We need some air in here!

In a previous posting I mentioned running into an issue with our water heater failing to draft when all the exhaust fans and clothes dryer running.  I did some research and through testing I thought the entire issue could be resolved by adding more “make-up air”.   Thus I added a 6″ duct into our mechanical room.  This was in addition to our existing 5″. You would think that after having ~48″ sq. of open hole to the outside would mitigate any possible negative pressure, but you would be wrong.  The water heater still failed to draft in worst case CAZ testing. I called a local HVAC contractor to evaluate.  They spent 3 hours trying different things, including adding 3′ to our chimney, but no joy!