Rheem Classic 90 Furnance - burners do not stay lit reliably

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Old 11-26-10, 04:33 AM
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Rheem Classic 90 Furnance - burners do not stay lit reliably

An attempt to describe the problem.
Draft Induction fan comes on (recently replaced fan and had furnace "cleaned" and serviced by HVAC Tech).
Electric igniter glows bright.
After pressure difference is good, gas solenoid valve opens and burner lights to blue flame.
Next the is a noticable pressure chang in the box (a whoosh) and the flame goes out in all burner tubes and gas valve closes.
Igniter stays lit and after a few seconds, gas valve reopens and burner relights. Sometimes it stays lit and cycle goes on like normal. Sometimes it "blows out" again. Sometimes after 2-3 cycles of these blowouts, igniter goes off, blower goes off and everything goes back to "rest" mode.
Then Thermostat kicks on again, since temp never increased, and the cycle repeats.

Is this a flame sensor issue? Tech cleaned the sensor with emory paper as part of the cleaning (the only part I saw before phone rang and I left him be).

Just spent $500 replacing draft fan, and now the furance works, but seems to struggle to cycle (lots on likcing and clunking in this process).
 
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Old 11-26-10, 05:45 AM
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Not a flame sensor issue since it's been cleaned.

I'd check the pressure switch to be sure it's staying closed after the inducer motor comes up to speed. Use an AC voltmeter with one test lead connected to chassis ground and use the other test lead to check for 24 VAC at the pressure switch connections.

One connection should have 24 VAC to it when the thermostat call for heat. The other should have 24 VAC when the inducer motor comes up to speed --- and check to be sure it stays at 24vac when the furnace is shutting off.

Also, please post the model number of the furnace from the rating plate in the burner compartment of the furnace.
 
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Old 11-27-10, 11:07 AM
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I'm most curious as to why and when the tech was there.
 
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Old 11-28-10, 07:51 AM
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Why Tech visited

ecman51 - Tech came out after Draft induction fan failed. Fan threw blades and was jammed or so out of balance would not get required airflow/pressure difference. He replaced fan. While I had him there, paid to have a cleaning/PM service call.

Since he left, have had the unstable burner issue.

Could this be the position of the flame sensor? It is clean, but could it be out of place and not in the flame enough?
 
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Old 11-29-10, 07:36 PM
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Rheem Classic 90 Model #

SP - Here is model # requested: RGRJ-07EMAES.

When you mention pressure switches, do you mean the ones that sense the pressure drop across the draft inducer fan? These are small cricles connected to fan and burner area by rubber flexible tubing. Will check voltage - once I replace my faulty voltmeter tomorrow - Ugh!

Thanks for the advice!
 
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Old 11-30-10, 12:23 AM
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Yes. If the pressure switches open it will shut off the burners. Even with the new inducer motor/fan, there are other things that could cause the pressure switch to open.

That's worth checking, anyway.
 
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