Why does my furnace keep turning on and off? 

Furnace Mind 1280x720 1

If your furnace keeps kicking on and off and your home never seems to stay comfortable, you’re not imagining it. This is a common issue known as short cycling.

Possible Reasons

  • Faulty thermostat
  • Oversized furnace
  • Dirty or broken flame sensor
  • Blocked exhaust vent
  • Furnace is Overheating

Your thermostat could be the issue.

One of the first things to check when the furnace seems to be short-cycling is the thermostat. The thermostat is the starting point and tells the furnace to turn on and how long to stay on, so even small issues can cause large comfort problems. Sometimes the problem is that the temperature is set too low, causing the furnace to cycle on and off. Or the thermostat might be faulty and misreading temperatures due to wiring problems, outdated technology(it’s old), or it’s dirty. 

At other times, it could just be in the wrong place. If the thermostat is located in direct sunlight or too high up, the thermostat can read the temperature wrong. If this is the case, then the thermostat needs to be moved to a better place. 

Your furnace might be oversized. 

This might seem crazy, but sometimes bigger isn’t always better. If your furnace is oversized, it can heat your home up to the desired temperature quickly and tell the thermostat to turn off. The problem is that if you add heat too quickly, it doesn’t give the warm air enough time to evenly heat your home. This can cause cold spots, resulting in uneven heating inside. Leaving you warm and cozy one second and cold the next, 

Dirty or Broken Flame Sensor

What is a flame sensor?

A flame sensor is there to keep you safe and make sure that there is a solid flame once the burners are turned on. Which is great, no gas filling up your home!  It works by sending an electrical current through a metal rod in the path of the burner. If the current connects great, a good flame. If it doesn’t, the furnace shuts off and stops the flow of gas. 

How to know if your flame sensor might be dirty or broken.

When a sensor gets dirty, your furnace may think it’s unsafe to keep running, which can cause it to shut off just seconds after it starts. If this happens, technically, the safety features are doing their job, but if the flame sensor is just dirty, you are left with a cold house. While fixing or cleaning the flame sensor may seem simple, we still recommend that you have an HVAC technician inspect it to double-check first that there isn’t actually a flame or gas problem. Well, that’s the flame sensor’s job anyway.

Blocked Exhaust or Faulty Pressure Switch

What’s a Blocked Exhaust? 

Another reason you might hear your furnace turn on and off without powering up is if the exhaust vent is blocked or dirty. All gas heaters have a vent that goes outside to release the bad stuff from burning natural gas or propane. The same way your tailpipe on your car sends everything from your engine out. There is a safety feature built into your furnace to make sure the bad fumes can escape. If they can’t, it will turn off, protecting you and your family from potential bad gases. 

Pressure Switch Issues. 

If your exhaust vents aren’t blocked, the pressure switch may be faulty. The pressure switch detects whether air can flow through the exhaust. If it is broken or faulty, it won’t allow the furnace to turn on. If you think this might be the case, have an experienced technician come and look at the unit. These repairs are tricky and, if done wrong, can be dangerous. 

Furnace Is Overheating

If your furnace is turning on and off every few minutes, it may be overheating. Overheating usually happens when airflow through the system is restricted, causing heat to build up inside the furnace. When this occurs, safety switches shut the system down to prevent damage. Here are a few reasons why it might be happening to your furnace. 

Dirty Air Filter

First and easiest, a dirty air filter restricts airflow into the furnace, preventing enough air from moving across the heat exchanger. This causes heat to build up and triggers safety shutoffs. Replacing a clogged filter is one of the most common and easiest solutions.

Cracked Heat Exchanger

The heat exchanger is what helps move the heat from the flames and into the air that will make your house warm. As furnaces age, the heat exchanger can crack, allowing heat to escape. When this happens, your furnace gets hotter than it would like, and it will shut off. Heat exchanger cracks can be dangerous and need a trained technician to diagnose and fix. 

Blower Motor Issues

Finally, the blower motor is what keeps air moving through the furnace and into your home. If the blower motor is set too low or blocked by debris, not enough air will move through the system. This causes heat to build up and the furnace to shut off.  Warning signs may include weak flow out of your vents, unusual noises like knocking, or a burning smell.

When to Call a Professional

There are other causes of furnace short cycling, but these are the most common. While replacing your air filter is usually safe to do yourself, we don’t recommend attempting other repairs without proper training.

If your furnace is turning on and off and you’re unsure why, give us a call. We’re here to help keep your home warm and comfortable.

NO SWEAT.