Furnace Woes: Why Do They Start And Stop?

If your furnace is turning on and off repeatedly, it could be what is known as ''short cycling'. This is when a furnace fails to reach the desired temperature and instead turns off and on in quick cycles. There are several reasons why this could be happening. It could be due to a malfunctioning flame sensor, a dirty air filter, blocked vents, or a poorly placed thermostat. It could also be that your furnace is too big for your home. If you are unsure of the cause, it is best to contact a professional for help.

Restricted airflow

Blocked or Leaky Ducts

Air ducts are responsible for guiding hot air throughout your home. If there is insufficient or poor airflow, it is likely that your air ducts are compromised. Wear and tear, the environment, and frequency of use can cause holes to develop in the ductwork, allowing warm air to escape. Buildups of dust or other debris on the ducts can also cause airflow issues as the air struggles to get through.

Closed or Blocked Air Supply Vents

Closed or blocked vents limit airflow, causing the furnace to overheat and shut off prematurely. Make sure all vents are open, even in rooms that are not in use, to prevent your furnace from shutting off.

Dirty Air Filters

Dirty air filters can cause short cycling. A clogged filter restricts airflow, causing the furnace to overheat. Regularly cleaning or replacing air filters can prevent this issue.

Dirty Blower Wheel

Dirt on the blower wheel can reduce efficiency, causing the furnace to short cycle. Regular maintenance can help avoid this issue.

Blocked Flue Pipe

A blocked flue pipe can prevent exhaust gases from escaping, causing the furnace to overheat and shut off.

Faulty Thermostat

A malfunctioning or improperly placed thermostat can cause short cycling. For example, if the thermostat is too close to a heat source, it can misread the temperature and cause the furnace to shut off prematurely.

Malfunctioning flame sensor

A malfunctioning flame sensor is one of the most common issues with furnaces. A flame sensor is a safety component of a furnace that is located on the interior of the burner assembly. It monitors and verifies whether a flame is burning inside the unit. If the sensor does not detect an active flame, it will shut the furnace off to avoid a potential gas leak or explosion.

• Furnace turns on and off very quickly (after 2-30 seconds)
• The flame sensor is cracked
• Corrosion or dirt is visible on the flame sensor
• Yellow burner flames
• Rusted flue lines
• High utility bills
• Constant thermostat adjustments

If you suspect that your furnace has a malfunctioning flame sensor, it is important to take action to address the issue. Here are some steps you can take:

• Check for any visible signs of damage or dirt on the flame sensor.
• Clean the flame sensor with an emery cloth or fine abrasive pad.
• If cleaning does not resolve the issue, you may need to replace the flame sensor. This can be done by following the manufacturer's instructions or by contacting a licensed HVAC technician.
• Ensure that your furnace is properly ventilated and that the exhaust vent is clear of obstructions.
• Regularly clean or replace your furnace air filter to prevent dirt and debris from building up and restricting airflow.

Poorly placed thermostat

A poorly placed thermostat can cause your furnace to start and stop repeatedly, a phenomenon known as "short cycling". Short cycling can be confusing as furnaces are meant to turn on and off repeatedly. However, short cycling is characterised by the furnace turning off and on in a very short period of time, roughly anywhere from 2 seconds to 2 minutes. This can happen due to poor thermostat placement, where the thermostat is located near a heat source (like a fireplace), a window, or on an external wall. This can cause skewed sensor readings, as the thermostat will consider the house to be hotter than it actually is and will therefore prematurely shut the system down.

To avoid this, the thermostat should be placed in a central location where your family spends time. It should be away from any sources of secondary heat, windows and doors, and external walls. If you need to move your thermostat, refer to the installation instructions that came with the device.

Overheating

To prevent overheating, it is important to regularly clean or replace air filters, ensure that all vents are open and unblocked, and maintain the blower wheel. It is also crucial to keep the area around the furnace clear and well-ventilated to promote proper airflow.

If your furnace is overheating, you may need to reset it or wait for it to cool down before it can resume functioning. In some cases, you may need to call a professional HVAC technician to diagnose and resolve the issue.

Blocked vents

A blocked vent will prevent your furnace from heating up, and it may shut down completely. This is because the vent blockage disrupts the airflow, causing a lack of oxygen for combustion. This can lead to a build-up of dangerous gases, such as carbon monoxide, inside your home. This is extremely hazardous, as carbon monoxide can cause illness and even be fatal. Therefore, many furnaces are designed to shut off automatically when the vent is blocked.

To prevent blockages, it is important to keep the area around the vents clear. Rake up leaves, trim any nearby bushes or vegetation, and clear away snow and ice. You should also regularly inspect and clean the vents to remove any debris or obstructions. A small brush or vacuum can be used to clear debris from the vent screen. If the blockage is more severe or difficult to reach, you may need to call a professional for assistance.

By taking these preventative measures and staying vigilant, you can help ensure your furnace functions safely and efficiently, and avoid the risks associated with blocked vents.

Frequently asked questions