Why Does My Furnace Keep Shutting On and Off?

Furnace Keeps Shutting Off

Normal heating systems don’t run constantly, but instead, cycle on and off several times an hour. However, when your furnace is shutting off much more often than usual, that spells trouble. This type of furnace problem is called “short cycling.” 

Without warm air coming consistently through your air vents, hot and cold spots will show up throughout your house. If your furnace keeps shutting off and on, it generally indicates a problem with some aspect of your heating system. 

Dirty Furnace Filter

This type of furnace problem is called “short cycling.” As Brampton’s furnace repair and install specialists, let us walk you through these common furnace causes.

1. Your Air Filter Is Clogged

If your furnace’s air filter clogs with dirt, the air flow to the heat exchanger becomes restricted and the component overheats. This, in turn, triggers the high limit switch (your safety shutoff).

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To solve this problem, just turn off the furnace and change the filter, if you have the disposable type. Dirty air filters can send dust and debris into your heating system, and a clogged air filter might prevent air flow altogether.

Permanent filters, which do a better job of protecting your furnace and are more eco-friendly, are designed to be cleaned and reused.

Whichever filter you use, be sure to inspect it monthly and change or clean it every 90 days minimum.

For even better airflow, open some of your vents and have your furnace blower wheel cleaned.

2. Your Thermostat Is The Problem

Check your thermostat to determine if it is turned on. Make sure it’s switched to “HEAT” and set at the correct temperature. (If the temperature setting is too low, your furnace will shut off more frequently than you’d like.)

Consider the thermostat location as well. It should be positioned away from sources of heat or cold (like in direct sunlight or drafty doors) and mounted about 1.5 metres above floor level.

Ensure it isn’t mounted too high because, as you may remember from science class, hot air rises. If the thermostat registers extreme temperatures, a shortened furnace cycle may result.

Another possible problem could be that your faulty thermostat is not sending a signal to your furnace. Check this out with a buddy. One person should stand next to the thermostat, the other next to the furnace. Slowly raise the thermostat setting and listen for a noise from the furnace. Lack of any sound indicates that you may need professional thermostat repair or replacement.

3. You Have A Dirty Flame Sensor

The flame sensor is a safety device in your gas furnace designed to detect the presence of a flame when the burners are on. Its primary function is to ensure that the burners are igniting properly and that a stable flame is established during the heating cycle.

The flame sensor typically consists of a metal rod, often made of stainless steel, positioned in the path of the burner flame. When the burners are active, the flame should engulf the sensor. The sensor generates a small electrical current when exposed to the flame, indicating to the furnace control board that combustion is occurring as expected.

If the flame sensor becomes dirty or coated with soot and debris over time, it can interfere with its ability to accurately detect the flame. When this happens, the furnace may perceive a lack of flame or an unstable flame, leading to safety concerns. As a safety measure, the furnace control board may shut down the heat exchanger to prevent the release of unburned gas into the living space.

If you suspect that you have this problem, get expert Scarborough furnace repair as soon as possible.

4. The Furnace Is Too Big For Your Home

Your furnace may simply be too large for the size of your home. An oversized furnace will heat the space too quickly, shut off, and abruptly start again when the house cools down.

This furnace cycling is not only annoying, it also wastes energy and wears your furnace out faster than usual. Furthermore, your home is never really a consistent, comfortable temperature.

This is one of many reasons that affect how long furnaces last. The best solution is to replace your furnace with a correctly-sized, energy-efficient model, especially if it’s over 15 years old.

5. Your Exhaust Vent is Blocked

A blocked exhaust vent can lead to the buildup of harmful combustion byproducts, including carbon monoxide, and may cause the furnace to shut down as a safety measure. 

If you suspect or discover that your furnace exhaust vent is blocked, it is important to address the issue promptly to ensure the safe and efficient operation of your heating system.

Turn off your furnace and open windows and doors to help ventilate the area. If you can’t locate the source of the block, or cannot safely remove any debris without damaging your home’s air vents, it’s best to call in a professional HVAC technician. They’ll be able to remove the blockage and install vent caps or covers to prevent and future blockages.

6. You Have a Faulty Blower Motor

Technician performing Furnace Maintenance

One reason your furnace keeps turning on and off might be due to a faulty blower motor. The blower wheel has blades that push air through your heating system, and a dirty blower wheel won’t work as efficiently (or at all).

Without a working furnace blower fan, your short cycling furnace will not have proper airflow, and thus won’t be able to get your home to the desired temperature.

If you notice a burning smell, higher utility bills, or if your furnace keeps shutting on and off, you should take a look at the blower motor. Keep it from getting clogged by regularly changing your air filters; a dirty air filter will bring debris and dust into the air vents and will eventually affect the blower motor.

7. Your High Limit Switch is On

The high limit switch in a furnace is a safety device designed to monitor the temperature inside the furnace. If the temperature exceeds a predetermined limit, the high limit switch will automatically shut down the burners to prevent overheating and potential damage to the furnace. When the high limit switch is activated, it is an indication that the furnace is experiencing a condition that could lead to unsafe operating temperatures.

There are many reasons why the high limit switch may be on. Check for and address any issues related to restricted airflow, a dirty heat exchanger, blocked vents, or malfunctioning parts. This may involve changing the air filter, cleaning the evaporator coil, inspecting the blower motor, and ensuring proper ventilation. 

If you are unsure about diagnosing or fixing the issue, or if the high limit switch continues to trip after you’ve reset it, it is recommended to contact a qualified HVAC professional. They can perform a thorough inspection, identify the root cause, and perform necessary repairs to ensure the safe and efficient operation of your furnace. 

Find Friendly Expert Furnace Repair or Replacement

Whatever the cause of your furnace short cycling problem, rely on Dr. HVAC for help. We’re the friendly experts for furnace repair or furnace replacement anywhere in the Brampton area.


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