Why your furnace keeps turning on and off

HVAC tips for Utah homes

A normal furnace cycle is steady. The thermostat calls for heat, the burners light, the blower comes on, and it runs until the house reaches the set temperature. On a cold Salt Lake night that can be a long run.

Short cycling is when the furnace fires up, runs for only a few minutes (or less) and shuts off, then does it again. The house may never feel quite warm. Your gas bill may creep up. And the constant starting and stopping wears out parts faster.

Sometimes the cause is simple. Sometimes the furnace is shutting itself down because something is wrong. Here's how to tell the difference.

Common causes

A dirty or too-restrictive filter

This is the most common one, and the easiest to fix. When air can't get through the filter, heat builds up inside the furnace. A safety switch called the high limit trips and shuts the burners off. It cools down, resets and tries again. A very dense filter in a thin slot can cause the same thing even when it's new.

Snow or ice blocking the vent pipes

Most newer high-efficiency furnaces vent through two plastic pipes out the side wall or roof. One brings in air and one carries out exhaust. After a heavy snow, or when drifts pile up against the house, those pipes can get covered or frosted over. The furnace senses it can't vent safely and shuts down. This is a very common winter call in the valley.

A dirty flame sensor

If the burners light and then go out within a few seconds, a dirty flame sensor is a likely suspect. The sensor has to "see" the flame or the furnace cuts the gas as a safety step. A light film builds up on it over time. It's a routine fix during service.

A frozen or clogged condensate drain

High-efficiency furnaces make water. It drains through a small tube, often to a floor drain or a pump. If that line clogs, or freezes where it runs through a cold garage or crawlspace, a switch stops the furnace to prevent flooding.

Thermostat problems

A thermostat mounted near a supply register, in direct sun, next to a drafty door or above a lamp can read the wrong temperature. It gets warm fast and tells the furnace to stop. Low batteries and loose wiring can also cause odd behavior. Some thermostats also have a cycle-rate setting that is simply set too short.

A furnace that's too big for the house

An oversized furnace heats the air near the thermostat very quickly, then shuts off before the rest of the house catches up. This is common in homes that were remodeled, insulated or had windows replaced after the furnace went in. It's not dangerous, but it's uncomfortable and inefficient. The fix is usually proper sizing at the next replacement.

Overheating from a bigger problem

Closed-off supply registers, a failing blower motor or a damaged heat exchanger can also cause overheating. These need a technician.

What you can safely check

1. Look at the filter. If it's gray or you can't remember when you changed it, change it. See our filter guide for which rating to use.
2. Check the vent pipes outside. Clear snow and ice away from the intake and exhaust pipes by hand or with a broom. Don't chip at them with a shovel. Keep the area clear through the rest of the storm.
3. Open your registers. Make sure supply vents and return grilles aren't closed or blocked by furniture or rugs.
4. Check the thermostat. Replace the batteries. Look at what's near it. If there's a heat source nearby, that may be your answer.
5. Look at the condensate line. If you see water pooling around the furnace or a drain line that runs through a cold space, note it for the technician.
6. Note the pattern. How long does it run before shutting off? Does it light and go out in seconds? Is there a blinking light on the control board? Many furnaces have a code chart inside the door. This information saves time on a service call.

Leave the gas valve, burners and internal wiring alone. Don't bypass a safety switch to "see if that's it." Those switches are there to keep the house safe.

When short cycling is dangerous

Most short cycling is a comfort and wear problem. Some of it isn't. Treat these as urgent:

• Your carbon monoxide alarm goes off. Get everyone out of the house and call 911 from outside. Don't go back in until you're told it's safe.
• You smell gas. Leave the house without flipping switches, then call your gas utility's emergency line or 911 from outside.
• People in the house get headaches, nausea or dizziness that improve when they leave. That can be a sign of carbon monoxide.
• You see scorch marks, soot or rust around the furnace, or the flame looks yellow and lazy instead of blue.
• The furnace keeps overheating even with a clean filter and open vents. Repeated overheating can crack a heat exchanger.

Every home with a gas furnace should have working carbon monoxide alarms. Test them when you change the clocks.

When to call a pro

Call if the short cycling continues after you've changed the filter, cleared the vents and checked the thermostat. Call if the burners light and drop out, if there's water around the furnace, or if the control board is flashing a code. And call right away for any of the danger signs above, once everyone is safe.

Our furnace repair page explains how we handle a no-heat or short-cycling call. If the furnace is old and this is the latest in a string of problems, our repair-or-replace tool can help you think it through. A replacement sized for your home, covered on our furnace installation page, fixes the oversizing problem for good. If the thermostat is the culprit, see thermostat services.

Hearing a new bang, whine or rattle along with the cycling? Our what's that noise tool can help narrow it down.