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Burners

TERRY1
TERRY1 Member Posts: 9
We have a customer servicing a Viessmann Vitogas 100 GS1-29 power vented boiler and he is getting flame inside the burner tubes. NG pressure is between min. 5 oz and max.14 oz, 3-1/2 oz at the manifold as required, burners have been cleaned, clean combustion air, cannot adjust volume of CA. What would be the most likely cause for this to occur and what could he do to rectify it? Too much draft? Has been operating flawlessly for many years up until last year. Any suggestion is much appreciated.