What Does a Spill Switch Do

A spill switch is a device designed to sense and correct unsafe venting of combustion gases from a gas-fired appliance into a building.

How does a vent safety switch work?

The vent limit switch, which is also referred to as a fan limit switch and is typically shaped like a small metal disc, prevents damage to your furnace by shutting the furnace down in the event that it overheats. This prevents the heat exchanger from warping and cracking from temperatures that are too high.

What causes flue gas spillage?

Anything that depressurizes the building or the area around the furnace can cause flue gas spillage. Improper air circulation, which leads to inadequate combustion air, can also lead to flue gasses spillage. … For example, many heating systems vent flue gases back into the house in the first few minutes after startup.