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The furnace pressure switch monitors the furnace pressure and closes when there’s a call for heat to complete the circuit. Also called the gas pressure switch, it can get stuck closed even when the furnace isn’t working right. When this happens, the startup sequence of the furnace will be stopped and the furnace will go into lockout mode until the issue is rectified. This gives off a pressure switch error code which can be rectified in several ways as explained below.
If your furnace shows a pressure switch code and the inducer doesn’t come on, check the switch by removing one of its wires. If this turns on the inducer, the pressure switch will be stuck closed. Slightly tap the pressure switch body to open it up or replace the pressure switch to fix it.
The furnace pressure switch diagram shown below is one from an Intertek furnace. They may differ in size and other aspects but their functions are the same. A furnace pressure switch with labor costs $50 to $150 depending on the size and brand of the furnace.
Furnace pressure switch troubleshooting
When the other parts of the furnace such as the motor are running well yet the pressure switch won’t open or close, you can check for problems by tracing the voltage across its terminals. The pressure switch uses 24 volts which come from the control board then goes though the safeties before reaching the pressure switch.
Trace this voltage by placing one lead on the ground connection, or a metal piece to the body of the furnace. Attach the other lead on the incoming terminal of the pressure switch. If the incoming terminal has 24 volts but the outgoing one doesn’t, the pressure switch is open.
You can also test this by using the leads across the terminals of the pressure switch. If the switch is closed, it should read 0 volts and 24 volts when its open.
If any of these figures occur inversely or not at all, the pressure switch has a problem.
If you carry out these tests and the pressure switch has no problem, you should look for the problem on the other parts of the furnace instead. A Hayward heater LO code, for example, means the limit string is open which may not necessarily be an issue with the pressure switch.
Furnace pressure switch stuck closed
There are many causes for a pressure switch getting stuck closed each with its fix. However, don’t attempt to fix the furnace pressure switch error if you don’t have some experience working on furnaces and heaters and similar home appliances.
Some furnace pressure switch symptoms that may make it stay closed include the following:
Obstructed flue pipe
The flue pipe leads the exhaust gases outside the building and it usually terminates on the roof. Often, birds, insects and rodents build their nests in the flue pipe thus blocking it in the process. When this occurs, the furnace will not be able to draw out the gasses from the heat exchanger. It senses a problem in the air flow and the pressure switch remains closed to avoid poisonous gases from remaining in the home.
Damaged pressure switch hose
The pressure switch hose is made of rubber and it can get damaged in many situations. For example, it can be chewed upon by rats or get brittle due to age. Whatever the cause of the damage, such a pipe won’t maintain the proper air pressure. The furnace will in turn keep the pressure switch closed to prevent damage to the rest of the parts or the leakage of exhaust gases into the home.
Clogged port on the pressure switch collection chamber
The pressure switch has a collection chamber that may get clogged thus preventing air flow through the port. Debris may collect on the port if it’s not cleaned frequently enough. The dirt either partially or completely blocks the air flow in the pressure switch thus keeping it closed due to the poor air flow.
Stuck or damaged diaphragm
The diaphragm of your pressure switch may be stuck or ruptured and will thus prevent the switch from opening. You can check that by lightly blowing air into the hose that leads to the pressure switch. If you don’t hear the diagram open and close then it’s stuck or damaged. It may also produce a flapping sound when it’s damaged. You can also check for voltage difference in the diaphragm using an ohmmeter.
Water in the pressure switch hose
Water vapor may condense inside the flue pipe and flow back into the furnace pressure switch tubing thus blocking it. This will often make the pressure switch get stuck open or closed no matter how many times you try to start the furnace. Ice on the venting system can also cause the same issue.
Dirty blower wheel on the inducer
Another reason why the pressure switch to your furnace may be stuck shut is a dirty blower on the inducer. The inducer motor turns on before the pressure switch checks whether it operates properly. If the pressure switch finds a fault with the inducer, it’ll remain closed and the furnace will not turn on.
Cracked heat exchanger
A crack in the heat exchanger will make the whole furnace malfunction. As such, when the pressure switch notes any issues with the heat exchanger or other parts of the furnace, it’ll remain closed until the issue is fixed. This prevents further damage to the furnace or any accidents that may occur.
Faulty control board
The control board of your furnace controls the functions of the blower motor, flame sensor, gas valves, and the ignition. If it’s faulty, some or all of these parts may stop working leading to a failed furnace. If it doesn’t control the pressure switch properly, it’ll stay closed or open.
How to fix a pressure switch that is stuck closed
To fix the above issues and restore the proper working of the pressure switch, do the following:
|Obstructed flue pipe||Clear the flue pipe|
|Damaged pressure switch hose||Replace the pressure switch hose|
|Clogged port on the pressure switch collection chamber||Clean the collection chamber to the pressure switch|
|Stuck or damaged diaphragm||Tap on the pressure switch lightly|
|Water in the pressure switch hose||Drain the pressure switch pipe|
|Dirty blower wheel on the inducer||Clean the inducer|
|Cracked heat exchanger||Replace the heat exchanger|
|Faulty control board||Fix or replace the control board|
Clear the flue pipe
Identify the point the flue pipe protrudes out of the building then check for any blockages. You should check the whole length of the flue pipe since blockages can occur on any part of the pipe.
Once you identify a blockage, insert a hooked wire or other object that can pull out the blockage away from the furnace. If you push the blockage towards the furnace, you might cause other issues.
Replace the pressure switch pipe
Replace the pressure switch pipe to restore a pressure switch that is stuck closed. While sealing it is an option, it’s not the best since the seal might leak or loosen out. You may seal it as a temporary measure as you find a replacement pipe.
Clean the collection chambers to the pressure switch
Use a wire or other stiff object to scrap any debris from the clogged port on the collection chamber. You don’t need to wash it since most of the debris there is dry and can be easily scrapped off.
Tap on the pressure switch
Gently tap the flat part of the casing of the pressure switch. If the diaphragm is simply stuck, it’ll get loose and work as expected. If damaged, you’ll need to replace the whole pressure switch to restore the normal working of the furnace.
Drain the pressure switch pipe
Place a bucket under the pressure switch pipe then disconnect it on one end to allow the water drain out. Depending on the outdoor temperatures, you may find a lot of water in the pipe or nothing at all.
Clean the inducer
A dirty blower wheel on the inducer should be cleaned by removing the inducer and cleaning it. It’s a delicate part and this procedure should thus be carried out by a professional if you’re not familiar with it.
Replace the heat exchanger
If the heat exchanger is cracked, the only viable option is to replace it with a new one. The heat exchanger separates the combustion chamber and the blower and transfers heat from the combustion chamber to the blower. If cracked, the burning will be less effective hence the need to replace it.
Fix or replace the control board
If the problem is a faulty control board, examine it to find out exactly what the issue is and if it affects the pressure switch alone or all the controls under it. If the issue can be rectified (such as through reseting), carry it out. Otherwise, replace the control board with a new one.
Replace the pressure switch
If the fixes above don’t restore the normal working of your pressure switch, it’s time to replace it with a new one. Since pressure switches have diaphragms which work based on gravity, you should always ensure the pressure switch is installed vertically rather than in any other orientation.
To avoid any issues with the replacement of the pressure switch, prioritize OEM (Original Equipment Manufacturer) switches rather than universal ones. The OEM ones will simply fit into the space left by the old one while the universal ones may require some tinkering to work.
Lastly, avoid using longer hoses than the one on the one connected to the old switch since you may create an area for water to collect in after condensation which will lead to pressure switch problems.
Replace a pressure switch as follows:
- Turn off the power and ensure the furnace isn’t running.
- Remove the rubber hose and wires connected to the pressure switch.
- Remove the pressure switch from its bracket. If the new switch comes with its own bracket, remove the old bracket as well.
- Connect the new switch to the furnace then reconnect the rubber pipes and wires as required.
- Turn on the power then test the new pressure switch.
This procedure shouldn’t take long but make sure you know exactly what you’re doing before attempting it. Otherwise, call for an expert who may help with other issues including a tripping high limit switch.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.