Why a furnace starts and stops every few minutes

A furnace that fires, runs for two or three minutes, shuts off, then fires again a few minutes later is short cycling. The house never quite gets warm, the burner wears through years of ignitions in a single season, and the gas bill goes up rather than down. It is worth treating as a fault even though the heat technically still works.

Start with the filter

A blocked filter starves the furnace of return air. With too little air moving across the heat exchanger, the temperature inside the cabinet climbs until a safety limit switch cuts the burner. The blower keeps running, the cabinet cools, the burner relights, and the loop repeats. This is the most common cause and the only one on this list a homeowner can rule out alone.

Pull the filter and hold it up to a light. If you cannot see light through it, replace it and give the system an hour. Most one inch filters need changing every 60 to 90 days in a working household, and faster with pets or an ongoing renovation.

Then check what is closed

• Supply registers shut in unused rooms. Closing more than a couple of them raises static pressure across the whole system.
• Furniture, rugs or storage sitting over a return grille.
• A humidifier bypass damper left in the wrong seasonal position.

Causes that need a technician

If airflow is clear and it still cycles, the remaining causes are diagnostic. A flame sensor coated in oxide stops proving the flame and drops the burner within seconds of ignition. A failing limit switch trips early. A furnace that is simply oversized for the house satisfies the thermostat before the ducts ever warm up, which no adjustment fixes and which only shows up in a load calculation.