Question: How can I test to see if a pressure switch is operating properly?
- Set your meter to volts AC, turn your furnace on so it is calling for heat. The draft inducer should start.
- There should be either two or three wires going into your pressure switch.
- Probe one lead of the pressure with one lead of your meter while touching the other lead of your meter to ground. The ground would be any bare metal part of your furnace. This must be bare metal. I always try to touch the other lead of my meter to the furnace’s switch box.
- If you have a two-wire pressure switch you should be getting 24 or more volts between both leads to ground. By this I mean if you touch one terminal of the pressure switch with one lead of your meter, and touch the other lead of your meter to a ground, the bare metal part of your furnace, you should get 24 or more volts (24 to 28 volts).
- If you do not get 24 or more volts with the furnace running then you have a pressure switch problem.
- A gas furnace pressure switch problem could be caused by the following:
- Your vent could be stopped up with a bird’s nest, wasp nest or debris.
- You could have a bad, dragging, dirty or slow running draft inducer.
- The tube that runs from your pressure switch to the draft inducer could be plugged up. Many times the tube is plugged up with water. If the pressure switch tube has water in it then this is a sign that your condensate drain line could be stopped up or not draining properly. You can disconnect the pressure switch tube and run a wet vac to hopefully clear the stopped up furnace condensate drain line.
- The draft inducer hole could be plugged up. I have seen the draft inducer holes get stopped up so bad that I had to use a small drill bit and drill to open the hole back up.
- On high-efficiency condensing furnaces, the water drain line could be stopped up, causing a water back up and blockage in the pressure switch tube. *Make sure you disconnect the pressure switch tube before using a wet vac because the high vacuum of the wet vac can damage the pressure switch. I usually take the drain hose loose from the condensing furnace and use a wet vacuum to open the drain back up.
- I wanted to post this because many people are telling me that they are sucking on the pressure switch tubing to test the pressure switch operation. I have heard from other forums that you should not suck on the pressure switch to test it because this can damage the diaphragm inside the pressure switch. It would be best to use a manometer or test with a voltmeter like we advise on this page. I wanted to pass this along so others will not damage pressure switches.
- Please click here if you are interested in seeing the furnace pressure switches that we sell
- Please send us your furnace’s model number if you would like for us to recommend a pressure switch or part for your furnace. Our email address is: Support@arnoldservice.com
- Below we have three really good Youtube videos made by GrayFurnaceman and Word of Advice TV that shows how to troubleshoot pressure switches on an 80% furnace and how to by-pass a furnace pressure switch. Thanks to GrayFurnaceman and Word of Advice TV for making these informative videos! Jumping or by-passing a pressure switch would be good to do if you were troubleshooting and wanted to make sure that the pressure switch was the problem. I hope these videos will help you troubleshoot your gas furnace pressure switch.
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