Problem: Can a draft inducer motor somehow get “weak” before it seizes up or shows some type of problem? My furnace code is telling me pressure switch/vent problem but I have checked both and they function fine. The problem is when the burners ignite the gas valve cuts off after only a second or two because the pressure switch much be opening for a split second (if I bypass the switch the burners stay ignited and everything works). With no vent problems and pressure switch working like it should I am down to thinking that either the inducer motor is not keeping up when the burner ignites and gives a little burst of pressure, or maybe the gas volume is set a little too high so that the burst of ignition is causing a pressure pulse that opens the switch. I’m not sure which one would be more common after running fine for 10 years. If I run the furnace for a few minutes with pressure switch bypassed, and then shut down, hook switch back up and restart, the system will ignite and run as it should, but later after being off for a while it will not stay lit and does the same thing and gives same code. This leads me to believe that when vent and system is hot it can draft better and switch will not open when burner first ignites so whatever the problem is, it is just barely giving enough pressure change to open the switch when ignited and system is cold. Inducer or gas volume or have you seen something in this situation that could be the problem? Thanks.
Answer: Excellent question! I have seen this problem happen before when I was doing service work. Most of the time it was caused by the pressure switch itself being the problem. The diaphragm inside the pressure switch gets tight over time or the diaphragm develops a small leak and will not sense the right pressure. Please click here if you would like to see the pressures switches that we sell. Please email us anytime with your furnace’s model number if you would like for us to look up part numbers for you. Our email address is: firstname.lastname@example.org We would Love to Help You Out and Have Your Business!
The second most common cause is the pressure switch tubing (line) being slightly obstructed with water or debris. If your Lennox furnace is a condensing furnace that produces water when it runs, then please make sure the water drain line on the furnace is draining well. Please make sure the pressure switch tubing where it attaches into the draft inducer is not obstructed with water, or the hole where the pressure switch tubing attaches to the draft inducer is not obstructed. If you are having a slight delayed ignition when the furnace first starts up then you might want to clean the burners with compressed air. Clean the pilot or furnace lighting mechanism to make sure the furnace lights smoothly. Too much gas pressure or too little gas pressure can cause a furnace to have delayed ignition. You can adjust the gas pressure if you think you might be getting too much or too little gas pressure. I show how you can do this on the following page: Please click here if you would like to see our Gas Furnace Troubleshooting Simplified Page. Please be careful when trying to adjust gas valve gas pressure. This can be dangerous when working with natural gas. A draft inducer’s bearings can get tight, or the inducer blower wheel can get dirty which could slow it down. Please make sure that the draft inducer wheel turns freely. A good test would be to watch to see how long the draft inducer wheel spins after the draft inducer goes off. The wheel should spin from 3 to 5 seconds after the power goes off to the draft inducer. If the draft inducer does not spin very long (only a second or two) then the bearings on the draft inducer might be tight and you will need to purchase a new draft inducer or a new inducer motor. If your draft inducer has a run capacitor you might want to check the capacitor to make sure it is reading in the right specs and not weak. A weak capacitor will cause the draft inducer to run slow. You said, the vent is not obstructed so this isn’t the problem. I would also suggest to look at the things on the furnace like the condensate trap and drain lines from the secondary heat exchanger. Please make sure the furnace is tilting forward slightly so the water drains out the front of the furnace. The installation instructions on these high efficiency furnace tells how many feet of vent pipe are allowed for venting. If the allowed vent feet are exceeded then you will have trouble with the pressure switch not staying closed. The 90 degree turns in the plastic PVC venting sometimes are calculated as 5 feet of piping. If you have exceeded the number of turns the furnace will not run right and the pressure switch will not work or stay closed. I remember one furnace I installed where I tried to use 2″ PVC for the venting and ended up having trouble with the pressure switch going off like your furnace is doing. I ended up having to tear the 2″ pipe out and install 3″ PVC vent. The venting is critical for these high efficiency furnaces to run right, to say the least! I hope that I given you some insight into what might be the problem. I am sorry we do not sell very many Lennox parts but I will be glad to try and help you out anyway I can. Steve Arnold, https://arnoldservice.com