Gas furnace ignites but then makes large knocking sound


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Old 01-14-20, 02:29 PM
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Gas furnace ignites but then makes large knocking sound

My Bryant 376B gas furnace will ignite, but after the pilot goes on for a moment it creates a large periodic knocking sound over and over again instead of the fan starting. I have my suspicions, but I wanted to ask if anyone has run across this before. The main blower is accessible and the fan spins. I cannot reach the inducer easily and would rather not take it apart.

Here is a link to the startup sequence https://youtu.be/K2jNJ1Abh20
 

Last edited by Chris Black; 01-14-20 at 02:36 PM. Reason: added link and model detail

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01-14-20, 06:15 PM
roughneck77
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Stop!
Why is it trying to light? The inducer doesn’t appear to be running.
If that thing lights off it’s going to roll out like crazy and possibly start a fire. Turn off the gas valve until you can determine what’s going on.
Have you jumped out something?
 
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Old 01-14-20, 04:41 PM
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You've posted in the boiler forum. Best to post it in the forum for forced air furnaces.
 
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Old 01-14-20, 05:50 PM
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Thank you very much! This should be enough characters now.
 
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Old 01-14-20, 05:57 PM
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Welcome to the forums.
Thread moved.

That is the intermittent spark ignition system causing that problem. More than likely the pilot assembly will need to be removed from the furnace and cleaned..... possibly replaced.

The link shows what it looks like..... example of spark ignited pilot assembly.

The pilot burns inside that metal housing.
Those wires are the control to turn off the spark and light the burner.
 
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Old 01-14-20, 06:15 PM
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Stop!
Why is it trying to light? The inducer doesn’t appear to be running.
If that thing lights off it’s going to roll out like crazy and possibly start a fire. Turn off the gas valve until you can determine what’s going on.
Have you jumped out something?
 
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Old 01-14-20, 06:25 PM
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Good catch. I didn't notice the inducer not running.

Name:  inducuer.JPG
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A very serious problem there.
Actually multiple problems between the inducer and the intermittent spark ignition.
 
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Old 01-14-20, 06:53 PM
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It’s very important to turn off the gas valve immediately. If the pressure switch isn’t preventing the burner from firing then other safety controls like the rollout and limit may not work either if that burner lights.
This can be very dangerous.
 
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Old 01-14-20, 07:47 PM
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Thank you. I already have gas and power cut off to the unit. Thank you for energetically working to get my attention on the potential safety risk. I appreciate the effort.

I did not want to say it at first, but I suspected the inducer fan was seized up (at best) or the associated motor was burned out (not the main blower). In your experience, is it possibly just seized up? I am hoping for this. If so, I can take it apart, free it and return it to working order without calling anyone out.

btw, I had already removed and cleaned the spark pilot assembly. It lights and heats up though my headlamp washes out the flame in the image.
 
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Old 01-14-20, 08:15 PM
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That unit shouldn’t even be attempting ignition at all with the inducer not operating.
You’ll have to take your meter and start investigating. It will involve working with live voltage.
The pressure switch should remain open and prevent the burner from attempting to light.
If the inducer is seized there is usually a reason why and just cleaning it will not fix the problem.
 
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Old 01-14-20, 08:16 PM
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On a call for heat..... the inducer starts and proves a vacuum..... then the igniter starts. Once the pilot lights.... the spark ignition stops. Your is not. Either the switch is bad in the pilot assembly or the pilot flame is not large enough.
 
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Old 01-15-20, 03:01 PM
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Any luck on troubleshooting?
 
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Old 01-15-20, 09:13 PM
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Yes. I opened up the inducer and it was rusted up pretty well but I could pry it free to turn. I'm lucky enough to live in a big city so I got a replacement draft inducer kit from a supply house nearby and replaced the inducer outright. It works fine now. The heater is over 30 years old. I expect it was just about time.

Thank you folks for the leads and advice. You saved me from getting a heater replacement.
 
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Old 01-16-20, 04:14 AM
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Please turn the unit off.
You still need to diagnose why the unit was attempting to fire with no inducer running. Until that’s figured out it’s unsafe to operate.
 
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Old 01-16-20, 07:10 PM
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Thank you. I will. Thankfully it is warming up here so I have the benefit of time now.
 
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Old 01-16-20, 07:51 PM
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Shouldn’t be too hard to diagnose. Use your meter. See if the pressure switch is stock closed. Or there’s some other electrical problem.
 
 

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