A furnace blowing cold air is running its blower without making heat, and the most common causes are a thermostat fan set to On, a clogged air filter that trips the high-limit switch, or an ignition problem that keeps the burners from lighting or staying lit. Check the thermostat and filter first, since those take two minutes and fix a good share of cases. If the furnace still turns on but no heat comes out, the problem is usually the flame sensor, the ignitor, the gas supply or a blocked vent, all of which a technician can find quickly. Yearly maintenance catches most of these before the first cold night.
Why Is My Furnace Blowing Cold Air?
When a gas furnace heats normally, the burners light first and the blower waits 30 to 60 seconds to start so it doesn't push cold air. If you're feeling cold air for more than a minute or two, the blower is running while the burners are off. That happens for a handful of reasons, starting with the easiest to fix.
The Thermostat Fan Is Set to On
If the thermostat's fan setting is On instead of Auto, the blower runs nonstop, even between heating cycles. When the burners shut off because the house reached the setpoint, the blower keeps pushing room-temperature air through the ducts, and it feels cold at the vents. Switch the fan to Auto and the blower will only run while the furnace is heating.
A Clogged Air Filter
A dirty filter restricts airflow, the heat exchanger overheats and the high-limit switch shuts off the burners to protect the furnace. The blower keeps running to cool things down, so you get cold air until the furnace resets and tries again. Replace the filter, wait about 15 minutes and see whether the heat returns.
The Burners Won't Light
If the burners never light, the blower may run on its own as the furnace cycles through failed ignition attempts and then locks out. A cracked or weak hot surface ignitor, a pressure switch that won't close or a failed gas valve are the usual causes. The won't turn on guide walks through the full startup sequence if you want to see exactly where it's stopping.
The Gas Supply Is Off
Check that the gas valve on the pipe leading into the furnace is open, with the handle running parallel to the pipe. Then check whether your other gas appliances, like the stove or water heater, are working. If none of them are, the problem is at the meter or with your utility.
A Blocked Vent or Condensate Line
High-efficiency furnaces vent through PVC pipes on the side of the house or the roof. Snow, leaves or a bird's nest blocking those pipes keeps the pressure switch from closing, and the burners never light. A clogged condensate trap inside the furnace can do the same thing. Check the outdoor pipes for obstructions, but leave the internal drain to a technician.
Leaky or Disconnected Ductwork
Sometimes the furnace is making heat and it's getting lost on the way. A disconnected duct in the attic or crawlspace dumps warm air outside the living space, and the rooms on that run get cold air or none at all. If one or two rooms are cold while the rest are fine, ductwork is the likely cause.
Furnace Turns On but No Heat, Then Turns Off
If the burners light, run for a few seconds and then shut off while the blower keeps going, the flame sensor is almost always the cause. A thin layer of oxidation on the sensor rod keeps it from proving the flame, so the control board cuts the gas as a safety precaution. The furnace retries a few times and then locks out. Cleaning or replacing the flame sensor is quick routine furnace repair.
If the furnace runs for several minutes with good heat before shutting off and blowing cold, it's more likely overheating and tripping the high-limit switch. Start with the filter, then make sure supply vents and return grilles are open and unblocked.
Furnace Blowing Warm Air, Not Hot
A gas furnace normally heats the air 35 to 70 degrees above the temperature coming in, so air at the vents usually feels hot, often well over 100 degrees. If it's only lukewarm, check a few things before assuming a problem.
Two-stage and modulating furnaces spend most of their time on a low setting that produces gentler, cooler-feeling air for longer stretches. That's by design and saves fuel. Heat pumps also deliver cooler air than furnaces, often in the 90s, which can feel lukewarm to someone used to gas heat.
If you have a single-stage gas furnace and the air has gone from hot to warm, dirty burners, low gas pressure, a weak blower setting or leaky ducts are the usual causes. A technician can measure the temperature rise across the furnace and tell you whether it's working as designed.
Electric Furnace Blowing Cold Air
Electric furnaces heat air with resistance elements controlled by relays called sequencers. When the blower runs but the air stays cold, the elements aren't getting power. Check the breaker panel first, since the elements often have their own large breakers separate from the blower. A tripped breaker, a failed sequencer, a burned-out element or a tripped limit switch are the usual causes.
If you have a heat pump rather than a furnace, the heat not working guide covers heat pumps blowing cool air, and emergency heat can keep you warm while you wait for a repair.
Furnace Not Blowing Hot Air: Easy Fixes to Try First
Start at the thermostat. Make sure it's set to Heat, the fan is on Auto and the setpoint is a few degrees above the room temperature. Replace the batteries if the display is dim.
Next, check the filter and replace it if you can't see light through it. Confirm the gas valve at the furnace is open and the outdoor vent pipes are clear.
Then reset the furnace once. Turn off the service switch near the unit, wait about 30 seconds and turn it back on. Before you do, note the blinking pattern on the control board light, which you can see through the window in the blower door. That code tells a technician where to look.
Should You Turn Off a Furnace That's Blowing Cold Air?
If the furnace is cycling through failed ignition attempts or overheating and tripping the limit switch, turning it off at the thermostat until it's repaired is a reasonable choice. Running it in that state won't heat the house, and repeated overheating wears on the heat exchanger. Shut it off right away and call for help if you smell gas or burning, or hear banging or grinding.
When to Call a Technician
Call if the furnace still blows cold air after you've checked the thermostat, filter, gas valve and vents, or if it locks out again after a reset. Ignitors, flame sensors, gas valves, pressure switches and control boards all involve gas or live electrical circuits.
Get Your Heat Back
MR. HVAC is family-owned and has repaired furnaces for more than 25 years. Our technicians find out why your furnace is running without heating, explain the fix in plain terms and give you the price before any work starts. Yearly furnace maintenance keeps flame sensors clean and catches weak ignitors early. If your furnace is still blowing cold air, schedule service and we'll get your heat back on.
