Bryant Furnace Has Pilot, no Flame

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Old 10-12-19, 02:24 PM
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Bryant Furnace Has Pilot, no Flame

I have a Bryant Pro plus 90 model 398AAW036080ABBA. It recently stopped responding, and replaced the main board, as it had shorted on the main connector and was all burned up. I've checked all the thermostat connections, and all is well there. On powering the furnace on, with a call for heat, it runs both blowers, sparks the pilot flame, and then waits. After about 90 seconds, the entire system stutters, and starts over without a spark. It runs another cycle and sparks again. This cycle repeats.

I followed the troubleshooting guide for the furnace, and it says that I should be getting 24volts on contact one for the gas valve. I am not. I've run out of ideas on what to test or swap next.
 
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Old 10-12-19, 04:09 PM
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Welcome to the forums.

On a call for heat only the draft inducer blower should start.
The main blower is on a turn-on time delay.

This is an intermittent ignition pilot so it should also use an ignition board.
What is the part number on the ignition board ?
What replacement board did you use ?
 
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Old 10-12-19, 04:27 PM
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I used this part from Repairclinic. It is the identical part number as what was in there, and matches visually. I don't think it has a separate ignition board. The only two boards are the main board that I replaced, and a smaller board for the inducer. I've checked the pilot assembly, and it continues to spark even after the pilot is lit. I strongly suspect the flame sensor is bad.
 
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Old 10-12-19, 09:19 PM
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That is a blower timer control board. There is another board in your furnace.
It will connect from that board to the pilot assembly.
 
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Old 10-13-19, 08:46 AM
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The wiring goes from the main board, through the gas valve relay, into the spark igniter, and straight to the pilot assembly. There are no boards in between.
 
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Old 10-13-19, 11:01 AM
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In the following link is the same as your furnace. Towards the bottom I left a diagnostic manual link.
You will need to follow that.

https://www.doityourself.com/forum/g...-activity.html
 
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Old 10-13-19, 12:45 PM
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I've already followed that guide, which is what led me to suspect the pilot assembly. The troubleshooting guide leads to the flame contact being the problem. I have on one order and I'll update once I've installed that.
 
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Old 10-14-19, 06:04 PM
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The new pilot assembly is installed. With a small adjustment to the flow on the gas valve for the pilot, the igniter/spark generator is operating correctly. It sparks and lights the pilot. After 60-90 seconds, I hear all the relays seemingly randomly firing, and the pilot light goes out. The furnace then goes through a full cycle, the same thing happens, and then we start again from the beginning. It seems like something is causing some sort of system panic when the gas valve should be opening. Does this mean it is a bad gas valve? Are any of the relays or other components serviceable without changing out the entire gas valve?
 
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Old 10-14-19, 06:17 PM
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Is there any way to force a main burner turn on, to verify whether it is the main board or the gas valve that needs to be replaced?
 
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Old 10-14-19, 06:57 PM
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The gas valve is easy to test.
Is it getting power? If so, what is the incoming and manifold gas pressure?
 
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Old 10-14-19, 07:08 PM
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I don't think there's a meter anywhere on the gas system for pressure, just the volume meter coming into the house. I get good flame on the pilot light, and good responses from the other gas appliances.
 
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Old 10-14-19, 07:09 PM
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There’s 2 taps on the gas valve, one for each side. You’ll have to use a manometer to check gas pressure.
 
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Old 10-14-19, 08:45 PM
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the secondary in this late 80s/early 90s unit is likely plugged and i wouldn't put any money into it.
 
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Old 10-15-19, 05:46 AM
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Would the heat exchanger being plugged keep the furnace from firing? Wouldn't it either cut out after the mains fired up, or just have terrible quality?
 
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Old 10-15-19, 05:54 AM
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A plugged heat exchanger can prevent ignition.
It can usually be diagnosed via draft through the heat exchanger.
 
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Old 10-15-19, 01:38 PM
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The pressure switch will still close with a bad secondary because it's measuring the pressure difference between the collector box downstream and the burner inlets.

Both sides are negative.

With it plugged, the negative pressure at the burner inlets drops but the negative pressure in the collector box increases, the switch senses this and still closes.

I guess negative pressure at the burner box only can be checked, but there has to be a frame of reference.

*

If it's really bad, you'll see rust on the back of the secondary with the blower pulled.
 
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Old 10-15-19, 03:41 PM
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I've got a professional coming later this week to get this sorted out. At this point I'd just be throwing expensive parts at it until it worked again. Thank you all for the help! I'll be sure to report back once I have it fixed.
 
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Old 10-15-19, 04:43 PM
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That unit is on the list of furnaces that could be affected by the class action suit. That affected the heat exchangers.
 
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Old 10-15-19, 07:35 PM
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I'll look into it, thank you.
 
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Old 10-15-19, 08:08 PM
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Old Yesterday, 04:51 PM
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Unfortunately, no one could locate the problem. I've been forced to update with a new furnace. Thank you all for the help!
 
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