Rheem Imperial 90 Plus (75,000 btu Propane furnace) 2 of 3 burners not igniting

TheDude

Rheem Imperial 90 Plus (75,000 btu Propane furnace)

Model: RGEB-07EC-FB



Issue:



Occasional "flash-over" or operation with the two left burners NOT ignited after ignition sequence does not ignite the left 2 burners of the 3 in the heat exchange.





The furnace will always light the right burner with the heater coil but the pilot tube that carries the flame over to the two left burners does NOT carry a flame across about half the time. Then the furnace continues to run, dumping un-combusted propane from the two left burners out the exhaust stack. Then the furnace will "flash-over" at some random point when the pilot tube finally carries over the flame(50% of the time). The problem disappears by removing the front cover plate and operationg without it (8" x 12" galvanized sheet metal burner cover with two round mica windows) and the ignition sequence operates flawlessly.



Not sure why the Flame Sensor does not shut down the furnace when the left burner does not ignite ? The safety system should always shut the furnace down without full ignition of all three burners.



The Exhaust System (fan) seems to cause too much draft in the burner area that prevents the Pilot Tube from carrying the flame to the two left burners (been cleaned and seems to work perfectly with the cover removed) or the propane pressure is too low (burners seem to run fine with a good flame when running properly), or I have no idea !



What is going on ?

lchmb

would need

To know your incoming pressure's to the gas valve and also your manifold pressure after that. The next item I would check would be the differential pressure on the unit side and compare it with what the manufacturer expects to see. As far as not dropping out on safety, that to would be a concern since most have a 5 to 7 second flame failure and if your running past that something is not working properly (pretty sure that model does have a flame rod) ....Did it get a new module installed at some point?

John Mills_5

Flame sensor

Originally, the ignitor functioned as the flame sensor. The metal rod which is likely over burner 3 was tapped into the ignitor wiring as a supplemental sensor. What Rheem found was that over time, the ignitor would not do well sensing flame and shut the furnace down. So this kit was available to be added if someone was having nuisance problems. The ignitor is still functioning as the main flame sensor so as long as that burner lit, the board  saw flame sense current.



Check for some kind of blockage in the crossover. Soot, dust, spider web. That is the usual culprit in these things not lighting fully. Back in 1991, there was an issue with these burners needing to be replaced. If the installing dealer didn't do that, might be wise to replace them.



Or the whole furnace, based on age. No cracks?

TheDude

Thx

Thanks for your help Guys, goods points. I had the HVAC tech come in this morning, head scratcher to him, had not seen this behavior before. The 21 year old Rheem will be put out to pasture today and replaced with a 80k btu 90% Ruud. The furnace has other non-conformance safety issues as well, so out she goes...



Thanks again