For what its worth, I have a Polaris condensing water heater.
Its a negative gas pressure regulator, and of course a direct forced vent.
After hundreds of codes and failed ignitions, I extended the outlet combustion vent by 30 feet and ran the intake air pipe inside of it to get some preheat on the incoming combustion air.
It too had no means of air fuel ratio, so I intuitively thought that more backpressure in the exhaust would downrate the combustion BTU's and reduce the airflow across the burner. It seems to have worked well now for 10 years without a code.