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Old 01-12-2022, 08:02 PM   #1
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Suburban Furnace Won't Stay Lit

I have a Suburban NT-20SEQ furnace that acts as follows:
1 - fan comes on when thermostat calls for heat
2 - igniter fires, lights burner, burner stays on for about 5-6 sec (igniter continues to spark when burner is on)
3 - burner turns off, and cycle repeats 3 times then locked out.

If I turn thermostat off, then on again, will repeat cycle. I took out the igniter (is it also a flame sensor for this model?) and cleaned it, but no different results. Seems like it should be a simple fix, but don't know what causes flame to go out. Have the grill off and fan is blowing hard the whole time.

Any recommendations?
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Old 01-12-2022, 08:49 PM   #2
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Check for mud dauber nests. Either in intake or exhaust pipes.
Blockage will cause shutdown.
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Old 01-12-2022, 09:07 PM   #3
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Spark Electrode has dual purpose:
*High Voltage Spark for Ignition
*Return Path for Flame Proving Signal

If circuit board does not receive that signal within 6-7 seconds after flame ignites then it drops DC to gas valve so raw fuel does not accumulate
3X then lockout is standard safety measure

Spark Electrode should stops firing when flame ignites
Continuing to spark after ignition suggest circuit board issue

**Could be spark electrode.
Cracked ceramic insulator allows flame signal to go ground
Broken wire inside the ceramic also inhibits return of flame signal
Loose/dirty electrode wire connection at circuit board will inhibit return also

(Ignition Spark is roughly 2000VDC----flame proving signal is only a few milliamps)
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Old 01-12-2022, 10:21 PM   #4
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Thanks, checked and both were clear of mud dauber nests.
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Old 01-12-2022, 10:24 PM   #5
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Thanks, I'll recheck the spark electode tomorrow.
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Old 01-12-2022, 10:37 PM   #6
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Check the igniter distance to the baffle..1/8 inch is the correct space.
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Old 12-08-2022, 04:45 PM   #7
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Having similar issue

Quote:
Originally Posted by Old-Biscuit View Post
Spark Electrode has dual purpose:
*High Voltage Spark for Ignition
*Return Path for Flame Proving Signal

If circuit board does not receive that signal within 6-7 seconds after flame ignites then it drops DC to gas valve so raw fuel does not accumulate
3X then lockout is standard safety measure

Spark Electrode should stops firing when flame ignites
Continuing to spark after ignition suggest circuit board issue

**Could be spark electrode.
Cracked ceramic insulator allows flame signal to go ground
Broken wire inside the ceramic also inhibits return of flame signal
Loose/dirty electrode wire connection at circuit board will inhibit return also

(Ignition Spark is roughly 2000VDC----flame proving signal is only a few milliamps)
Since PILLBOX never closed this thread, I would ask them to clarify if the igniter ended the problem. We have a barely used QF35 - very clean tube, orifice, etc. NO BUGS! It has a "stub igniter". Porceline looks good. Only "clicks" when igniting.
Any troubleshooting help is welcome - I will post fix when found, Thanks
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Old 12-08-2022, 04:49 PM   #8
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Quote:
Originally Posted by kdb623 View Post
Since PILLBOX never closed this thread, I would ask them to clarify if the igniter ended the problem. We have a barely used QF35 - very clean tube, orifice, etc. NO BUGS! It has a "stub igniter". Porceline looks good. Only "clicks" when igniting.
Any troubleshooting help is welcome - I will post fix when found, Thanks
Spark electrode should only click when igniting ...once flame lights it should stop sparking

BUT the electrode is ALSO used as return path for Flame Proving signal.....if circuit board does not sense the change in current(milliamp) then it shuts down gas valve
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Old 12-09-2022, 07:23 AM   #9
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Problem resolved

I was the OP, and did end up resolving the issue by getting a new control board.

Suburban SB521099 3G Furnace Fan... https://www.amazon.com/dp/B00NBPYF7U?ref=ppx_pop_mob_ap_share
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Old 12-10-2022, 07:29 AM   #10
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Thanks for submitting your fix!
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Old 12-10-2022, 10:48 AM   #11
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Same symptoms if the lp gas is not turned all the way on.
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Old 12-11-2022, 05:13 PM   #12
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Quote:
Originally Posted by Fiesta48 View Post
Same symptoms if the lp gas is not turned all the way on.
In this particular case we have bench tested the unit with full propane tank. Same symptoms. Beginning to suspect board. Disappointing because unit is just over 2 years in service and rarely used. Will report when repair is confirmed
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