Furnace Keeps Shutting Off After a Few Minutes? Causes and Fixes

A furnace that keeps shutting off is usually overheating because of a clogged air filter, losing its flame signal because of a dirty flame sensor, or being cut off by a blocked flue or vent. Replace the filter, open all vents, then check the thermostat. If it still shuts off, clean the flame sensor or call an HVAC technician.

Furnaces shut off on purpose when something is wrong. The trick is to work out which safety system is doing it. This guide shows how, starting with the checks that cost nothing.

Why does my furnace keep shutting off?

A furnace cycle has a clear order: the thermostat calls for heat, the burners light, the flame sensor confirms the flame, and the blower runs. Anything that breaks the chain ends the cycle. The usual suspects are:

• A dirty air filter. Airflow drops, the inside overheats and the limit switch shuts the burners off. The blower often keeps running to cool the furnace, so you may feel lukewarm air before it stops.
• A dirty flame sensor. This thin metal rod proves the burners are lit. When it is coated with soot, the control board decides there is no flame and cuts the gas, often within seconds.
• Blocked airflow. Closed or blocked supply vents, a covered return or a crushed duct have the same effect as a dirty filter.
• A blocked flue or vent pipe. A bird nest, snow or ice can block the exhaust or intake. The pressure switch then stops the furnace.
• A clogged condensate drain. High-efficiency (90%+) furnaces drain water from the exhaust. A blocked drain trips a safety switch.
• The thermostat. Weak batteries, loose wires or a thermostat in direct sun, near a register or in a draft can end a cycle early.
• An oversized furnace. A furnace too big for the house heats it too fast and shuts off, again and again.
• A tripped rollout switch. This switch trips if flame escapes the burner area. It can point to a cracked heat exchanger or a blocked vent. It is serious.

How long should a furnace run?

Many furnaces run about 10 to 20 minutes per cycle in cold weather. A cycle that stops at the thermostat setpoint is normal. A furnace that stops in under five minutes, repeatedly, is short cycling.

How to fix it

1. Replace the air filter. Check it first. If it is gray and clogged, swap it. A new filter usually costs somewhere between about $5 and $25, depending on size and type.
2. Open every vent and register. Make sure rugs, furniture and curtains are not blocking supply vents, and that the return grille is clear.
3. Check the thermostat. Set it to Heat and a temperature at least five degrees above the room. Replace the batteries. If the screen is blank, see what to do about a blank thermostat.
4. Look outside. Find the white PVC pipes (or metal vent) on the side of the house. Clear snow, ice, leaves or a nest from the openings. Do not poke anything into the pipe.
5. Watch one full cycle. Stand by the furnace and watch through the small window if there is one. Note whether the burners light, how long they stay on and whether the blower runs after.
6. Clean the flame sensor. See the next section.
7. Check the condensate line. On a high-efficiency furnace, look for a clear tube leading to a drain or small pump. Water backed up in it can trip the furnace.
8. Reset once, not repeatedly. Turn the furnace off at the switch for a minute, then on. If it runs, the problem may be intermittent. If it shuts off again, stop.

How to clean a flame sensor

This is the most common DIY fix when the burners light and quickly go out. If you are not comfortable working near gas appliances, call a technician.

1. Turn off power to the furnace at the switch or breaker, and turn the thermostat to Off.
2. Remove the front panel. Find the flame sensor, a thin rod with a bend, mounted near the burners by a single screw.
3. Remove the screw and slide the sensor out. Do not touch the ceramic base.
4. Rub the metal rod gently with a very fine abrasive pad or fine emery cloth until it is shiny. Do not use coarse sandpaper.
5. Wipe it with a clean dry cloth, put it back and secure the screw.
6. Restore power and run a cycle.

If the furnace still shuts off, the sensor may be worn out, and a replacement part is an inexpensive fix. A technician can confirm it.

What if the furnace will not restart?

Many furnaces lock out after several failed ignition attempts and try again after about an hour. Turning the power off for 30 seconds and back on can clear a lockout. If it fails again and again, do not keep resetting it. Look at the blinking LED on the control board (visible through the small window) and write down the pattern, which helps a technician. The code chart is usually on the back of the access panel. The case where the furnace tries to start, but only clicks, is covered in a furnace that clicks but will not start.

Is it worth repairing or replacing?

Filters, sensors and igniters are modest repairs. A cracked heat exchanger, a failing control board or an old furnace that needs several parts is a different decision. As a rough guide, many furnaces last 15 to 20 years. If yours is in that range and the repair quote is large, compare it with the cost of a new unit, and ask about efficiency. A technician can also check whether your furnace is too big for the house, because oversizing causes short cycling that no repair will cure.

When to call a professional

Call a licensed HVAC technician if:

• The rollout switch is tripped, or the furnace shuts off with flames rolling out.
• You see soot, scorch marks or yellow, flickering flames.
• A carbon monoxide alarm sounds. Leave the house and call 911.
• The furnace keeps shutting off after a new filter, a clean sensor and clear vents.
• The condensate drain or the flue needs clearing and you cannot reach it.
• The furnace is old, or the heat exchanger may be cracked.

If your heat is out completely and you are not sure who to call, choosing the right heating pro lays out the options.

What not to do

• Do not bypass or jumper a safety switch.
• Do not reset a rollout switch again and again.
• Do not block the furnace flue or intake with snow or stored items.
• Do not use the oven to heat your home while the furnace is down.

Bottom line

Start with the filter, the vents and the thermostat, and clean the flame sensor if the burners light then die. Those fixes handle most short cycling. A safety switch that keeps tripping is a warning, not something to override. If your furnace runs but blows cool air, see what to check when the furnace blows cool air. If it shuts off along with a breaker, read a furnace that keeps tripping the breaker.