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Test for the Furnace Differential Pressure Switches

 A differential pressure switch is designed to sense the negative pressure created by the draft inducer motor during the furnace startup and to shut down the furnace ignition if the air pressure is inadequate. A furnace differential pressure switch is a safety device located near the draft inducer motor of a gas forced-air furnace. The switch is there to prevent the furnace from running unless the correct venting air pressure is present.

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During combustion, the combustion blower creates an air pressure that is less than atmospheric (negative pressure) between the inlet side of the combustion blower and the inside of the burner box of the furnace. The draft inducer motor is a blower that creates a flow of combustion air through the furnace's heat exchanger to make sure all combustion exhaust byproducts are vented outside the home via the flue vent.  The differential pressure switch, which is normally open (switch is off), senses the drop in pressure and closes to complete the circuit (switch is on). The proper degree of negative pressure differential is necessary to maintain furnace operation. Without the pressure switch, there is the potential for exhaust gases to fill the living space should the vent pipe get blocked or if certain parts fail.

A differential pressure switch that fails to turn on could be caused by a number of problems, including: 
Failure of the draft inducer motor
Restricted intake air vent
Restricted combustion air vent
Leaks around assemblies
Clogged condensate drainage
Electrical failure of the pressure switch

How to Test a furnace differential pressure Switch
The differential pressure switch on a furnace may fail or get stuck in an open position, and there are few simple checks you can complete before testing the switch for electrical failure. Testing the switch itself requires a multimeter. 
1. Turn off the power to the furnace by switching off the disconnect switch located near the furnace. If there is no disconnect switch, turn off the furnace's breaker in the home's service panel (breaker box). 
2. Remove the main access panel on the front of the furnace. 
3. Inspect the hose(s) connected to the pressure switch to make sure it is connected at both ends and is in good condition. If the hose is cracked, replace it. 
4.  Remove the pressure switch hose(s) after carefully noting where it is connected (if there is more than one hose, remove and reinstall them one at a time). Make sure the hose is not obstructed inside. Do not blow into a hose that is connected, as this can damage the pressure switch. 
5. Use a flashlight to inspect inside the hose port on the pressure switch and make sure there are no obstructions. Remove debris with a small screwdriver or similar tool, being careful not to poke into the switch body. Do not blow into the inlet to remove the obstruction. Reinstall the hose(s).
6. Inspect the ventilation slots on the furnace cover to make sure they are clear of debris. Clean the vents, if necessary. 
7. Check the vent pipe for blockage, if the furnace is a condensing type. The vent pipe is a PVC (plastic) pipe typically extending out the side wall or through the roof of the house. 
8. Turn off the power to the furnace and remove the access panel. Disconnect the wires connected to the pressure switch by pulling the wire connectors off of the switch terminals.
9. Set a multimeter to test for resistance (ohms). Touch each tester probe to one of the switch terminals. The tester should read 0 or close to 0 (indicating no resistance). If the reading is higher, the switch has failed and must be replaced by a qualified technician.
10. Reinstall the furnace access panel, and restore power to the furnace. Test the furnace operation.

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