Pressure switch is the safety device that proves the ventor motor is working (motor above burners connected to vent). It has vacuum lines that are connected from the pressure switch to the ventor motor that measure a neg air pressure. All furnaces will have different pressures required to operate properly, most times the pressure will be written on the switch measured in inches water column.
Problems with pressure switches
Vacuum lines port plugged
You will get open pressure switch error code, It can easily be fixed then you should reset the furnace to delete error code.
Over time moisture will build up in the vacuum line port then dry up causing it the plug off. I have found the build up on the motor side of the vacuum line that was bad enough to shut down the furnace causing no heat. The vacuum line can be removed and the hole cleaned out with a small Alan key or anything small enough to fit in the hole with out breaking off. After cleaning it out you would need to shut power off at the furnace power switch to delete error code.
Pressure switch fails closed
Normally this would indicate a failed pressure switch, The furnace will test to make sure the switch is open before starting the ventor motor. The contacts in the switch are either stuck together or moisture has entered the switched and seized it in the closed position. The switch will need to be replaced then you should make sure the vacuum lines are sloped back to furnace so water will not enter the switch again.
Pressure switch fail open
First and most obvious would be failed ventor motor, check 110 volts to motor and if it has seized.
The most common problem on high efficient furnaces would be frost or water in the vent. First off the furnace being high efficient will produce water due to low vent temperatures and condensation in the vent pipe. Motor ventor motor has been replace recently then make sure orfice plate after motor is in place
Water I do not like horizontal vent pipes on furnace but sometime they must be done. If the pipe sages a little and water collects it will block off the pipe, Very important to have the pipes properly supported. When the Vent pipe is repaired it should drain back to the furnace where the water will be routed to a floor drain.
Frost the exhaust on the furnace is warm and moist, sometime the intake which is very close to the exhaust will suck in this warm moist air and mix it with cold air producing frost. Frost will plug up the intake pipe causing the furnace to fail, the exhaust vent should be extended 8 inches past the intake. Also you should be careful of fences and other obstructions the exhaust could rebound back to the intake pipe.
Anytime I am working on a furnace or during maintenance I will test the pressure switch for faulty contacts. I will start the furnace up and when the burners are on lightly tap the pressure switch if it stops the furnace then it should be replaced since contacts are not making a good connections internally.
How to test the pressure switch
Pressure switches are on the safety circuit of the furnace so that means it is 24 Volts a/c. Test from common to either side of switch when motor is running you should have 24 volts on each side. Once it is established you have power then you can connect meter similar to the way I have on the picture below. Connect Positive to one side and Negative to the other, when the ventor motor is running the meter will read 0 Volts. If you have 24 Volts, the pressure switch is open and not proving the ventor motor. Or you test any Voltage between 1 to 23 Volts then you have bad contacts and the full power is not getting through causing the furnace to fail. On a few furnaces I have found the ventor motor seams to be running fine and power is good but fails on pressure switch. When this happens it is best to record the Voltage from one side of the pressure switch to the other because the motor could drop rpm for very short time. The short drop in power is enough for circuit board to notice but not enough for you to notice.
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