensor cannot see the flame, so the control board shuts off the gas after 3 to 8 seconds), a failed hot surface igniter (the igniter does not glow, the gas never lights), or a gas supply interruption (the gas valve opens but no fuel flows). To reset the lockout, turn the thermostat to OFF or turn the furnace power switch off, wait 30 seconds, and restore power. The furnace will attempt its startup sequence again. If it reaches lockout a second time, the underlying problem — the flame sensor, the igniter, or the gas supply — must be repaired. Do not keep resetting a furnace in lockout. Each reset sprays unburned gas into the heat exchanger, and each failed ignition attempt leaves that gas sitting in the combustion chamber.
3. Limit Switch Tripping: Overheating Shuts Off the Burners
A furnace that starts normally — the air is warm — but then turns cold after several minutes while the blower continues to run is cycling on the limit switch. The heat exchanger is overheating because airflow is restricted. The limit switch opens at roughly 180°F to 200°F, cutting power to the gas valve. The burners shut off, but the blower continues to run to cool the heat exchanger. After several minutes, the heat exchanger cools, the limit switch closes, and the burners relight — if the thermostat is still calling for heat. The cycle repeats: warm air, then cold air, then warm air again.
The most common cause of limit switch cycling is a dirty air filter. Replace the filter. If the filter is clean, check that all supply registers are open and that no return grilles are blocked. Closing registers in unused rooms increases static pressure and reduces total airflow — exactly the condition that causes limit switch cycling. After replacing the filter and opening all registers, the furnace should run a full cycle without tripping the limit switch. If it continues to cycle, the blower wheel may be dirty, the blower motor may be failing, or the limit switch itself may be faulty.
4. Pilot Light Out or Ignition Failure (Older Furnaces)
Older furnaces with standing pilot lights — typically manufactured before the mid-1990s — cannot fire the burners if the pilot light is out. The thermostat calls for heat, the gas valve opens, but there is no pilot flame to ignite the gas. The blower may start (depending on the furnace’s control system), but the air is cold because no combustion is occurring. The pilot light is a small blue flame visible through a sight glass on the front of the furnace. If it is out, relight it by following the instructions on the furnace’s rating plate.
If the pilot lights but goes out when you release the reset button, the thermocouple — a small metal probe that sits in the pilot flame and generates a tiny electric current to hold the gas valve open — has failed. A thermocouple costs $10 to $20 and takes a technician 30 minutes to replace ($150 to $250 total). If the pilot will not light at all, the pilot orifice may be clogged, or the gas supply to the furnace may be interrupted. Check that the gas valve at the furnace is ON — the handle should be parallel to the pipe.
5. Dirty Flame Sensor: Burner Lights, Then Goes Out
A furnace where the burners light — you hear the whoosh of ignition — and the air is warm for a few seconds, then the burners shut off and the air turns cold, has a dirty flame sensor. The flame sensor is a thin metal rod in the burner flame path that generates a microamp current when in contact with the flame. That current tells the control board the burners are lit. When the sensor is coated with soot or silica, it cannot detect the flame. The control board shuts off the gas valve after 3 to 8 seconds. The blower continues to run to clear the unburned gas.
Cleaning the flame sensor is a 10-minute job: turn off power to the furnace, locate the flame sensor (a single wire on a porcelain insulator, mounted in front of one burner), remove the single screw, pull the sensor out, and rub the metal rod with a dollar bill or fine steel wool. Reinstall and restore power. A clean flame sensor should hold the burners on. If the burners still go out, the flame sensor may be cracked or failing electrically. Replacement costs $150 to $300.
6. Ductwork Losing Heat: Warm Air Cooling Before It Arrives
If the furnace fires normally — the air at the supply plenum above the furnace is hot — but the air at the registers in the rooms is cool or lukewarm, the ductwork is losing the heat before it reaches the rooms. This is most common with ducts running through an unconditioned attic or crawlspace. The hot air travels through uninsulated or poorly insulated metal ducts in a 30°F attic and loses 20°F to 30°F of its temperature before it exits the register. The furnace is producing hot air. The ducts are cooling it before it arrives.
Insulating accessible ducts in unconditioned spaces with R-6 or R-8 duct wrap insulation resolves this. Sealing duct joints with mastic prevents cold attic air from being drawn into the ducts through leaks and mixing with the heated air. A duct system that loses 30% of its heat to the attic costs the homeowner that 30% in wasted fuel every winter. The furnace is not the problem. The ducts are.
FAQ: Common Questions About Furnace Blowing Cold Air
The air is cold when the furnace first starts, then gets warm. Is that normal?
Yes. The blower starts before the heat exchanger reaches full temperature to prevent a blast of hot air and to reduce thermal shock on the heat exchanger. The air will be cool for the first 30 to 90 seconds, then warm up quickly. Modern furnaces with ECM blowers do this intentionally — the blower ramps up slowly as the heat exchanger warms. Older furnaces with a fan-limit switch delay the blower start until the heat exchanger reaches roughly 110°F to 120°F, so the first air is already warm. Neither pattern indicates a problem.
Why is my furnace blowing cold air continuously even after I turn it off?
The blower continues to run after the thermostat is satisfied or turned off because the fan-limit switch is in cooldown mode. The heat exchanger is still hot — above the fan-off setpoint, typically 90°F to 100°F — and the blower runs until the heat exchanger cools below that temperature. This is normal. If the blower runs for more than 10 minutes after the burners shut off, the fan-limit switch may be stuck in the closed position or the limit switch’s fan-off setting may be too low. A technician can test and adjust or replace the fan-limit switch for $150 to $300.
Check the Thermostat Fan Setting First
A furnace blowing cold air is most commonly a thermostat set to FAN ON, a furnace in lockout after failed ignition, or a limit switch tripping from a dirty filter. Switch the thermostat fan to AUTO. If the air was never warm, the burners did not light — check for lockout, check the pilot light or igniter, and reset the furnace once. If the air was warm and turned cold, the limit switch tripped — replace the filter and open all the registers. If the air is warm at the furnace but cold at the registers, the ducts are losing the heat — insulate and seal them.
The thermostat fan setting fixes the problem immediately if it was the cause. The $10 air filter fixes the problem immediately if the limit switch is cycling. The dollar-bill cleaning of the flame sensor fixes the problem immediately if the burners are lighting and going out. Start with the zero-cost checks. The furnace is usually not broken. It is usually protecting itself from a condition you can fix in 5 minutes.
References: U.S. Department of Energy, Furnaces and Boilers, energy.gov/energysaver/furnaces-and-boilers.