Posted By Merv Newman
Old ducts are not much of a problem. They should have been removed but apart from the odd dead mouse, or bats, I wouldn't worry. Cover them over with sticky tape.

Merv

Posted By Ian Blenkharn
Do cover the inlets & outlets, where possible close all the dampers, and close off any external duct connections.

If the main plant has been removed and the ducts left open there will be a continuing breeze that will in time accumulate debris and create a dust and fire hazard, and respiratory problems to those who are sensitive to dusty allergen-rich indoor air.

I studied a substantial London tower block some years ago, on suspicion of problems with the air qualiy from their new a/c system only to discover it was the old system that was causing the problems.

Posted By Norman Elkington
Just one more consideration.

Any fire dampers that may have been incorporated in the original ducting as it passes through fire walls (if any) ought to be routinely checked/tested. Alternatively, have the redundant ductwork sealed at these points.