Burners shut off when blower comes on

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Old 01-17-18, 07:07 PM
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Burners shut off when blower comes on

Looking for a second opinion. A customer called me and said their gas furnace was not working. When I arrived right away I noticed the inducer motor was not turning on. The inducer motor was getting 120 V however the fan was not turning on. I replaced the inducer motor and the furnace works for two days when the customer called and said it was not working again. I went back and there was a code on the circuit board stating the pressure switch was open when it should be closed. I bypassed the pressure switch and the unit worked fine. Therefore I assumed it was the pressure switch and replaced it as well. The furnace again worked for a day before it quit working again. The furnace goes through It sequence and turns on but right as the blower turns on or shortly there after the burner turns off. After testing about everything I could think of, the only potential issue I found is that when the blower kicks on there is a voltage drop at the inducer motor. It starts at 121V and when the blower comes on it drops to 115V and the pressure switch opens causing the burners to turn off. It is an International Comfort Product with a dual pressure switch. With the help of a friend we used a combustion analyzer to test the exhaust and the CO across the coil and everything tested fine. We came to the conclusion that it was an issue with either the circuit board or the blower motor. We changed the circuit board and still have the same issue. Waiting on the blower motor to come in but looking for any other advice from someone who may have had a similar issue.
 
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Old 01-17-18, 07:54 PM
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Welcome to the forums.

I've never changed a pressure switch. I've never found one defective. They are super reliable.

You changed the draft inducer. The second thing you should have done is connected a manometer to the rubber line. You'll probably find the vacuum to the pressure switch to be very close to its trip point. You may have a vacuum leak in the inducer.

Some replacement units can cause problems. Sometimes the gaskets don't seal right. Sometimes there is a pinhole leak and sometimes the inducer doesn't move the same amount of air that the original one did.

The voltage drop measured at the control board when the blower runs sounds about normal.
 
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Old 01-18-18, 04:18 AM
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Using the manometer, I was getting around 1.6 in/wc on the pressure switch hose and it is rated at 1.38
 
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Old 01-18-18, 04:45 AM
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Also, the amount of time the burners run varies with each cycle/reset. But at the longest maybe 5 seconds after blower comes on, the burners shut off.
 
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