Furnace Short Cycling: Why It Turns On and Off (and What to Do)

Short cycling means the furnace starts, runs for only a few minutes, shuts off, and then does it again. And again. The usual causes are a clogged filter, a dirty flame sensor, overheating from blocked vents or returns, a thermostat problem, a furnace that's too big for the house, or a blocked flue or intake. Some of those you can check in a minute. Some need a technician.

Common causes

Here's what we find most often when a furnace in a home north of Houston keeps turning on and off.

A dirty or clogged filter

This is the big one. A clogged filter chokes off airflow, so heat builds up inside the furnace. A safety switch called the high limit switch senses the heat and shuts the burners off. The furnace cools down, starts again, overheats again, and the cycle repeats.

A dirty flame sensor

The flame sensor tells the furnace the burners actually lit. When it gets coated, the furnace can't confirm the flame, so it shuts the gas off a few seconds after lighting. Then it tries again. This is a common cause, and it's a job for a technician.

Overheating from blocked vents or returns

Closing a lot of supply vents, or blocking a return with furniture, has the same effect as a dirty filter. The air can't move, the furnace overheats, and the limit switch shuts it down.

Thermostat placement or settings

A thermostat near a supply vent, a lamp, a sunny window or a kitchen can read warm too quickly and shut the furnace off early. Weak batteries and some thermostat settings can cause short runs too.

A furnace that's too big

An oversized furnace heats the house so fast that it reaches the set temperature in a few minutes and shuts off. The house cools a little, and it starts again. It wears parts out faster and leaves rooms uneven. This one doesn't go away with a filter change.

A blocked flue or intake

If the vent pipe that carries exhaust out of the house is blocked, or the intake can't pull in air, the furnace's safety controls will shut it down. This is a safety issue. Don't try to clear it yourself. Call a technician.

Short cycling with a new filter

Changed the filter and it's still doing it? A few things can explain that. The new filter may be the wrong size, or a much more restrictive type than the furnace was set up for. Airflow may still be blocked somewhere else, like closed vents, a blocked return or a dirty coil. Or a part, like the flame sensor or the limit switch, is the real problem. Once the filter is ruled out, it's time for a technician.

Short cycling and blowing cold air

If the furnace starts, blows cold air, and shuts off, the burners may not be staying lit. A dirty flame sensor is a common reason. It can also be an ignition problem, an overheating shutdown, or a thermostat with the fan set to ON instead of AUTO. Check the fan setting first. If it's on AUTO and you still get cold air, call us.

Is it dangerous?

Most of the time short cycling is a wear problem. All that starting and stopping is hard on parts and can lead to a bigger repair. But overheating and venting problems are safety concerns, so don't ignore it.

If your carbon monoxide alarm goes off or you smell gas, leave the house and call 911 or your gas utility from outside. Don't try to troubleshoot it.

What you can safely check

These are safe for any homeowner:

• Look at the filter. If it's dirty, replace it with the same size.
• Make sure supply vents are open and returns aren't blocked by furniture or rugs.
• Put fresh batteries in the thermostat and check that it's set to HEAT with the fan on AUTO.
• Notice where the thermostat sits. If it's near a vent, lamp or sunny window, mention that when you call.

Please don't open the burner compartment or try to clean the flame sensor yourself. Gas parts are a job for a licensed technician.

When to call

If the furnace keeps short cycling after a new filter and the checks above, call us. A technician will run the furnace and watch the whole cycle to find what's shutting it down. More on what we check during heating and furnace repair.

If the furnace is old and was too big for the house to begin with, repairs may not fix the cycling. In that case we'll talk through furnace replacement and show you both options. And if the heat has quit completely in a cold snap, here's what to do when the heat goes out.

We work in Tomball, Spring, The Woodlands, Magnolia, Conroe, Cypress, Humble, Kingwood, Porter and New Caney. Open 24 hours, Monday–Saturday. Closed Sunday. Call (832) 589-0813 or request service.