Furnace keeps starting and stopping

As winter approaches, the last thing you want is an unreliable furnace. Many homeowners face the perplexing issue of a furnace that continuously starts and stops, a condition known as short cycling. This not only affects your comfort but can also raise your energy bills and lead to premature wear and tear on your furnace. In this article, we delve into the reasons behind short cycling and provide actionable solutions to ensure your furnace runs smoothly.

What is Short Cycling?

Short cycling is a term used to describe when a furnace turns on and off more frequently than it should. Ideally, a furnace should complete a full heating cycle, from igniting to reaching the desired temperature and then shutting off. However, with short cycling, the furnace shuts off before the home is adequately heated, only to restart again shortly after.

Common Causes of Furnace Short Cycling

Understanding the root causes of short cycling can help you identify the best course of action. Here are the most common reasons why your furnace may be short cycling:

1. Overheating

Overheating is a primary cause of short cycling. Furnaces have built-in safety features that shut off the burner if the system gets too hot. Causes of overheating include:

  • Dirty air filters: A clogged filter restricts airflow, causing the heat exchanger to overheat.
  • Blocked vents: Closed or obstructed air vents can also lead to overheating.

2. Faulty Thermostat

A malfunctioning thermostat can lead to erratic furnace behavior. Issues may include:

  • Incorrect placement: If the thermostat is near a heat source or in direct sunlight, it may inaccurately read the room temperature.
  • Electrical issues: Wiring problems can cause the thermostat to lose its connection to the furnace.
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3. Improper Furnace Size

A furnace that is too large for your home will heat it quickly and shut off, only to turn on again soon after as the temperature drops. This is inefficient and can severely shorten the lifespan of your furnace.

4. Flame Sensor Problems

If the flame sensor is dirty or faulty, it may cause the furnace to shut off as a safety precaution, assuming that no flame is present when there is one.

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How to fix furnace keeps starting and stopping

If your furnace is short cycling, it’s important to approach the situation methodically to ensure safety and prevent further damage to the system. Here are the steps you can take to troubleshoot and potentially fix a furnace that keeps starting and stopping:

1. Check the Air Filter

The first and easiest step is to inspect the air filter.

  • Action: Locate the filter, usually found in the blower compartment, and check if it’s dirty.
  • Fix: If it’s clogged with dust and debris, replace it with a new one appropriate for your furnace model.

2. Inspect the Thermostat

A faulty thermostat can cause short cycling.

  • Action: Check if the thermostat is in the correct mode and set to a temperature that calls for heat. Ensure it’s not located in a place where it can get false readings, like near a heat source or in direct sunlight.
  • Fix: If the thermostat is battery-operated, replace the batteries. If it’s an older model or appears to be malfunctioning, consider upgrading to a newer, more reliable thermostat.

3. Examine the Vents and Registers

Blocked vents can lead to overheating.

  • Action: Walk through your home and make sure all vents and registers are open and unobstructed.
  • Fix: Remove any furniture, rugs, or curtains blocking the airflow.

4. Look at the Flame Sensor

A dirty or faulty flame sensor can cause the furnace to shut off as a safety measure.

  • Action: Locate the flame sensor, which is usually found in the burner assembly, and visually inspect it.
  • Fix: Gently clean the sensor with fine emery cloth, steel wool, or a soft-bristled brush if it’s dirty. If it’s faulty, it will need to be replaced.
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5. Check the Exhaust Vent

A blocked exhaust vent can cause the furnace to overheat and shut down.

  • Action: Inspect the exhaust vent for blockages like bird nests, debris, or ice.
  • Fix: Carefully remove any obstructions you find. If the venting system appears damaged, call a professional.

6. Evaluate the Heat Exchanger

An overheating heat exchanger can lead to short cycling.

  • Action: This is a bit more complex and usually requires a professional to inspect the heat exchanger for cracks or other damage.
  • Fix: If there is damage, it’s often a significant repair that requires professional assistance.

7. Inspect the Control Board and Limit Switch

The furnace’s control board and limit switch are safety features that can cause shut down if they’re not working properly.

  • Action: These components usually require a professional’s diagnostic tools to test.
  • Fix: If faulty, they will need to be repaired or replaced by a professional.

8. Consider Furnace Size

An improperly sized furnace can frequently cycle on and off.

  • Action: If you suspect your furnace is too large for your space, have an HVAC professional perform a load calculation.
  • Fix: If the furnace is indeed too large, the only real fix is to replace it with a properly sized unit.

9. Schedule Regular Maintenance

Annual maintenance by a professional can prevent issues like short cycling.

  • Action: Book a service appointment with a certified technician.
  • Fix: The technician can perform a comprehensive check and tune-up.


A furnace that keeps starting and stopping is more than a nuisance; it’s a sign that your heating system needs attention. By understanding the causes of short cycling and taking proactive steps, you can extend the life of your furnace, improve your home’s comfort, and keep energy costs down. Remember, when in doubt, it’s always best to consult with a professional to ensure safe and effective solutions.