| Bad/Shorted Switch | Inducer Fan Running Too Slow | Inducer Fan Motor Is Bad |
| Blockage In Inducer Fan | Blockage In Flue/Chimney | Crack/Hole In Suction Tube |
| Bad/Loose Wire Connection | Water Built Up In The Inducer Fan Housing (High Efficiency Furnaces) | Loose Suction Tube |
The table above gives you quite a few causes of pressure switch failure that are easily confirmed or eliminated as the problem either audibly or visually. You should start with checking those things since they are the easiest, don't require special tools, and cost nothing.
First, as the unit begins to try and run (meaning after the inducer has ran for 10-30 seconds) do you hear the gas valve "click" open? The gas valve will not open if the pressure switch doesn't work so the "click" sound of the gas valve opening tells us that the pressure switch is fine and the igniter is likely our problem since we should also notice it didn't try and light just prior to the "click" we heard. If we don't hear the click...
Listen to see if you can hear anything rattling around in the inducer fan. If so, see if you can remove it. (Be sure to turn the power off first) If you hear the motor whining, grinding, or running seemingly slow then you may need to replace it and the switch is likely doing what it's supposed to do.
Look to see if the switch hose is tight and in tact. If it's loose, try to tighten it up a bit and if it's cracked or has a hole, see if you can seal it or cut away the cracked material and reconnect. Be careful as the inducer housing may be hot if the unit has ran recently. Again, this would show the switch doing it's job and it should work now that the hose is fixed.
Look for any blockage in the flue you can see. If one is found, remove it and your problem should be solved.
If you've taken a few minutes to rule these things out and still have a problem, then we can confirm the switch is bad by testing the circuit with a voltmeter and maybe get the unit working temporarily for a little heat at least.
If you have a voltmeter, shut off the power to the unit. Remove the wires from the pressure switch and test for resistance (ohms) across the two terminals. If it measures "0" (or close to), then the switch is closed when it shouldn't be (shorted or stuck). If it measures "infinite" (O.L.) the switch is not shorted and could just be sticking open. Either way we'll need to replace the switch but perhaps we can still get it to work temporarily and get the heat back on for a bit. To do this...
Remove the hose from the inducer housing (again being careful since the inducer housing may be hot if the unit has ran recently) and gently suck and blow on the end of it. You should be able to hear the switch opening and closing. Do this a couple of times and then reconnect the tube to the inducer housing and see if it will work. Sometimes the switch will get stuck and just needs a little help loosening up. Even if this works, replace the switch soon. It is quite likely the switch will stick again and it's always better to fix it on your terms than the typically inconvenient timing it will choose to breakdown again.
Replacing a Pressure Switch
The actual work involved in replacing a pressure switch is quite simple and doesn't require any specialty tools that you don't likely have laying around the house.
Turn off the power to the unit
Disconnect the wires from the switch
Disconnect the hose from the switch
Remove the switch from the bracket (or bracket too if your new one has it mounted already)
Fasten the new switch back in place
Reconnect the hose to the switch
Reconnect the wires to the switch
How Much Does a Pressure Switch Cost?
A single stage pressure switch whether from the factory or universal can cost anywhere from $20-$50 while pressure switches for a 2 stage unit can run in the $30-$75 range.
Finding and Buying a Replacement Pressure Switch
Like many furnace components, the pressure switch is not an item typically found in the "big box" stores like Lowe's and Home Depot. To find a replacement, you will likely have to do one of two things: contact the contractor supply house that sells your brand of furnace or obtain a universal replacement from a contractor supply house or online.
Furnaces are much like cars and have parts that are specific to their brand. That is why if you want a direct factory replacement, you have to find the supplier that carries your brand. It's a "don't call a Chevy dealer for Ford parts" sort of thing. Once you've found the supply house that carries your brand, be prepared to tell them what part you are looking for and have your unit's model and serial number handy so they can help you. That said, many of these suppliers are "contractor only" and will not sell to the general public so the more prepared you are, the more likely you are to get them to sell to you or at least give you a part number so you can look online for it. Or you can group purchases with friends to accumulate small quantities such as a box, because there may be no distributors in some areas, and large quantities are discounted are also applicable to furnace pressure switch factories.
A universal pressure switch, however, may be easier to find and purchase at any supply house or online. I keep this LEFOO LF31 universal furnace pressure switch handy so that if my or a family member's unit breaks down at an inopportune time, I can make the repair without going hunting for one. These are nice because they can handle single stage and dual stage units as well as be set to various pressures. They also contain brackets that make mounting possible in more than one unit. The instructions walk you through the process of setting up the switch to suit your unit's needs.
