Burner won't stay lit


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Old 03-03-12, 09:08 AM
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Burner won't stay lit

At the recommendation of a furnace tech, I replaced the 50E47-060 control board on my Trane XL90 with a 50E47-843 board. My problem persists.

The thermostat calls for heat.
The furnace kicks on normally and runs for a few minutes,
then the burner goes out.
When this happens, the green light on the board also goes out.
The draft inducer and main blower keep running.
After 20-30 seconds, the yellow led comes on (diagnostic), then
turns green (normal operation), goes through the startup procedure
and the burner relights.
This may happen 4 or 5 times before the thermostat is satisfied.

Would really appreciate any help. HVAC dude charged me 140 bucks
to tell me the board was bad. Wanted $300 for a new one.

Thanks,
Clay
 
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Old 03-03-12, 09:18 AM
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sounds like a venting/ pressure switch problem.
 
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Old 03-03-12, 09:19 AM
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Here's what the tech guy said

Removed and cleaned flame sensor, tested at 3.6 microamps
Cleaned the drain trap
Pressure switch ok, rated at 1.2" w.c. tested 1.0"+ w.c.
Measured 24v to gas valve
Checked ignitor voltage, 123v constant

Hope this helps.
 
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Old 03-03-12, 09:20 AM
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still sounds like a pressure switch/ venting problem
 
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Old 03-03-12, 09:34 AM
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So what do I check next?
I have a good Simpson VOM and I know how to use it.
 
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Old 03-03-12, 10:01 AM
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Make sure that the condensate line is completely clear.

Not sure what kind of technician would tell you to change the board yourself.
 
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Old 03-03-12, 10:33 AM
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Sorry, I didn't make myself clear. He wanted to change it, but was going to charge me $300 plus installation, AND he wasn't sure that was the problem. He didn't have one and was gonna have to order it. I knew I could get one cheaper and as I work with electric equipment every day, I knew I could change it. Turns out it WASN'T the problem.

I just took the condensation line loose and blew it out. It's clear.
I took the pvc vent pipe loose at the furnace and hooked a shop vac to
it. It's clean.
Went up on the roof and looked down into the other end of the pvc vent pipe,
looks clean from there too.
Jumpered out the pressure switch and fired it up. Ran for 4 minutes and
the burner kicked off same as before. Pretty much rules out the pressure
switch doesn't it?
 
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Old 03-03-12, 12:58 PM
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The xl90 appears to be a two stage model with a variable speed ventor motor.

There should be a second pressure switch or monitoring device. When there's a call for high heat, the inducer should speed up until the second pressure switch closes.

Does the board have a W2 terminal?

Does it always stop working at the same point? (time it)



Here's the spec sheet: http://www.google.ca/url?sa=t&rct=j&...linFLA&cad=rja
 
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Old 03-03-12, 01:12 PM
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In general, it's never a good idea to bypass safeties.
 
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Old 03-03-12, 02:52 PM
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No, mine is a single stage model, #TUC080B942A1. I didn't leave
the jumper on, just ruled the pressure switch out with it.
 
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Old 03-03-12, 03:46 PM
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It could be the high limit, but the inducer shouldn't remain on when that trips.

It could also be a rollout switch, but again, the inducer shouldn't remain on when it opens and I'm pretty sure that most don't automatically reset.

------------------------------
This is a long shot, but...

Check the electrical polarity, ground, and line voltage. Ground is required to prove ignition.

Check the flame current when it cuts out.

Are there any flame disturbances when the blower comes on?
 
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Old 03-03-12, 03:47 PM
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With the fan & venter staying on, it makes me suspect the furnace is hitting limit.
 
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Old 03-03-12, 03:59 PM
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Possible causes of cycling limit: (verify that limit is opening prior to investigating)
- Dirty filter
- Dirty blower or secondary heat exchanger
- Bad limit
- Too many vents closed
- Furnace is severely overfired -> regulator in gas valve not working correctly (google "gas meter clocking") Technician required to check this.
- Blower speed set incorrectly. (unlikely, unless it always had this problem. Factory setting should be okay)

Check temperature difference between supply and return.
 
 

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