A properly functioning furnace should heat your home efficiently without frequent interruptions. However, if you notice your furnace cycles on and off too often or shuts down unexpectedly, it could indicate an issue that needs attention. Below, we'll cover common reasons why your furnace might be shutting off and how to troubleshoot the problem.
How Your Furnace Should Operate
Understanding how a furnace operates can help diagnose issues when something goes wrong. Typically, a furnace follows this process:
The thermostat signals the furnace to turn on when the indoor temperature drops below the set point.
The ignition system starts, and the burners ignite to produce heat.
The blower fan distributes the warm air throughout the home.
Once the set temperature is reached, the thermostat signals the furnace to shut off.
If your furnace doesn't follow this cycle smoothly, there could be an underlying problem. Below are some of the most common issues homeowners encounter.
1. Clogged or Dirty Air Filter
A clogged air filter restricts airflow, causing the furnace to overheat and shut off as a safety measure. If your furnace runs then shuts off and starts again, replacing a dirty filter might solve the problem.
How to Fix:
Locate your furnace's air filter, which is usually near the blower compartment or return air vent. Over time, dust and debris accumulate, blocking airflow and causing the furnace to overheat. If the filter appears clogged or covered in dust, replacing it with a clean one of the same size and type can restore proper airflow and furnace operation.
To prevent future issues, it's recommended to check and replace the filter every 1-3 months. A clean filter ensures the furnace runs efficiently, maintains indoor air quality, and reduces unnecessary strain on the system.
2. Malfunctioning Flame Sensor
A flame sensor detects the presence of a flame and signals the furnace to keep running. If it's dirty or faulty, the furnace blower comes on but won't ignite, or it may shut off after a few seconds.
How to Fix:
Turn off the power to the furnace before performing any maintenance. Locate the flame sensor, which is a thin metal rod near the burners. Over time, dust and carbon buildup can interfere with its function. Gently clean it using fine sandpaper or a soft cloth to remove any residue.
After cleaning, reinstall the flame sensor and restore power to the furnace. If the issue persists, the sensor may need replacement from an HVAC professional like Bardi.
3. Faulty Thermostat Settings or Placement
Incorrect thermostat settings or placement near heat sources can cause the furnace to cycle on and off irregularly.
How to Fix:
Ensure the thermostat is set to heat mode and that the temperature setting is above the current room temperature. If you have a programmable or smart thermostat, review the settings to ensure it's not turning the furnace off too soon.
Thermostat placement is also important. If it's located near a vent, direct sunlight, or another heat source, it may misread the temperature and cause the furnace to turn off prematurely. If necessary, consider relocating the thermostat to a central part of the home for accurate readings.
4. Blocked Vents or Registers
If air can't circulate properly due to closed or blocked vents, the furnace may overheat and shut down.
How to Fix:
Check all supply and return vents to ensure they are fully open and free from obstructions such as furniture, rugs, or curtains. Blocked vents restrict airflow, leading to overheating and frequent furnace shutdowns. Cleaning out any dust or debris around the vents can also improve airflow and system efficiency.
5. Limit Switch Issues
A limit switch shuts off the burner if the furnace overheats. If this switch is faulty, your furnace cycles on and off rapidly or won't stay on.
How to Fix:
Turn off the furnace and locate the limit switch, which is usually near the blower compartment. If the switch is stuck or appears damaged, try resetting it by turning off the power for 30 seconds before turning it back on.
If the issue persists, the limit switch may need to be replaced. Our skilled professional HVAC technician can determine if a replacement is necessary.
6. Pilot Light or Ignition System Failure
If you have a gas furnace and the pilot light or electronic ignition system isn't functioning properly, your gas furnace keeps shutting off and needs to be reset frequently.
How to Fix:
Check if the pilot light is lit. If it's out, follow the manufacturer's instructions to relight it. For furnaces with electronic ignition, listen for clicking sounds or check for error codes on the control panel.
If the pilot light won't stay lit or the ignition system fails repeatedly, professional servicing may be required to diagnose and repair the issue.
7. Short Cycling Due to an Oversized Furnace
If your furnace is too large for your home, it will heat the space too quickly, causing it to shut off frequently. This results in inefficient operation and higher energy bills.
How to Fix:
If your furnace shuts off after 30 seconds, it might be oversized for your home. Contact Bardi for an HVAC technician to evaluate your furnace size and determine if downsizing or adjusting settings can improve performance.
8. Faulty Blower Motor or Capacitor
A malfunctioning blower motor or capacitor can cause the furnace not to turn on, but the fan works. The blower is responsible for circulating warm air, so if it fails, the furnace may overheat and shut off.
How to Fix:
Listen for unusual noises coming from the blower motor, which may indicate mechanical failure. If the fan is running but no heat is coming through the vents, the motor or capacitor may need replacement. An HVAC professional can diagnose the issue and perform necessary repairs.
9. Gas Supply Issues
If there's an issue with the gas supply, the furnace may struggle to ignite or stay lit, leading to frequent shutdowns.
How to Fix:
Ensure the gas valve is open and supplying fuel to the furnace. If you suspect a gas leak, shut off the furnace immediately and call a professional for safety and repairs.
Preventative Maintenance Tips
To avoid frequent furnace shutdowns and repairs, consider these maintenance steps:
Change the air filter regularly.
Schedule annual professional inspections.
Keep vents and registers clean and unobstructed.
Monitor your thermostat settings and replace batteries as needed.
Clean the flame sensor annually.
When to Call a Professional
While some furnace issues can be resolved with basic troubleshooting, others require professional assistance. Contact Bardi if:
The furnace repeatedly shuts off despite troubleshooting.
You notice unusual smells or hear loud noises from the unit.
The furnace isn't heating your home adequately.
Conclusion
If you find yourself asking, "Why does my furnace keep running?" or dealing with frequent shutdowns, the issue likely stems from one of the common problems outlined above. From dirty filters and thermostat issues to ignition system failures, identifying the cause can help restore your furnace's normal operation. Regular maintenance and timely repairs can keep your heating system running efficiently throughout the colder months.
