Gas boiler will not start intermittently

Purseon

Older high efficiency Lennox boiler intermittently shuts down during startup, but when it does start, it runs well. It shuts off immediately after I see flame through the view port. I've checked several things and replaced some old parts that were suspect but wondering where to check next?

So far, I have replaced the gas valve and installed a new igniter but the issue remains. The flame sensor is the hot surface igniter itself, but problem is still there even with the new igniter. Could the burner unit itself, which appears very old and a bit falling apart, be causing a short of the flame sensor circuit, thus triggering a shutdown? Any advice on how to troubleshoot the flame sensor or any other possible causes of a shutdown after ignition, would be appreciated.

mattmia2

who adjusted the gas valve and checked the combustion after the gas valve was replaced?

is there any error code flashing on the control? the connection of the burner back to the control needs to be good for it to properly prove flame.

HVACNUT

What model furnace? Pulse?

realliveplumber

Op said it was a boiler?

The comment about the "burner unit being old and a bit falling apart" is worrisome.

Purseon

@mattmia2 I installed the new gas valve, per the boiler install manual and it was within specs with factory gas valve settings. What I observed is that the new gas valve did not fix the intermittent start issue but it did fix another issue which was low gas output of the old gas valve. The result is that the boiler heats to a higher operating temp now within specs whereas before it barely crossed the 120 degree mark. The control unit does not have error codes. All it has is one green and a few red lights to show the startup sequence. It gets to the ignition stage and fails on the flame stage, although I can clearly see a flame in the view port. So I am leaning towards flame sensor as the cause and trying to understand how the flame sensor works since on this model it is built into the igniter. Model of the boiler is Lennox GWB9-100IH-2, which I am beginning to believe was actually manufactured by Utica-Dunkirk, since all the parts come from them.

Purseon

@realliveplumber given your comment, I pulled the burner unit yesterday and it is in such bad shape I ordered a new one from Utica. The burner unit consist of two metal tubes, one inside the other, and the outer tube was literally split in half, very roughly, as if it had rusted through or perhaps burned through? I am guessing it was the original unit from 2007 when the boiler was installed, but I was surprised it was that bad. If the flame sensor sends the micro voltage from the igniter through this broken part to ground, I can understand why it may not be getting a good ground each time, thus intermittently failing to detect the flame.