Calcing combustion air by combining 2 sources

Stray

Hi,
An existing hot water/fuel oil boler is in a closet within living space, with louvered doors. I'm trying to turn it into a confined space (ie putting solid, insulated/gasketed door) due to noise concerns.

The boiler is Weil-McClain 151K btu/h with a Beckett AFG burner. Previously, someone installed a Field Controls "Air Boot" on the burner which is ducted with 4" flex 25' to the outside. This brings in outside air direct to the burner, which I'm sure was for efficeincy/infiltration/comfort reasons.

I contacted Field Controls, and they don't recommend using the air boot for the sole combustion air source. They also cannot provide cfm flows, since this (rightly so) relys on Beckett's fan.

I can duct an 8" round (50.24 sqr inches) duct about 10 feet into the confined space, and put a motorized fresh air damper (like Hoyme SF1) on the end to provide additional combustion air into the space.

What I don't know is how to go about calcing whether or not the combination of the passive 8" duct and the 4" ducted air boot/burner fan can supply enough combustion air.

How can I figgure calculations for the CFM capability of the air boot/burner combination?

Any help appreciated!!