A pressure switch is a safety component on a furnace. It has sole responsibility for ensuring that the exhaust blower or inducer is operating properly. This is why you will find a hose connected directly to the inducer housing. Each pressure switch has a designed working pressure rating measured in “inches of water column”. This is a term relating to the particular pressure needed to be produced by the inducer the switch is attached to through the hose.
There are several different electrical connections. One is common and the other two options would be “NC” (normally closed) and “NO” (normally open). This relates to whether or not the switch opens or closes when the inducer runs. When the inducer runs and the pressure reaches the preset amount, the switch allows a low voltage (24 volts) current to pass through it and back to the control board allowing it to start the ignition process.
If your furnace quits working and you believe the issue to be the pressure switch, you should check a few things before you go out and buy a new one. The first would be to remove the hose from the pressure switch and attempt to blow through it. If you can’t do this, have a hard time, or hear a “gurgling” sound from water, the switch may not be the root of your issues. Check the nipple that the hose attaches to on the inducer housing and be sure it is clear. If you hear a “gurgling” sound, this means you have a backed-up condensate drain and need to remove and flush this out.
Another issue that I see a lot of “do-it-yourselfers’” do is fail to notice that the inducer motor itself is not running. In that case, a new pressure switch will not help you! If you have an 80% efficient furnace, you need to remove the flue pipe from the top of the inducer because you may have a bird stuck in there and blocking the flue gas. This will cause the pressure switch to trip and I have even seen them stuck in the inducer wheel, locking the motor up. If this happens you will hear the motor trying to start and if you touch it, be careful because it will be red hot.