Signs of a Bad Pressure Switch in a Furnace

A well-designed furnace has safety features incorporated into the equipment to prevent problems. Among the safety devices in a furnace is the pressure switch. When the furnace begins the ignition sequence, the pressure switch is among the components that must close a circuit to allow the furnace to start up normally. Although the pressure switch can be a safeguard against some issues in your furnace, the problem is sometimes the pressure switch itself. Know how to identify signs that the switch is faulty.

Hot water heater , gas furnace and air conditioning unit
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You may see symptoms of a bad pressure switch in your furnace.

Furnace Won’t Ignite

If your furnace won't ignite, there could be problems with the blower motor, the igniter, the gas or power supply and many other components of the system. But a failed ignition sequence is also a sign of a bad pressure switch. If the switch is stuck in the open position, it will not allow the current to complete the circuit needed to ignite the furnace and turn on the blowers.

Burnt Terminals

There are electrical terminals on your furnace pressure switch where the device will send the electrical signal to the control board to verify that it is ready to ignite the furnace. If you inspect these terminals closely and notice that the terminals are burnt or discolored, then you can assume there is a problem with the component.

Wrong Voltage

You can use a volt and ohmmeter to determine the voltage of the pressure switch easily. You simply have to hook one probe to the pressure switch terminal and the other to a grounded piece of metal on the body of the furnace and check the reading. You should ideally get somewhere between 24 and 28 volts on your meter if the pressure switch is in good condition. If you have no reading or the numbers are far from this range in either direction, there is likely a problem with the switch.

Other Causes

A pressure switch may appear to be faulty when it is simply doing its job. The pressure switch indicates whether the air pressure blowing through the system is adequate. If the pressure is not high enough, the switch opens up and breaks the circuit to the rest of the furnace, shutting it off or preventing it from starting. A clog in the ductwork or extremely dirty filters could potentially make you make the wrong diagnosis about a properly working pressure switch.