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What Are Flue Gas Spill Switches

While some components of a heating system make sense to the average Olympia homeowner – think blower fan, thermostat and air ducts – others are more esoteric and prone to bouts of head scratching.

So, you may find yourself asking “what the heck is a flue gas spill switch?” Read on for the answer.

As you know, gas heating appliances produce heat by means of combustion. The gas line feeds gas into the appliance, the gas is ignited, and the burning gas produces heat. It’s a simple concept that goes all the way back to our caveman ancestors building fires to keep warm, and it is the same process in gas furnaces, boilers and water heaters.

In addition to producing the cozy heat we love in the winter time, this combustion process also releases gases. Known collectively as “flue gases,” some of these – carbon monoxide being the most notorious – can be very toxic. This is why we have flues or chimneys in our northwestern homes– to give these gases a means of egress.

A flue gas spill switch is designed to shut down the furnace if these gases start seeping out. It is made up of a sensor or series of sensors that detect heat outside the flue, not unlike the flame sensor in your furnace. If flue gases start to escape and pass by the sensor, the sensor heats up and signals the furnace to shut down. This cuts off the power and gas, so that no more flue gases can leak into the home and create a health concern.

If your furnace has been abruptly shutting down, it could be your flue gas spill switch trying to tell you that you have a leaky or cracked flue. If this is the case, you want to have it repaired right away. The constant off and on is not good for the furnace, and more importantly, those flue gases can be exceptionally hazardous to your family’s health.