Furnace Not Working? How to Find the Cause and Fix It

Furnace Not Working at All? Check These First

When there is no heat and no sound, the fault is usually one of these simple checks, and most of them cost nothing. Start at the top: the first ones take seconds.

1. The thermostat. Check the thermostat settings: HEAT, a few degrees above the room, and a lit screen. A heater not turning on with a blank screen usually has dead batteries in the thermostat.
2. The air filter. A clogged filter makes the furnace overheat and shut itself down, and after a few tries it stops trying.
3. The power switch. Most furnaces have a switch like a light switch on the wall or the side of the unit, and it gets turned off by mistake.
4. The breaker. Reset a tripped furnace breaker once. If it trips again, leave it off.
5. The front panel. A safety switch behind the panel cuts the power if the panel is not pushed fully shut.
6. The gas. The valve handle on the gas pipe by the furnace is open when it runs in line with the pipe. If the stove has no gas either, call the gas company.
7. The fuse on the control board. Many boards carry a small plug-in fuse like a car's. With the power off, a blown one shows a broken strip inside. Replace it once with one of the same rating, and call if it blows again, because something is shorting it.

Home heater not working even after all seven? Then the furnace itself has a fault, and the tool above narrows it down from what you can see and hear. If the furnace stopped working overnight, it has often locked itself out after several failed starts, which is also the usual story when a heater stopped working with no warning, and the blinking light and reset section explains how to clear it.

Tip: keep a spare filter by the furnace. It costs a few dollars, and a clogged one is behind many common furnace problems, including a furnace that shuts itself down.

Furnace Not Turning On or Not Kicking On

A furnace that is being asked for heat runs through the same steps every time, so gas furnace troubleshooting starts with that sequence. Knowing them turns a gas furnace not turning on into a question of which step is missing.

1. The thermostat calls for heat.
2. A small draft inducer fan starts and clears the burner box.
3. A pressure switch proves that the inducer is moving air.
4. The ignitor glows, or a spark or pilot is ready.
5. The gas valve opens and the burners light.
6. The flame sensor proves there is a flame.
7. After a short warm-up, the main blower starts.

If you hear nothing at all and the status light is dark, a furnace not coming on is not getting the call or has no power; if the light is blinking a code, it has locked itself out. If the small fan runs and clicks but nothing lights, a furnace not igniting has its fault at the pressure switch or the ignitor. If the burners light and go out, it is the flame sensor. A furnace not kicking on only when it gets very cold outside is often struggling with a long, cold vent or a weak ignitor that fails on the hardest starts. The same steps apply whether the heater won't turn on at all or you have a heater not kicking on only on the coldest nights.

Tip: listen for the small fan first. If it never starts, the fault is before the burners: power, the call for heat, or a lockout that the status light will show.

Furnace Won't Turn On but the Fan Works

A blower that runs while no heat comes on is two separate things. The blower can be on because the thermostat fan setting is ON, and the burners can be off because one of the steps above failed. Set the fan to AUTO first, so that the blower only runs when the furnace heats, and then watch whether the start sequence happens.

Tip: count the steps you can see and hear. Telling a technician "the small fan runs, then it clicks, then nothing" saves the time they would spend finding that out.

Furnace Fan Not Working, or Not Turning Off

When the burners light but no air comes out, the main blower fan is not running. The furnace then overheats within minutes and its limit switch stops the burners, so it looks like short cycling. A furnace blower not working is usually the motor or the capacitor that starts it. Replacing the blower motor runs $300 to $800, the same part the AC repair cost calculator prices for cooling, since one blower serves both.

The first suspect when the furnace fan won't turn off is a setting: a thermostat fan on ON runs it all the time. On AUTO, the blower normally runs a minute or two after the burners stop to move the last of the heat. One that keeps running much longer is cooling an overheated furnace, and one that never stops even with the thermostat off points to a stuck relay on the control board, a $300 to $600 repair.

Tip: if the burners run with no blower, turn the furnace off. Running it that way overheats the heat exchanger, the most expensive part in the furnace.

Furnace Blowing Cold Air or Not Blowing Hot Air

Four common causes explain a furnace blowing cold air when heat is on, and the first is not a fault. The first air through the vents after the blower starts has been sitting in cold ducts, so a minute or two of cool air is normal.

The second is the thermostat fan set to ON. The blower then runs between heating cycles too, and room-temperature air from a vent feels cold on your hand. With the fan on AUTO, a furnace blowing cold air between cycles stops doing it.

The third is a flame that does not last. The burners light, the flame sensor cannot prove the flame, and the board shuts the gas off within seconds, while the blower keeps moving unheated air. The fourth is a furnace that overheats: with a clogged filter too little air crosses the heat exchanger, a limit switch cuts the burners, and the blower keeps running to cool it down.

Tip: feel a vent through one full heating cycle before you decide you have a gas furnace blowing cold air instead of heat. The answer is in when the air turns cold.

Furnace Running but No Heat

A furnace not blowing hot air while it runs has one of three problems, and the window in the front panel shows which. No flame means an ignition failure: an ignitor that does not glow, a pilot that is out, or a gas valve that does not open. A flame that appears and vanishes means the flame sensor. A steady flame with cool air at the vents usually means very low airflow or a duct that has come apart in the attic or basement, which leaves the furnace not heating the rooms that duct feeds.

Tip: when the furnace kicks on but no heat reaches one part of the house, look for a closed damper or a loose duct before blaming the furnace.

Heater Blowing Cold Air in the House: Heat Pumps and Electric Furnaces

Many homes call a heat pump "the heater", and a heat pump is a common reason for a heater blowing cold air that is not broken at all. In cold weather it runs a defrost cycle for a few minutes to melt frost off its outdoor coil, and the air indoors feels cool while it does. Steam from the outdoor unit is the sign, so a house heater blowing cold air for a few minutes on a freezing day is usually defrost, not a fault. Central heat blowing cold air from a heat pump for a long time while the house cools is different, and that one needs a technician. Sizing and backup heat for heat pumps are covered by the heat pump size calculator.

An electric furnace has no flame to fail. It heats with resistance elements switched on in stages, so an electric furnace blowing cold air has lost an element, its relay or one of its breakers while the blower still runs, which makes it a heater not blowing hot air. Electric furnaces often have more than one breaker, so check all of them; an electric furnace not turning on at all often has one tripped.

Tip: before you call, find out which heating you have. An outdoor unit running in winter means a heat pump, and the fix is different from a gas furnace.

Furnace Short Cycling or Keeps Shutting Off

Short runs, over and over, instead of long steady ones, are what furnace short cycling means. When a furnace keeps turning on and off like this, the length of each run sorts out the furnace short cycling causes below.

• Seconds. The burners light and go out almost at once. That is the flame sensor, covered just below.
• A few minutes, then off, then on again. The furnace keeps shutting off because it overheats. Change the filter and open every supply vent and return grille.
• Short runs, but the house still warms. The furnace may be too big for the house, or the thermostat may sit near a supply vent, a lamp or a sunny window.

A heater short cycling for any of these reasons wears its ignitor and its relays faster and wastes fuel on every start. Oversizing is a sizing question rather than a repair, and the furnace size calculator shows what size the house needs.

Tip: time three runs with a clock before you call. The run length is the first thing a technician will ask about.

Furnace Ignites Then Shuts Off

The flame sensor is the usual cause if the furnace turns on then shuts off within seconds. It is a thin metal rod that sits in the flame. A light coating builds up on it over the years, and when it can no longer prove the flame, the board shuts the gas off a few seconds after lighting. Cleaning the rod fixes most of these, and a replacement sensor, when it is needed, is a $80 to $250 repair.

Tip: if it lights and goes out three times in a row, stop. Repeated failed starts lock the furnace out, and resetting it again changes nothing until the sensor is clean.

Furnace Pilot Light Won't Stay Lit, and Signs of a Bad Furnace Ignitor

Older furnaces keep a small pilot flame burning all the time. If the furnace pilot light won't stay lit, the usual part is the thermocouple, a sensor the pilot heats so that the gas valve stays open. When it wears out, the pilot lights while you hold the button and goes out when you let go. A thermocouple is a $100 to $300 repair.

Never relight a pilot if you smell gas: leave the house and call the gas company from outside. Otherwise, to relight furnace pilot lights, and for lighting the pilot light on a furnace each fall, most valves use the same steps. Turn the gas knob on the valve to OFF and wait the few minutes the label asks for, so any gas clears. Turn it to PILOT, hold it down, and light the pilot with the built-in spark button or a long lighter. Keep holding for as long as the label says, often up to a minute, so the thermocouple heats, then release it and turn the knob to ON. The label inside the furnace door wins if its steps differ, because valves differ.

Newer furnaces have no pilot. They light with a hot-surface ignitor, a small element that glows bright orange, or with a spark. A bad furnace ignitor shows as no glow, a glow at one end only, or a hairline crack. A furnace ignitor not glowing at all after the small fan runs has either failed or is not getting power. Hot-surface ignitors are brittle and should not be touched with bare fingers. A replacement is a $150 to $250 repair.

Tip: watch one start through the window before you call. Whether the ignitor glows, and whether a flame follows, is the most useful thing you can tell a technician.

Furnace Flame Sensor, Limit Switch and Pressure Switch: What Each One Does

These three are safety controls. Each one stops the furnace on purpose when something is wrong, so a furnace that shuts down because of one of them is protecting the house.

• Furnace flame sensor. Proves the burners are lit and shuts the gas off if they are not. Usually it needs cleaning, not replacing. Replacement: $80 to $250.
• Furnace limit switch. Shuts the burners off when the furnace runs too hot, usually because of poor airflow. Most reset on their own once the furnace cools. Some have a small button, and how to reset a furnace limit switch of that kind is to press it once; a switch that trips again points to the airflow, not the switch. Replacement: $100 to $375.
• Furnace pressure switch. What does a furnace pressure switch do? It proves the draft inducer is pulling exhaust out before the burners may light. A blocked vent pipe, a cracked hose to the switch or a weak inducer makes it refuse. The furnace inducer motor itself is a $400 to $1,100 repair; we have no trusted price for the switch alone.

Tip: never bypass a safety switch to get heat. Owners do it, and it removes the one thing standing between a fault and a fire or carbon monoxide in the house.

Furnace Leaking Water in Winter

A high-efficiency furnace, the kind with white plastic vent pipes, makes water every time it heats. That furnace condensation runs through a trap and a condensate line to a drain, often by way of a condensate pump. So a furnace leaking water when heat is on almost always means one of those has clogged, come loose or stopped. A condensate pump that has failed is a $100 to $450 repair.

A standard furnace with a metal flue pipe should not make water. If you find water leaking from furnace parts on one of these, the usual source is a humidifier mounted on the ductwork. A furnace exhaust pipe dripping water on a standard furnace is condensation in the flue, and a flue that drips may also not be venting properly. With the air conditioning running, the water is coming from the cooling coil above the furnace instead, and that is covered on the AC leaking water page.

Tip: turn the furnace off before you clean up the water. Water near the control board and the wiring is the real risk, not the puddle.

Furnace Blinking Light, Error Codes and How to Reset a Furnace

Most furnaces made in the last few decades have a small status light on the control board, visible through a window in the lower panel. A steady light or a slow, even blink means normal. A pattern, such as three blinks and a pause, is one of the furnace error codes. The chart that translates it is printed on the inside of the furnace door, and each maker uses its own numbers, so read your own door, not a chart from another furnace.

How to reset a furnace: turn the power switch off, wait about a minute, and turn it back on. That clears a furnace lockout after several failed starts. Some furnaces also have a rollout switch with a small reset button near the burners. A rollout switch trips when flame comes out of the burner box, which should never happen, so do not reset it yourself: turn the furnace off and call.

Tip: read and write down the code before you reset. A reset clears it, and the code is what tells a technician where to start.

When to Call a Technician, and When to Leave the House

Leave the house first and call the gas company or 911 from outside if you smell gas, if a carbon monoxide alarm sounds, or if anyone feels dizzy or sick with the heat on. Do not switch anything on or off on the way out.

Call a technician, and turn the furnace off until they come, for a rollout switch that has tripped, soot around the burners, a bang when it lights, a hot electrical smell, or a code that comes back after the filter is changed. A cracked heat exchanger is the expensive one: it can leak carbon monoxide, so the furnace stays off until it is fixed. The repair is $1,000 to $3,000, and on an older furnace a new furnace usually wins. Part prices and the repair-or-replace test for a furnace are on the furnace replacement cost calculator.

Dust burning off the heat exchanger after the summer is the usual reason a furnace smells like burning on the first cold day, and the smell fades within a few runs. A furnace making noise is easiest to sort by when it happens: a bang as the burners light is delayed ignition from dirty burners, and squealing or grinding that follows the fan is the blower motor or its bearings. Both are calls, not checks.

Tip: do every free check on this page before the visit. A call that ends with a new filter or a switch turned back on still costs the diagnostic fee.