Why does my gas furnace cycle on and off?

Is your gas furnace switching on and off more often than it should? This constant cycling doesn’t just mess with your comfort, it’s also a drain on your wallet and can wear out your furnace faster. Let’s explore some common reasons why this happens and how you can help your furnace stay on track.

Important: The info here might not apply exactly to your furnace model. We do our best to give useful tips for all sorts of DIY appliance repairs, but different models and setups can mean different troubleshooting steps, error codes, parts to swap, and so on. For spot-on advice, we highly recommend checking your appliance’s manual or reaching out to us if you’ve got any questions.

Dirty or clogged air filter

The air filter’s job is to capture dust, dirt, and debris, but over time, it can get clogged, restricting airflow. When the furnace can’t get enough air, it can overheat and shut itself off as a safety precaution. Here’s how to fix it:

1. First, turn off your furnace to avoid any electrical problems.
2. Remove the air filter from its compartment (typically found near the blower or where the return air duct enters the furnace).
3. If the filter is dirty, replace it with a new one. Some filters are reusable, so if yours is, give it a good cleaning (you can wash it under warm water and let it dry completely before reinstalling).
4. Before putting the filter back, check to make sure the airflow direction (often indicated by an arrow on the filter) is correct.
5. Turn the furnace back on and check if the issue is resolved.

Pro tip: Make it a habit to check and replace your filter every 1–3 months to prevent this issue from happening again.

Thermostat issues

If the thermostat is not properly calibrated, it might think the temperature is reaching the set point too quickly or inaccurately, triggering the furnace to turn off prematurely. Here’s how to troubleshoot thermostat issues:

1. Start by checking the thermostat setting to make sure it’s correctly set to the desired temperature.
2. If your thermostat uses batteries, try replacing them to rule out a power issue.
3. Check the thermostat’s location. If it’s placed near a heat source (like a lamp or a vent), it might cause it to read temperatures incorrectly.
4. If it’s still not working properly, consider upgrading to a programmable model for better energy efficiency and control.

Overheating furnace (flame rollout)

If your furnace is overheating, it might be shutting down automatically to prevent damage. This could be caused by a blocked vent, dirty burners, or a malfunctioning blower motor. Overheating is one of the most common reasons your furnace will cycle on and off. Here’s how to check and fix overheating issues:

1. Start by inspecting the air vents in your home. Make sure they’re not obstructed by furniture, curtains, or other items that could block airflow.
2. Next, check the furnace burners. Over time, dirt and debris can build up, causing the burners to operate inefficiently. Use a vacuum or blower to clean around the burner area. Be gentle, as you don’t want to damage any parts.
3. If the blower motor is malfunctioning, it could be struggling to circulate air properly. You can test the blower motor using a multimeter to check if it’s receiving power and if it’s operating correctly. If you find an issue, the motor may need replacement.
4. Additionally, check for any signs of dirt or debris on the blower fan and clean it as needed.

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Faulty gas flame sensor

The flame sensor’s job is to detect whether the burners are igniting properly. If it senses no flame or detects a problem, it signals the furnace to shut down to prevent gas from continuing to flow without ignition. Sometimes, the sensor can become dirty or wear out, leading to premature shutdowns. Here’s how to troubleshoot and fix a faulty flame sensor:

1. First, turn off the furnace and shut off the gas supply to avoid any risk.
2. Locate the flame sensor, which is typically found near the burners. You may need to remove a panel to access it.
3. Gently clean the flame sensor with fine sandpaper or steel wool. If there’s a buildup of dirt or soot, this can prevent it from detecting the flame properly.
4. Once cleaned, test the sensor with a multimeter to check if it’s receiving voltage. If it’s not, or the issue persists, you may need to replace it.

Malfunctioning pressure switch

The pressure switch in your furnace monitors whether the exhaust system is functioning properly. If it detects any blockage or a drop in pressure, it will shut the furnace down to avoid venting hazards. If the pressure switch malfunctions, it can cause the furnace to cycle on and off, even when there is no real issue. Here’s how to troubleshoot and fix a pressure switch:

1. Check the venting system for any blockages. Look for debris, bird nests, or anything that might block the exhaust or intake pipes.
2. Ensure that the hoses connected to the pressure switch are not cracked or leaking. If you see any damage, replace the hoses.
3. Test the pressure switch itself using a multimeter. Set your multimeter to measure continuity and check if the switch closes when the furnace is running. If the switch is not functioning properly, it will need to be replaced.
4. If you can’t find any blockages or issues with the hoses, but the pressure switch still seems faulty, it’s best to replace it to avoid further complications.

Blocked or leaking ductwork

When the air can’t circulate properly, the furnace can overheat, triggering a shutdown. If you notice your furnace running for only short periods, it’s a good idea to check the ductwork for potential issues. Here’s how to troubleshoot blocked or leaking ductwork:

• Start by visually inspecting the ducts in areas where they’re accessible (like the basement or attic). Look for any obvious signs of damage, such as holes, dents, or disconnected joints.
• Check for air leaks by running your hand along the ducts when the furnace is on. If you feel air escaping, that’s a sign of a leak.
• You can seal small leaks with duct tape or mastic (a type of sealant). For larger holes, you may need to use a patch kit or replace the affected sections of ductwork.
• If the ducts are clogged, such as with dust or debris, clean them using a vacuum with a hose attachment or hire a duct cleaning service.

Read more: 12 simple space heater safety tips for this winter

Use good-quality replacement parts

When you’re replacing parts on your furnace, using high-quality replacements help it last longer and work right.

• Find out exactly what part you need by finding your furnace’s model number. If you need a hand, here’s how to find your appliance’s model number.
• Buy genuine or OEM parts from www.appliancepartspros.com.
• Follow the manufacturer’s instructions when putting in new parts.

Read more: 12 things you should know about portable space heaters

Where to get extra help

We’re here to help you keep your place warm and cozy. For quality parts and expert advice, you can count on AppliancePartsPros.com. Check out our repair help section for more in-depth guides and resources. 

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If you have any questions or need more help, don’t hesitate to contact us. Together, we can keep your furnace running smoothly all through the chilly season.