A gas furnace pressure switch closes to allow current to pass through and start the ignition of the furnace. If the pressure switch is stuck, it does not send enough power to the inducer, and therefore, the ignition sequence does not begin.
Simply put, your gas furnace is made up so that when you power the furnace and adjust your thermostat, the control board gets a call for heat. The control board of the furnace is a complicated system, which is built to sense that the thermostat has been adjusted and to increase or decrease the heat. As soon as the control board senses the command, it checks the pressure or the centrifugal switch to see if the switch is open and non-energized. The pressure switch is in turn connected to the combustion air blower. The main function of the switch is to sense whether the combustion air blower is running. The air blower, also otherwise known as the induced draft motor, has three main functions. It cleans any residual gases or other materials from the combustion chamber, blows in as a natural draft to remove byproducts of burning gases and, finally, provides combustion air to the burners.
Here’s how you can fix a stuck gas furnace pressure switch.
Step 1 – Inspect the Control Board
The first step is to examine the control board and check if the switch is stuck due to an obstacle in its path. The switch is connected to hoses and the water in these can sometimes cause a little bit of a problem. Check the switch for any kind of a mechanical jam. It is also possible for the switch to get jammed because the switch itself is faulty. While you are inspecting the control board, also check for faulty wiring.
Step 2 – Inspect the Pressure Switch
Use an ohmmeter to check if the pressure switch is opening or closing properly when heat is called for. First, turn the power of the furnace off. Next, take off one of the power hoses and blow on it. Now use the ohmmeter to check whether the pressure switch is functioning properly. Suck on the power hose and then check the switch again, using the ohmmeter. Check the secondary exchanger and the inducer and see if the exhaust ports are blocked. Also check for garbage in the condensate trap. Next, check if you can see the plunger between the switch, check if the plunger pulls in when you suck on the vacuum tube.
Step 3 – Repair the Pressure Switch
If your pressure switch is stuck, it is most likely a problem in the running of the furnace. The plunger may not be pulling in completely and may only bob around when the pressure builds up inside the furnace. Using a cloth and a wire brush, remove any blockages and dirt from the secondary exchanger and the inducer. Remove the drain tubing and clean it properly by blowing out on it. However, when you are blowing out on the drain tubing, blow into the secondary exchanger as well to maintain the pressure. Also, adjust the condensation trap so that the water can reach the drain and furnace gases can be trapped.