Aqua Balance 155-C Low Fire combustion issue

NickNt

Good morning all,

Trying to troubleshoot a Aqua Balance machine on the side in my spare time.

155-C, was hearing mini boom when starting and occasionally would smell gas. Shut it all down, peeked at the manual, removed heat exchanger cover, and found the gasket to the burner plate was all dry rotted and the plate itself was cracked and damaged. Replaced with new plate and new gasket per manual. Using combustion Analyzer, I can't seem to get my PPM number during low fire testing below about 45 PPM. Spec is 7 PPM. And my CO2 spec is 8-9 and i'm lucky if i get 7% on my analyzer. During low fire testing the PPM will spike to a few thousand but then comes back down to 50-150ish. Oxygen often times is about 7%. But if I leave it in low fire for say about 5 minutes, the PPM will creep up slowly and my 02 will go down to 5.4% or so. High Fire PPM seems to be on spec, it's mostly low fire. And during normal operation for heat on startup, the system will spike to about 500 or so and settle back down to about 70 PPM. Gas inlet pressure is correct, Spec is 3.5 min and 10.5 max per the label on the machine. I read a constant 8-9 In H20 static and dynamic. Ignitor is new, burner plate/gaskets are new, refractory piece in exchanger is new. My flow rate from meter to unit during low and high fire respectfully is 254 seconds and 25 seconds, which equates out to 14,173 Btuh and 144,000 Btuh while label is 15,500 and 155,000 Btuh. Anyone have any ideas? I've tried tuning it but as I said this is the lowest I seem to be able to get it. Also, should mention, I did test my intake air and it's at 20.9% as it should be. If anyone has any pointers on which way to go I'd appreciate it. Thank you in advance.