Why Your Furnace Continues to Turn...

It’s the middle of a frigid cold winter evening, and your furnace continues to turn on and off—though your home remains cold. What’s happening?

This annoying problem is referred to as short-cycling, and it’s a typical indication that something isn’t operating as it should be. Short-cycling occurs when your furnace shuts down before it finishes a full heating cycle. Most of the time, this is caused by constricted airflow, a malfunctioning thermostat, or mechanical problems.

At Damiani’s Comfort Design, our background-checked HVAC technicians have helped thousands of homeowners across the San Antonio Metro area resolve short-cycling and other heating problems—quickly and professionally.

What’s a Normal Furnace Cycle?

On a typical cool day, your furnace should cycle on and off about three to eight times per hour, with each cycle lasting around 10 minutes. During extremely cold weather, those cycles may become longer and more frequent.

A complete furnace cycle consists of:

• The burner firing
• The heat exchanger heating the air

If your furnace is shutting off after a few seconds, it never gets to finish this process—meaning poor heating and wasted energy.

Common Causes of Furnace Short-Cycling (and How to Fix Them)

1. Dirty Air Filters

Air filters can help clean indoor air by filtering out dust and dirt. If this occurs, the blower motor in the furnace can overheat and activate the high limit switch, which will shut down the system.

Fix: Replace your air filters every 1 to 3 months.

2. Blocked or Closed Air Vents

Supply or return vents that are clogged will cause overheating and result in an automatic shutdown. Even for unused rooms, it is crucial to keep vents open to ensure correct airflow.

Fix: Make sure all vents are open and not obstructed by furniture or drapes.

3. Malfunctioning Blower Motor

If your furnace is older, the blower motor might be worn out. When this motor fails, your furnace can’t circulate warm air properly—causing short-cycling or shutdowns.

Check: Place your hand near a vent while the furnace runs. If there’s little airflow, the blower may be the issue.

Fix: Contact an HVAC technician for diagnosis or replacement.

4. Faulty Thermostat

An improperly working or badly placed thermostat might send incorrect signals to your furnace, causing early shutdown. Thermostats near heat sources or direct sunlight may misread indoor temperatures.

Fix: Check placement and functionality. Replace if needed.

5. Wrong Furnace Size

If your furnace is too powerful for your home, it will heat spaces quickly and shut off before the air is evenly distributed—leading to short-cycling.

Solution: Replace with a properly sized system for your home.

6. Dirty Flame Sensor

The flame sensor is a safety device that halts gas flow if no flame is detected. A dirty sensor might falsely read that there’s no flame and shut off the system.

Fix: Clean the flame sensor. While it’s a simple task, it’s best handled by a professional if you’re unsure.

Prevent Furnace Problems with Regular Maintenance

Prevent heating headaches before winter by investing in seasonal furnace maintenance. Our Comfort Club Membership offers scheduled tune-ups, ensuring your system runs efficiently before cold weather hits.