Troubleshooting a Non-Condensing Gas Furnace Pressure Switch

by Phil Rains
If you work in the HVACR business, you have probably had the opportunity to troubleshoot non-condensing gas furnaces in the field, whether packaged units, or furnaces combined with air conditioners or heat pumps (split systems).  These furnaces are typically rated around 80% AFUE (a measure of efficiency), and are vented to the outside via a metal vent/pipe configuration, or in the case of a packaged system, a side discharge outlet/hood.
On occasion, the non-condensing furnace control board (called various names like integrated ignition control, furnace control board, DSI, etc.) has a diagnostic LED light, especially on modern furnaces. Abnormal heating operation can often be indicated by the diagnostic LED light on the control board if the unit encounters an internal fault.
Occasionally, the furnace will go into a “hard” lockout and turn off the diagnostic LED. If this occurs, check the power supply to unit for proper voltage. Check all fuses, circuit breakers and wiring. Disconnect the electric power for five seconds. If the LED remains off after restoring power, replace the control board.
More often, the furnace will have an “external” lockout which occurs if the control determines that specific faults occur such as, but not limited to, a measurable combustion cannot be established within several consecutive ignition attempts, flame is established but lost during run, the flame rollout protection device opens, a drop in flame signal occurs, the primary limit switch opens, or if flame is detected with the gas valve de-energized. These situations occur sporadically with most gas furnaces.
The other faults that are often more common today concern the pressure switch.
A pressure switch is a safety device to shut down a furnace if proper combustion air flow is not being provided. All pressure switches in a non-condensing gas furnace are placed there for safety purposes. Generally, the single pole type of switch typically used allows electrical current to flow when the pressures are at an acceptable operational level based on the manufacturer design.
 A pressure switch problem that often occurs is the pressure switch is closed prior to furnace operation. When the control board senses a call for heat from the thermostat (24VAC on the "W" terminal) one of the first things it does is check to see that the pressure switch is open or non-energized. This is the normal state for most pressure switches on furnaces. If the control board senses that the pressure switch is open it then sends power to the inducer blower and then checks to make sure that the pressure switch closes. If it is then closed after a short time period (based on furnace manufacturer design) the control board proceeds with the ignition sequence.
 If the control board senses a closed pressure switch before it has energized the inducer, its program determines that something is wrong and for safety does not continue with the ignition process.
 This situation could be caused by something that is causing the switch to stay closed. Blockage in the hose or hoses going to the switch could be the problem. You also could have a defective switch. It is possible (but less likely) that you have a short in the wiring or (even less likely) that you have a defective furnace control board. You can check that the pressure switch opens and closes with an ohmmeter when you blow or suck on the pressure hose. Of course, perform this check with power removed from the furnace.
Another more common pressure switch problem encountered that can arise with furnaces is that the pressure fails to close after the inducer has started to run. This blower is called an "inducer blower." When the furnace gets a call for heat, it starts up the inducer to start blowing air (either sucks it into the burners ahead of the flame or blows it out after the flame—dependent on the manufacturer design). Since the whole point is to create a "draft", the furnace control must determine when this has been accomplished. The pressure switch “tells” the control board that there's sufficient air movement to go ahead and fire up the ignitor and then open the gas valve. In most cases, the pressure switch (normally open when the system is off) will close due to negative pressure created by the inducer running in a short period of time. Different pressure ratings exist for different pressure switches. These ratings often appear on the side of the switch. This is so replacement of a bad switch can be exactly what is there originally.
A pressure switch that is stuck open can be caused by many issues such as a plugged vent, a heat exchanger surface /passage problem due to carbon deposits (usually on LP gas furnace more that natural gas), the inducer slows down due to age, a pressure switch hose is plugged with debris, or simply a bad pressure switch is now in the furnace.
It only takes a small restriction to keep the pressure switch from closing.
One of the first steps in troubleshooting the open switch is to test it to see if it is defective. Remember, electrical devices (switches in this case) typically wear out over time. Using a voltmeter, place one probe on a lead wire going to the pressure switch and "ground" the other probe. To "ground" is to attach the probe to metal that is part of the furnace. The meter, which you should have set to volts AC, should register approximately 24 or more volts (24 to 28 volts is normal). A two wire pressure switch you should be getting 24 or more volts between both leads to ground. If you do not get 24 or more volts with the furnace running then you have a pressure switch problem.
One of the most common problems involves the tubing (typically rubber) where the hose slips onto the draft inducer. It may attach at the top, bottom or center of the draft inducer via a small nipple device. This point will get clogged rather easily. A simple test is to pull the hose off the pressure switch and blow into it. You should be able to blow air into the inducer easily. You can also pull the tube from the pressure switch and listen for air flow when the draft inducer turns on as well.
If you cannot blow air into the inducer, or hear air flow when the inducer is running, take a stiff wire, a piece of coat hanger, or a drill bit to clear the inducer nipple. You simply place the wire, or bit into the nipple on the inducer and tap it gently with a hammer until it breaks free. In extreme cases, you may have to actually drill the debris out with a drill and bit, but not most of the time.
Phil Rains copyright(c)2009
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About the Author: Phil Rains is Master Trainer/Technical Developer for He has over 35 years of HVAC and Refrigeration experience in installation, service, and training. He is NATE-certified in 5 areas, a member of ASHRAE and RSES, and ACCA EPIC-Certified in Residential and Commercial Design. He also holds a Universal Classification in EPA 608.More About Phil
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