The amount of time a furnace runs in a cycle depends on several factors, but it’s typically 10 to 15 minutes. If your furnace runs for a shorter amount of time, then turns off, it’s short cycling. There are several reasons why it may do that.
The Furnace Is Shutting Off to Prevent a Gas Leak
A furnace has a flame sensor that detects flames or natural gas. If the sensor is covered with dirt and dust or has corroded over time, it may be unable to detect flames and may cause the furnace to shut off to prevent a gas leak.
Your furnace may be short cycling because the hot surface lighter is damaged. If that’s the case, or if the pilot light shuts off, the furnace will turn off to keep gas from accumulating.
The Furnace Is Shutting Off to Prevent Overheating
If the air filter or furnace exhaust pipe is clogged with dirt, dust or other contaminants, that can block airflow and cause the furnace to overheat. Furnaces are designed to shut off automatically when they get too warm in order to protect internal components from damage.
Make sure that the supply and return vents are open and that there is no furniture blocking them. Check your air filter on a regular basis and replace it according to the manufacturer’s recommendations.
The exhaust vent removes poisonous carbon monoxide from the house. Rodents, birds and insects can get into a vent and cause a blockage. Ice can also block an exhaust vent. If carbon monoxide can’t escape because the vent is blocked, the furnace can overheat and shut off. A blocked vent can also lead to a gas leak in the house.
The Furnace Is the Wrong Size
Sometimes a house has a furnace that is too large for the amount of space it serves. If your furnace is too big for your house, it will provide more heat than necessary, then quickly shut off when it receives a signal from the thermostat. That can lead to uneven heat distribution, with some rooms or parts of rooms warm and others cold. It can also lead to high utility bills.
The Thermostat Isn’t in an Appropriate Location
The thermostat should be placed in a central location. If it’s near a window, a door, a vent or an appliance that generates heat, it can get an inaccurate temperature reading. If the thermostat’s reading is higher than the actual temperature, the furnace will shut off sooner than it should.
Consult a Professional
It can be difficult to figure out why your furnace is short cycling. If you aren’t sure of the reason, have an HVAC technician conduct an inspection, make any necessary repairs or refer you to another professional who can help with vent cleaning or other services.