Furnace pressure switch
A furnace pressure switch is a safety device located near the draft inducer motor that will prevent operation of the furnace if correct venting air pressures are not detected. The furnace pressure switch is designed to sense the negative pressure created by the draft inducer motor at furnace start up and to shut down furnace ignition if proper differential air pressures and venting are not maintained.
The Draft Inducer and Pressure Switch Operation
The draft inducer motor is a blower that creates a flow of combustion air through the furnace's heat exchanger(s) to make sure all combustion exhaust byproducts are vented outside the home by the flue vent. During combustion the combustion blower creates an air pressure that is less than atmospheric (negative) between the inlet side of the combustion blower and the inside of the burner box of the furnace. The pressure switch which is normally open senses the drop in pressure and closes to complete the circuit. The proper degree of negative pressure differential is necessary to maintain furnace operation. When abnormal conditions exist such as such as draft inducer motor failure, restricted intake air vent, restricted combustion air vent, leaks around assemblies, or clogged condensate drainage, then sufficient differential pressure will not exist and the pressure switch will open and shut down the furnace.
Types of Pressure Switches
The furnace pressure switch has a slightly different design depending on if it is used in a conventional furnace or a high efficiency condensing furnace. If used with a single stage conventional furnace the pressure switch will have one hose leading to the body of the draft inducer fan.
Single stage condensing furnaces will have will have two hoses on the pressure switch, one for sensing the pressure at the draft inducer / burner enclosure and the other for sensing proper venting pressure at the condensate collector box.
Two stage furnaces may have two pressure switches and modulating furnaces may have.
The pressure switch on a furnace may fail or get stuck in an open position. To test for proper functioning of the furnace pressure switch, you will need a multimeter (voltmeter/ohm-meter), a thermostat at a setting calling for heat, and the switch’s power wiring leads disconnected from the pressure switch terminals.
- For each of the switch’s two terminals, the pressure switch ohm meter should read 0 or close to 0 (no resistance) when the ohm meter leads are placed on each of their respective switch terminals.
- A resistance reading of infinity or a large number means pressure switch failure.
- Another method is to use a volt-meter where the voltage should read between twenty-four to twenty-eight volts DC between each terminal and ground.
- You should also check for a clogged hose port at the draft inducer or cracked or broken hose(s) running from the pressure switch to the draft inducer or condensate collector box which can cause a tripped switch (failure) condition.