When a furnace is blowing cold air, the first thing to work out is whether the burner is lighting at all. If it never lights, or lights and drops out after a few seconds, the blower is simply pushing unheated air through the ducts. If the burner runs normally and the air still reaches the rooms cool, the heat is getting lost between the furnace and the registers. Nearly every cause below falls into one of those two camps, and several take only minutes to check.
A little cool air is normal. On most gas furnaces the burner lights first and the blower waits until the heat exchanger has warmed up, but the air sitting in the ducts has cooled since the last cycle, and it comes out first. The blower also runs for a minute or two after the burner shuts off to pull the leftover heat out of the furnace, and that air gets steadily cooler. If the air turns warm within a minute or so of the blower starting, that part is working as designed.
1. The thermostat fan is set to ON instead of AUTO
This is the easiest fix and a very common reason a furnace seems to run with no heat. With the fan set to ON, the blower runs all the time, whether or not the burner is firing. Between heating cycles you feel air at room temperature, or cooler if the ducts pass through a cold basement or attic, and it is easy to mistake that for a furnace that has stopped heating. Set the fan to AUTO and the blower will run only while the furnace is making heat.
While you are there, confirm the mode is HEAT and the setpoint is a few degrees above the room temperature. A blank screen or a low battery warning means it needs fresh batteries. On a smart thermostat, check that a schedule or away mode has not quietly lowered the target.
2. A clogged filter is tripping the high-limit switch
A furnace needs a steady stream of air across its heat exchanger to carry the heat away. When the filter is packed with dust, airflow drops, the heat exchanger runs hotter than it should, and a safety device called the high-limit switch shuts the burner off. The blower keeps running to cool things down, so lukewarm or cool air comes out of the vents. Once the temperature falls, the burner relights, overheats and shuts off again, and the house never quite warms up.
You can check this yourself. Turn the system off at the thermostat, slide out the filter (usually where the return duct meets the furnace, or behind a return grille) and hold it up to a light. If you can barely see through it, replace it with one of the same size, airflow arrow pointing toward the furnace. Our guide on how often to change a furnace filter covers the schedule. Closed supply registers and furniture against return grilles cause the same problem, so open those up too. If the filter was clean and the furnace still cycles this way, the restriction is somewhere you cannot reach, such as a dirty blower wheel, a weak blower motor or undersized ductwork. Repeated limit trips also stress the heat exchanger, so that is worth a diagnosis.
3. A dirty flame sensor
If you look through the small window in the furnace door and see the burners light, burn for a few seconds and go out, the flame sensor is the first suspect. It is a thin metal rod that sits in the flame and lets the control board confirm the burner is lit. Over time a film builds up on it, the signal weakens, and the board shuts the gas off because it cannot prove there is a flame. The furnace tries again, fails the same way, and after a few attempts locks itself out.
Cleaning or replacing a flame sensor is a routine, inexpensive repair that a yearly tune-up often catches early. It means working inside the burner compartment, so leave it to a technician. What helps is noting how long the burners stay lit and the blinking light code on the control board, usually visible through a small window in the lower panel.
4. Ignition failure: hot surface igniter or pilot
Most modern gas furnaces use a hot surface igniter, a small element that glows bright orange just before the gas valve opens. The normal sequence: the thermostat calls for heat, the small inducer motor starts and clears the flue, a pressure switch confirms the draft, the igniter heats up, and the gas valve opens. Igniters are brittle and wear out. When one cracks, it stops glowing and the gas never lights. Older furnaces may have a standing pilot instead, and if it goes out the main burner cannot light. The relighting steps are on a label inside the furnace, but a pilot that keeps going out usually has a worn thermocouple or a draft problem behind it, so that is a job for a technician.
After a few failed tries (often three, though it varies by manufacturer) the control board locks out and stops trying. Many boards retry on their own after about an hour. Turning the furnace off at its service switch, which looks like a light switch on or near the unit, for a minute and back on also clears a lockout. Try that once. If the furnace goes through the same failed sequence again, leave it off and call. Resetting it over and over will not fix a bad igniter, and it hides what the board is trying to tell you.
5. A gas supply problem
Sometimes the furnace is fine and the gas is not reaching it. Every gas furnace has a manual shutoff valve on the pipe next to it: when the handle runs in line with the pipe the valve is open, and when it sits crosswise it is closed. Valves get closed during other work, such as a water heater replacement, and occasionally stay that way. If you find it closed, do not open it yourself. Someone closed it for a reason, and the furnace and piping should be checked before gas goes back to them.
If no gas appliance in the house works, the problem is likely at the meter or with the utility, so call the gas company. If you heat with propane, check the tank gauge; an empty tank is easy to overlook, and the supplier will need to restore service.
If you smell gas at any point, stop troubleshooting. Get everyone out, do not turn lights, appliances or the thermostat on or off on the way, and call the gas company or 911 once you are outside.
6. A blocked condensate drain or snow-covered vent pipes
High-efficiency furnaces, the kind with two white PVC pipes running out through a side wall or the roof, pull so much heat from the exhaust that water condenses inside them. That water drains through a trap and a small tube to a floor drain or a condensate pump. If the trap clogs, the line freezes or the pump fails, water backs up and a pressure switch or float switch shuts the furnace down. Look for water around the base of the furnace, a condensate pump that is full and not running, or a drain tube that has kinked or slipped out of the drain.
The two PVC pipes need to stay clear too. One brings in combustion air and the other carries exhaust out, and the furnace checks for a proper draft through them before the burner lights. If snow or ice covers either one, the pressure switch never confirms the draft, so the furnace will not light, or lights and then shuts down. After a storm, check where the pipes leave the house: drifting snow, a plow pile or snow shoveled off a deck can bury them, and in hard cold, frost can build up around the exhaust.
Clear the snow by hand or with a soft brush, and do not bang on the pipes or push tools into them, since PVC cracks easily in the cold. Some furnaces restart on their own once the draft is restored; others need the one-time power reset described above. Exhaust that cannot get out has to go somewhere, which is why clear vents also come up in our article on carbon monoxide warning signs.
7. Leaky or disconnected ductwork
Sometimes the furnace makes plenty of heat and it simply is not arriving. Supply ducts that run through an unheated attic, crawlspace or garage lose heat quickly if they are uninsulated or leaking, and a joint that has pulled apart can dump hot air straight into that space. The giveaway is unevenness: rooms near the furnace get warm air while one room, or one end of the house, gets air that feels barely warm.
In a basement you can usually see the ducts. Look for separated sections, crushed or kinked flexible duct, and tape that has peeled away. Leaks should be sealed with duct mastic or foil tape made for ductwork, not cloth duct tape, which dries out and lets go. Attic and crawlspace ducts are harder to inspect safely, so those are better checked during a service visit.
If you heat with oil
Roughly one home in five in the towns OniFix serves west and north of Boston heats with oil, and oil furnaces have causes of their own. The first is an empty tank. Check the gauge on top, usually a float inside a small clear dome. Running dry can pull sludge from the bottom of the tank into the filter and nozzle, and the fuel line often has to be bled before the burner will fire, so even after a delivery you may need a service visit.
The second is a burner lockout. When an oil burner fails to light, its primary control shuts it down, and the red reset button on that control restarts it. You can press it once. If the burner does not start, or starts and locks out again, do not press it a second time. Each attempt can spray oil into the combustion chamber without lighting it, and if that oil later ignites all at once, the result can be a puffback that spreads soot through the house, or worse.
When to stop troubleshooting
The checks in this article are safe for anyone: thermostat settings and batteries, the filter, the service switch, the vent pipes, the condensate drain, the oil tank gauge and a single reset. The no-heat checklist covers the same ground in a quick format, along with the breaker. Past that point, stop and call for service if:
- you smell gas or a carbon monoxide alarm sounds (get everyone outside first, then call 911 or the gas company)
- the furnace locks out again after one reset, or a breaker trips a second time
- you see soot, scorch marks or water pooling around the furnace
- the next step would mean removing the burner compartment panel, bypassing the door safety switch, or touching the gas valve, igniter or wiring
If you have worked through these checks and the furnace still blows cold, the next step is a proper diagnosis. Our furnace repair page explains how a visit works, including the estimate you get before any work starts.