The pressure switch in a gas furnace is an important safety mechanism that ensures the furnace is drafting correctly. If the furnace doesn’t draft properly, it leads to dangerous combustion fumes being trapped inside the combustion chamber and heat exchanger instead of being vented outside. This can cause carbon monoxide to leak into your home. The fumes can also cause a delay before the furnace ignites, producing a small explosion or starving the flames of oxygen so that they escape or “roll out” of the combustion chamber.

The pressure switch on your furnace can fail due to various issues. The pressure switch itself can sometimes wear out and fail to activate, but more commonly, it will stop working due to some other problem with the furnace. If the pressure switch ever fails, it will prevent the furnace from being able to start. Let’s now take a closer look at how the pressure switch works and what can cause it to fail.

How a Furnace Pressure Switch Works

A pressure switch is like any other switch; it makes a connection to allow electricity to flow across it. The pressure switch is activated or closed by the furnace’s inducer fan, which then allows electricity to flow so that the furnace can start. The inducer fan draws air up and out of the furnace, and this creates a suction or air pressure that closes the switch. Once the pressure switch activates, it signals the gas valve to open so that the furnace can then light.

The inducer fan works to draw the combustion fumes out of the combustion chamber and heat exchanger, which is essential as the fumes would otherwise cause a delay in ignition. Whenever your furnace starts, you will hear the inducer fan run for around 30 seconds to a minute before the furnace actually ignites. Now that we’ve covered how the pressure switch works and what it does let’s look at the most common reasons that a pressure switch will fail.

Inducer Fan Issues

As we said, the inducer fan is what creates the pressure needed to activate your furnace’s pressure switch. If the inducer fan fails, then the pressure switch won’t activate, and your furnace won’t run. In addition, the inducer fan motor can wear out over time and eventually stop working. This means that your furnace will only run once you have the inducer motor replaced. As the motor ages, it can also sometimes cause the inducer fan to run too slowly and not create the amount of suction needed to activate the pressure switch.

Exhaust Flue or Heat Exchanger Blockage

Your exhaust flue can sometimes become clogged due to leaves and debris. Animals also sometimes nest in the flue over the summer, which can also result in it becoming blocked. Sometimes, the ports that lead into or out of the heat exchanger become clogged with soot, dirt, and grime. If the heat exchanger or flue is blocked, the furnace will be prevented from drafting properly. This means that the inducer fan will need help to create the suction needed to activate the pressure switch. These issues can easily be avoided with annual furnace maintenance, as the technician will inspect the flue and heat exchanger to ensure they aren’t blocked, and the furnace is drafting properly.

Suction Tube Issues

The pressure switch has a small suction tube that works to sense the difference in air pressure so that the switch can activate. If the suction tube is loose, cracked or has a hole in it, it will prevent the pressure switch from activating. The solution to this issue is to have the suction tube repaired or replaced so that it again works properly.

Electrical Issues

The pressure switch itself can sometimes get overloaded due to a power surge and burn out. In this situation, the only option is to have the switch replaced. More commonly the switch will fail due to a loose electrical connection. If the wires that connect to the switch are loose, it will sometimes prevent electricity from flowing even when the switch is closed.

Pressure switch issues can be quite difficult to diagnose, which means you will definitely need to hire a furnace technician to determine the specific cause of the issue and how to fix it. If you suspect your pressure switch has failed or you’re experiencing any other furnace issues, MD Air Conditioning & Heating is ready to help. We specialize in all types of furnace repairs as well as installation and maintenance. We also install and service air conditioners, heat pumps, and indoor air quality equipment. Give us a call today to schedule any residential or commercial heating or cooling service in the San Antonio area.

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