It may be surprising that just these three systems — mechanical, electrical and plumbing (MEP) — can make up as much as 15% of the total environmental footprint of a building's materials. Even more important is the operation of these systems, particularly mechanical and plumbing. In many cases, pipes and ducts are run willy-nilly after a home has been designed. The frequent result: high energy and water bills. These details are examples of how MEP systems must be integrated into a home's design and then installed according to the design for optimal performance.
You can't undercut doors enough to provide adequate return air flow with HVAC forced air systems (with the door closed, of course). There are four different ways to provide adequate return: dedicated return ducts, above-door offset transfer grilles, high-low transfer grilles, and jump ducts.