Wondering why your furnace keeps turning on and off? A healthy furnace should complete 2 to 4 heating cycles per hour, each lasting 5 to 15 minutes. If it’s cycling more frequently, there’s likely an issue—one that can lead to higher energy bills and poor heating.
Let’s explore the common causes of furnace short cycling and how to fix them.
How Does a Furnace Blower Work?
Before we get into the causes, it helps to understand how a furnace blower operates:
- The blower pulls cool air from your home through return ducts.
- The air is warmed by passing over a heat exchanger.
- Heated air is then pushed through supply ducts to heat your home.
- Once the set temperature is reached, the blower turns off until the next cycle.
Why Your Furnace Keeps Turning On and Off
1. Dirty or Clogged Air Filters
Clogged filters restrict airflow and cause overheating, forcing the system to shut down early as a safety measure.
Fix: Check and replace the filter every 1–3 months.
2. Blocked Vents or Ductwork
Obstructed vents make the system work harder and can trigger short cycling.
Fix: Make sure all supply and return vents are open and free of debris or furniture.
3. Thermostat Issues
A malfunctioning or poorly placed thermostat may misread temperatures and cause your furnace to short cycle.
Fix:
- Set the thermostat to “Heat” with the correct temperature.
- Avoid placing it near heat sources or drafts.
- Replace it if problems persist.
4. Faulty Fan Limit Switch
This switch controls when the blower turns on/off. If it’s faulty, it can interrupt cycles prematurely.
Fix:
- Try resetting by turning off power for 30 seconds.
- If issues continue, call a professional to replace it.
5. Dirty or Faulty Flame Sensor
The flame sensor ensures the burner is lit. If dirty or corroded, it may cut off the gas too soon.
Fix: Clean it gently with sandpaper or replace it if needed.
6. Overheating from Poor Airflow
Restricted airflow from dirty filters, blocked vents, or a failing blower motor can cause the furnace to overheat and shut off.
Fix: Regularly inspect and clean air filters and vents to ensure consistent airflow.
In a Nutshell – Why Your Furnace Keeps Turning On and Off
Short cycling often signals a problem with airflow, thermostat settings, or sensor malfunctions. Regular maintenance can fix many issues. But if your furnace still short cycles after basic troubleshooting, call an HVAC professional to inspect and resolve the problem.
FAQs
1. Can short cycling damage my furnace?
Yes. It adds wear and tear, leads to higher energy bills, and shortens the system’s lifespan.
2. How often should I replace my furnace air filter?
Every 1–3 months, depending on air quality and filter type.
3. When should I call an HVAC expert?
If replacing the filter or clearing vents doesn’t solve the issue, contact a professional for a thorough diagnosis.
