Furnace Short Cycling

Common Symptoms

You might be experiencing this problem if you notice any of these signs:

• Furnace runs 3-5 minutes then shuts off
• Constant on-off cycling
• Never reaches thermostat temperature
• Uneven heating throughout home
• High heating bills
• Burners ignite then shut off quickly
• Home doesn't stay warm
• Excessive furnace wear

Step-by-Step Diagnosis

Follow these steps to identify the exact cause of your problem:

1. Time how long furnace runs before shutoff
2. Note time between cycles
3. Check air filter condition
4. Verify all supply vents are open
5. Check for blocked return air vents
6. Inspect flame - should be steady blue
7. Check thermostat placement and calibration
8. Verify proper thermostat heat anticipator setting
9. Look for obstructions around furnace intake
10. Check if home reaches set temperature
11. Note if problem is worse in mild weather
12. Inspect for closed doors blocking airflow

Most Common Causes (Ranked by Likelihood)

DIY Solutions

Step-by-Step Instructions:

1. Replace air filter immediately
2. Open all closed supply vents in home
3. Remove furniture blocking return vents
4. Turn off furnace power at switch
5. Locate flame sensor rod near burners
6. Remove flame sensor (usually one screw)
7. Clean sensor with fine sandpaper or steel wool
8. Wipe clean and reinstall
9. Check thermostat - move if near heat source
10. Adjust thermostat differential if programmable
11. Keep interior doors open for airflow
12. Run furnace for 24 hours after repairs

When to Call a Professional

Don't attempt DIY repairs if you experience any of these situations:

• Problem continues after filter replacement and cleaning
• Furnace is less than 5 years old and cycling
• Home is much smaller than furnace capacity
• Recent furnace installation or replacement
• Limit switch keeps tripping
• Yellow or flickering flames
• Rumbling or banging during operation
• Multiple previous service calls for same issue

Prevention Tips

Stop this problem before it starts with these preventive measures:

• Get proper load calculation before furnace replacement
• Replace filters monthly during heating season
• Never close more than 20% of vents
• Keep interior doors open or install jump ducts
• Install programmable thermostat with longer cycles
• Schedule annual pre-season tune-up
• Consider two-stage furnace to reduce cycling