Why Did Your Gas Furnace Just Switch Off?

Posted on: 28 June 2021

Imagine this scenario: you're enjoying comfortably warm air from your HVAC vents when suddenly the breeze turns cold and the system shuts down. Upon further investigation, you discover that your furnace has shut down. Depending on the model, you may be able to open up a panel and reset the unit, but what caused it to shut off in the first place?

Modern furnaces contain various safety features that allow them to shut down when needed. These features may trip due to potentially hazardous health conditions, or they may shut the furnace down to prevent damage to the unit itself. Whatever the case, understanding why your furnace is shutting off can help you to diagnose and repair the underlying problem.

Too Hot or Too Cold?

Two common reasons for your furnace to perform an emergency shutdown boil down to its operating temperature. Gas furnaces work by performing combustion in a sealed chamber and extracting heat from the exhaust fumes via an exchanger. The heat exchanger performs the vital role of containing harmful exhaust gases while still allowing you to utilize the warmth generated by combustion.

Hot air reaches the vents in your home because a blower pushes it away from the area around the heat exchanger. This process also cools the exchange unit, preventing it from overheating and cracking. If the temperature around the heat exchanger becomes too hot, the limit switch will trigger and shut the furnace down to avoid potential damage and hazardous exhaust leaks.

On the other end of the scale, a flame sensor detects the state of the combustion process itself. If the flame is too cold or not burning at all, then you're at risk of both potential gas leaks and carbon monoxide leaks due to backflow. As a result, your furnace will also shut down when your flame sensor detects a problem.

Poor Airflow

While any gas furnace can shut down due to either of the conditions above, high-efficiency furnaces may also turn off due to airflow problems. These furnaces use a secondary blower (a draft inducer) to clear exhaust gases from the combustion chamber. These furnaces use a pressure switch to detect the negative pressure induced by the draft blower and confirm that it is correctly operating.

If something is obstructing airflow or the draft inducer is malfunctioning, your furnace will shut down. This safety protects you from harmful gases building up in the combustion chamber and ensures your furnace operates as efficiently as possible. A high-efficiency furnace will not function without adequate airflow, so you'll need to fix the root cause to restore your furnace.

You can figure out the immediate cause of the shutdown on most furnaces by reading an error code from the control board. This code will tell you the sensor that triggered the reset, but you'll need to investigate further to discover the underlying cause for the necessary furnace repair. A qualified HVAC technician can help you find the problem and repair it permanently.