Checking for Sufficient Combustion Air

We certainly need to follow code requirements to verify sufficient combustion air to the appliance, but, unfortunately codes are prescriptive based as opposed to performance based.

In other words, does the mathematical calculation used to size the combustion air intake work under actual operating conditions?

One quick and easy way to verify sufficient combustion air intake (In the real world) is to fire up the heating system with the home under normal winter heating or summer air conditioning operation (ie. all doors and windows closed), and allow the readings on your combustion analyzer to stabilize.

While watching the O2 or CO2 and CO readings, open up a door or window from the mechanical room to the outside.  Should any of the readings change, there is a problem.

Obviously, a door or window should not be controlling the combustion process.

A change in the readings may not tell you exactly what the problem is, but can be useful in narrowing down the possibilities.