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Furnace Safety Switches: A Quick Guide

smoke-detector-flameIf you’ve ever worried about the safety of your furnace, you’re not alone. In our area, furnaces go unused for so long that many people are afraid to start their systems at the beginning of the season. In general, modern furnaces are safe, but it’s certainly ideal to have your furnace serviced at the beginning of the heating season. You should also keep smoke detectors and carbon monoxide detectors throughout the home.

Most of the protection from these risks, though, comes from the safety switches installed within your heater. These switches protect your home from unsafe conditions in a variety of ways (which does not mean you should skip annual service)!

The Flame Sensor/Thermocouple

The flame sensor is a small rod that has one job—to detect whether or not there is a flame present. In older furnaces, this is called a thermocouple, and it works a bit differently. The thermocouple monitors the ongoing flame of a standing gas pilot, while the flame sensor is only needed intermittently when the heater cycles on. They both, however, connect to the gas valve.

When a modern-day furnace calls for heat, a gas burner should light up after a hot surface or an electronic spark ignites it. If the electronic ignition system is not working properly, natural gas could leak from the system—if not for the flame sensor. This safety component shuts off the gas valve so that the furnace will not operate without a flame.

The Fan Limit Switch

The blower fan of the furnace serves a lot of different functions. Of course, it moves heated air into the ducts and around the home. It also sucks in air for use with the furnace. Finally, it ensures enough heat moves out of the furnace air plenum to keep the system from overheating. However, a fan can malfunction. If so, the furnace plenum is in danger—if not for the fan limit switch, which shuts off the system before it can overheat.

A second function of the fan limit switch is to keep the fan from running when the furnace plenum is not hot enough. This happens with assistance from the thermostat relay.

The Thermostat Relay

When you turn on the heat at the thermostat, the furnace blower fan does not immediately start up. This could be dangerous, as the flame sensor has not yet had time to detect whether a flame is present. Instead, a thermostat relay ensures the fan does not start up until the furnace has started up.

Air Pressure Switch

One more furnace safety switch we’d like to discuss is the air pressure, something that is not a feature on all furnaces but that you should certainly look out for when purchasing a new one. Carbon monoxide poisoning is a very real possibility when you have any gas appliance. A gas furnace vents carbon monoxide and other byproducts of the combustion process through the flue pipe that leads outdoors, but this process could go wrong.

If there is a leak in the flue pipe, carbon monoxide could move into your home, and this toxic gas is not easy to detect. An air pressure switch detects when there is not enough pressure moving through the flue vent, in cases like these, and shuts off the system to keep you safe.

For furnace repair in Beaumont, TX, you can count on the professional technicians at Air Services Unlimited.

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