A pressure switch is used to activate electrical current in a furnace or air conditioning unit once the set amount of pressure is either applied or released. In the case of a furnace or boiler, the switch is modeled as safety device to shut down the unit should the proper combustion air flow--which in this case is what provides the pressure--not be obtained.
Types Of Pressure Switches
Pressure switches in general can be either hydraulic or pneumatic. In the case of HVAC products, switches are normally pneumatic. Most recognizable manufacturers make their own pressure switches for their respective units. In many cases, there's a single switch that works on most, if not all, of a company's product line. Amana, for example, makes a three-wire, one-hose pressure switch containing an orange wire (common), yellow wire (normally open) and blue wire (normally closed). This device is considered a replacement for more than 200 different Amana furnace models. The three-wire, one-hose setup is common, though subtle differences make cross-brand substitutions inappropriate. Rheem makes a setup, for example, that fits more than 25 Rheem/Ruud, Coleman or York furnaces.
Replacing A Pressure Switch
It's important to note that what often appears as a pressure switch problem may not actually be so. Remember that the switch's function is all based on air flow. Accordingly, anything that disrupts air flow--such as plugged exhaust or intake vents, heat exchange surfaces coated with soot or a clogged switch hose (which can be caused by anything from dirt to spiders nesting)--can all cause fluctuations in pressure that alter the proper function of a pressure switch. As for the actual replacement, all electrical power to the furnace must be turned off. The burner compartment door should then be removed, and wires removed from the terminals on the pressure switch. The hose that attaches the pressure tap to the pressure switch itself should then be removed as well. Finally, the screws holding the switch to the front of the blower partition should be undone, allowing for the switch's removal. Reassembly is a simple reversal of this process.
Who Makes Pressure Switches?
Though not an all-inclusive list, manufacturers of pressure switches include: