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You need to run combustion analysis multiple times over a 20-30 min. period while monitoring the manifold gas pressure with all the rest of the house appliances off then repeat with everything under full load. Also, measure the surface temp of your burners from cold start to yellow-tipping. Classic yellow tipping is caused by low primary aeration with low manifold pressure.

Understand that as a burner heats up, the port loading changes. Also, the gas being discharged out the orifice into the burner mixer tube entrains primary air. At higher rates, more air gets drawn in. Also, denser gases, such as LP vs. NG draw in more air. If the air feed is subject to heating, yes, it can draw in less MakeUp Air as it warms. This is how a gas direct vent fireplace starts off with a clean, short, fake blue flame then stretches into a natural lazy yellow flame as the coaxial vent warms. If the air intake warms, it can reverse draft along its conduit surface against the flow of air thus reducing MUA.

@RollCNY

I would assume doing an accurate heat loss takes getting familiar with a buildings dimensions and build spec, and then calculating, so a contractor would be doing several hours free work to get the quote accurate.

That’s the problem with home renovations, we generally only hire mechanics, not engineers. I might get blowback on this one, but in a perfect world an engineer is sizing and specifying the boiler, not a mechanic. The two jobs very much work in hand-in-hand but have different roles.

Professionally, I scope, bid, and manage facility improvements for a Fortune 500 Company. Currently, I’m working on a large (200+ton) chiller replacement, one of seven in the building. By hiring an engineer for the chiller project we saved a lot of money. The engineer determined that our building’s pipes only had so much capacity, and a larger chiller would not get us more capacity. This stinks as we are short on capacity and replacing the pipes would be a significant challenge.

For home improvements, good luck hiring an engineer. A mechanic replacing a boiler can do the sizing, but this tends to be by table-based rules of thumb (EDR) with a larger than needed safety factor built in - you can read a whole bunch of posts here about EDR still over sizing boilers.

But, the contractor who tells you need an engineer and charges appropriately isn’t getting the job. Thus there’s no market for engineers and only a tiny one for even architects in the home improvement world.

My gripe is that I don’t get taken seriously by contractors despite specifying this stuff for a living. I’ve come to learn my limitations as an engineer and not a mechanic and hire when it requires troubleshooting I can’t do, is beyond my skill, or I’m not in position to deal with consequences of failure - I hired a contractor this fall specifically to break open a valve nut after I gave it a lot stress to the point I was worried about breaking it. I wouldn’t have been able to repair the pipe if I broke it, but they would.