keeprite has short prepurge

[sparta]

I was having small explosions taking place in the chamber, after professional inspection of the furnace ,it was determined the heat exchanger was bad,broken eylets. the exchanger was replaced. However right after the replacement ,after a few heat calls the explosions started again intermittently. After cleaning of the burners and pilot assembly, it happened again intermittently.
I now believe the problem is the prepurge cycle is to short,upon a call for heat the vent motor starts up and within 3 to 5 seconds the pilot lights. Is this too short of a prepurge. how long should it be. Is the circuit board what gives the delay for the prepurge prior to ignition trail or is the diaphram in the pressure switch weak.
I noticed a diode on the board (R1),is broken ,could this be the problem in the prepurge being so short.
your help is much appreciated

[4l530]

I dunno about all that. There used to be furnaces that were designed with no prepurge at all. There was one called Quattro that lit the pilot before the inducer even started, it would light & prove the pilot, then start the inducer.

R1 sounds more like a resistor than a diode, it could actually be a jumper that was cut intentionally.

JMO.

[Wheelbaron]

not enough info. We need Model and Serial if possible. Who replaced the HX and why did they not take care of this problem?

[sparta]

FURNACE IS A KEEPRITE DC90 HIGH EFFECIENCY, MODEL NUGMO75,SERIAL NUMBER L9636-44470.THE CONTRACTOR TOLD ME THE HEAT EXCHANGER WAS FAULTY THUS CAUSING THE PROBLEM, AFTER HE WAS DONE HE FIRED THE FURNACE A FEW TIMES AND ALL FIRED PROPERLY, AFTER HE LEFT IT FIRED ONE OR TWO MORE TIMES,THEN IT HAPPENED AGAIN,IT HAPPENS ONCE EVERY 6 TO 7 STARTUPS.
tHE INLET PRESSURE WAS 7 WC AND OUTLET WAS 3.5 WC.
AND R1 IS A RESISTOR,AND THERE LOOKS LIKE THERE WAS SOME RUST ON IT WHICH CORRODED ONE SIDE OF WIRE THAT WAS SOLDERED TO THE BOARD.
ALL THE THREADS THAT TELL YOU WHAT MAY BE THE PROBLEM WHEN YOU HAVE THESE SO CALLED EXPLOSIONS HAVE ALREADY BEEN ADDRESSED.
I NEED SOMETHING NEW TO GO WITH

[The Penguin]

Hey Numb Nuts - the all cap post is hard to read And I won't bother - plus it means you are shouting

I recomend you adjust yer attitude

[vmc1161]

Hey Numb Nuts


How did you know his nuts were numb...:eek: :D


.

[The Penguin]

How did you know his nuts were numb...:eek: :D


.

He typed in all caps

[HeyBob]

Call a contractor back to repair the problem. This is not a DIY forum. Delayed ignition is dangerous, shut it off until it's repaired.


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