A properly functioning single-stage furnace should run heating cycles that last 15 to 20 minutes and have two or three cycles per hour. If your furnace shuts off before it has run a complete heating cycle, it may not be bringing your home to the desired temperature or distributing the heat evenly. Here are five reasons why your furnace may continually shut off prematurely and what you can do about it.

1. Restricted Airflow

A furnace must draw cool air into the system and then push the warmed air back into your home. One of the biggest reasons why a furnace may shut down prematurely is because it cannot circulate air effectively. Consider the biggest causes of restricted airflow that most furnaces will eventually experience.

Clogged Air Filter

The common 1- or 2-inch air filter needs to be replaced every 30 to 90 days. When neglected, the filter will clog, restricting the airflow through it. This happens because it removes many of the common airborne contaminants before they can enter the system. How quickly this happens depends on how you use your furnace combined with your indoor air quality.

When the filter clogs, the system can’t draw enough air through the system to absorb the heat from the heat exchanger. This then trips the upper limit switch, causing the furnace to shut down. Check your filter every month to ensure you catch a dirty filter before it restricts the airflow through the system.

Other Airflow Restrictions

Your furnace produces heat as it burns its fuel, which then transfers to the air moving over the heat exchanger, which in turn carries it back out into your home. However, when the air can’t move effectively back into your home, the heat builds up in the system. Common airflow restrictions that prevent heat from leaving the system effectively include blocked or closed supply vents.

Ideally, keep all of your supply vents open. Check how much clearance your vents have, aiming for at least 6 inches above and around the vent to ensure air can flow from it freely. This not only allows the heated air to flow out but also to circulate effectively around your home.

Clogged Exhaust Pipe

The circulating air is not only the air that must move through the system. Rather, your furnace will draw air in to allow the fuel to burn and then push exhaust gases out.

The exhaust pipe will sometimes get clogged. Animals and nests may have gotten lodged inside the pipe; these are common causes of a clogged exhaust pipe. If you can safely get on your roof, check the inside of the exhaust chimney while the unit is off. Otherwise, call a professional to inspect the pipe to ensure that it’s clear.

Dirty Circulating Fan Wheel

Your circulating or blower fan is responsible for both drawing cool air into the system and pushing the heated air back out. Unlike fans you typically see around your home that have individual blades, this is a slatted wheel that moves air as it spins.

Even when you change air filters regularly, some contaminants will find their way into the system and build up on the wheel over time. As this happens, it will move less air until it’s moving so little that it trips the upper limit switch and shuts down your system. The best way to prevent this is to get routine maintenance for your furnace. As part of your maintenance, your technician will clean the circulating fan wheel, maintaining the volume of air it moves as it spins.

Malfunctioning Blower Motor

The blower motor is responsible for spinning the circulating or blower fan. This motor will eventually wear out over time and may need to be replaced. When the motor gets near the end of its service life, it may not spin as quickly, reducing the volume of air the circulating fan wheel moves.

In addition to cleaning your circulating fan wheel, a technician will test the blower motor itself during maintenance. They’ll ensure that it’s spinning at the proper speed, and they’ll balance the wheel and even lubricate the motor bearings if needed.

2. Old or Faulty Thermostat

Thermostats are the brain your furnace needs to operate effectively. However, they do not last forever but have an expected service life of 10 to 15 years. As the unit nears this age, the temperature sensors may not work properly, registering a higher temperature than is actual, shutting down the system prematurely.

Additionally, you may have other problems with your thermostat. This can be anything from improper placement, such as directly over a supply duct, to having faulty components that register inaccurate temperatures or send improper signals. If you believe you have a faulty thermostat, you can either replace it with a new one or have a professional perform a diagnostic to check for trouble.

3. Dirty Flame Sensor

The flame sensor is a critical safety mechanism contained in the burn chamber. It’s a temperature sensor that makes sure there is sufficient heat to ignite the fuel that comes into the burn chamber. If it doesn’t sense the heat, it’ll turn off the gas to prevent a buildup of fuel in the system. Over time, the flame sensor can corrode or build up soot if not cleaned regularly. To prevent this, a technician will clean the flame sensor during routine maintenance. They’ll also test the function to ensure that it’s sending the proper signal at the right time.

4. Bad High-Limit Sensor

One of the risks a furnace poses is the heat it generates as the fuel burns. Ordinarily, that heat dissipates through the circulating air and the exhaust that moves air out of your home. However, when there’s an airflow restriction, that heat can build to dangerous levels, potentially damaging the heat exchanger, which can lead to a toxic carbon monoxide leak. The high-limit switch monitors the temperature in the plenum and shuts down the system if it senses the temperature is rising unusually. That sensor can go bad over time, which can make it register too high a temperature. This is also something that your maintenance technician will test during routine seasonal maintenance.

5. Oversized Furnace

If you’ve recently had your furnace replaced, there could be an issue with its size. When it comes to furnace sizes, this refers to the heating capacity of the unit. If it produces too much heat, it can build up in the system, causing it to overheat and shut down.

This is why it’s so important to have a professional technician determine the proper size for your system. They will use what’s called the Manual J calculation to determine the heat load of your home. This not only considers your home’s size but also your home’s construction, its environment, how many occupants are in your home and several other factors.

When you need quality HVAC services around Jacksonville, rely on the same company people have trusted since 1966. The J&W Heating and Air + Plumbing team proudly provides AC and furnace repair, maintenance and installation together with indoor air quality solutions, thermostat installation and ductwork services. Call to schedule your furnace repair appointment with one of our trusted technicians today.

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