Just got back from a call on a gov. bldg that has the make-up air ducted together with the combustion air vent. The system is old, but the metal duct is in good shape.
the two ducts join right before the vent to the outside.
There is a water heater and two 80 % furnace in an 8 x 12 sealed room sharing an 8 x 20 combustion air vent with the buildings fresh air make-up vent.
Isn't this a code violation?