How to Diagnose a Bad Furnace Pressure Switch (5 Easy Things to Check in 2021)
Channel: Fox Family Heating and Air Conditioning
Date: 2020-12-26

Hey HVAC technicians! Today we’re going to talk about How to Diagnose a Bad Furnace Pressure Switch.  I wanted to expand on our recent gas furnace troubleshooting series by going into each part of the sequence of operation of a furnace.  In this video, You'll learn what the pressure switch does and why it’s important.  And towards the end of the video, I’m going to give you 5 easy things to check when you’re troubleshooting a furnace pressure switch.  That's coming up here on Fox Family Heating & Air. 

How to troubleshoot a furnace pressure switch 

Let's assume the inducer motor is running properly. But the pressure switch doesn’t seem to be closing.  With your meter, you can trace the 24 volts coming from the control board, through the safeties, and on to the pressure switch.  Place one lead on ground, or a solid piece of metal attached to the furnace.  Place the other on the incoming terminal of the switch.  If you have 24 volts on the incoming terminal, but not at the terminal leaving the switch, you can assume the pressure switch has not closed. 

Remember, the pressure switch doesn’t close until the inducer motor comes on and provides the necessary suction for the pressure switch to close. The required suction is listed on the pressure switch. 

Pressure switch not closing 

When we’re troubleshooting a furnace pressure switch, we can do a few things.  We can take our manometer and make sure the inducer motor is creating the vacuum by hooking up the meter's hose directly to the collection chamber that the pressure switch tubing is connected.  Take that pressure switch hose off and put your manometer's hose on the same port. 

Once you put the hose on and start the system up, the inducer comes on, and the manometer should start reading the induced draft as it begins to rise.  This number on the meter needs to be greater than the number on the pressure switch.  

5 easy things to check  

Some of the more common reasons I've seen pressure switches either fail or not close to allow the rest of the system fire up are: 

1. A clogged port on the collection chamber to the pressure switch. 
2. An obstruction in the flue 
3. A diaphragm that’s ruptured or stuck 
4. The pressure switch hose is damaged 
5. The pressure switch hose has water in it 

Installing a furnace pressure switch 

During the pressure switch installation, you want to make sure it's mounted in the correct position.  The pressure switch you took out was in a vertical position for a reason.  Diaphragms don't activate as easily when they have to fight gravity.

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00:00 Intro
0:43  The sequence of operation for a furnace
1:21  First the inducer motor starts
1:58 What the furnace pressure switch does
3:09 How to troubleshoot a furnace pressure switch
5:05 5 things to check on your furnace pressure switch
8:55  Installing a furnace pressure switch
9:58  Recap

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