Darren,

I just reviewed the installation instructions from three B vent manufacturers.

DuraVent:
"Connector Rise. Plan a minimum of one foot vertical connector rise
coming out of each appliance."

AmeriVent:
"Wherever possible, install vertical vents directly above
appliances before beginning any lateral runs. "

And I could find no such requirements for MetalBestos.

But, then again, I could very well have missed the specific instruction.

__________________
The apostrophe troll.