Quote Originally Posted by frank View Post
Tut, Tut, Brian. How mis-leading

Imagine a ceiling void in an office being used as a return air path for the ventilation - positive or negative pressure ???
It's got to be negative in relation to the surrounding area otherwise the air wouldn't move through it would it ?

The air pressure within the office space is at a higher pressure than that in the ceiling space which which why the air can be extracted through the ceiling vents/grilles or whatever.

If the ceiling void was at a positive pressure in relation to the office space then the air would travel from the ceiling into the office space.

M'Lud