Well Rick, it's as Spaw says...The more you know about something the worse this gets ...

I spent many years as a metal worker and many happy months making ducting - If they were to get it right just ONCE.

Perhaps we should comandeer the Mudcat Sitcom thread and do an episode where everything is right for a change.
The hero takes of the grill and finds that he can barely get his arm in the duct - he reaches inside and gets his hand caught in the first joint - after much struggling and screaming he manages to extract his hand and it's dripping with blood.

Oh, and by the way has anyone EVER seen a room in which the walls move in?

Bert.