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People tend to overestimate how tight a spray foam building is. Usually is better than regular construction but can still leak a fair bit if a couple of details are not handled well. Even if super tight, a power vent unit doesn't exhaust all that many CFM of air and it will take a heck of a tight building to get it any measurable depressurization in a large workshop. Power vent units are also much less sensitive to building pressure.

If the boiler does end up enclosed in a small room, than it needs a high / low vent to the open workshop space provided that workshop space is big enough.

Manuals are awesome.

Read carefully. Install more carefully.

100K You need min , 100" square of opening . You may have to add a few..

Air in a Can , can be a option..

1000,btu fire needs min one square" fresh air .. Code book .

The problem is circulators pumping into the ponpc. This is dropping pressure, probably sub zero into the top of the system. Adding auto vents at the highest part of the system will introduce more air into the system not expel it.