What is the purpose of the room pressure differential requirement?
In a forced air system, the rooms are essentially continuations of the ducts. If a door between the supply register and the return air grill is closed, it is like shutting a damper within the duct. The pressure in the room builds, and four things happen: 1) the amount of conditioned air entering the room is reduced, causing it to be cold or hot depending on the season; 2) the increased pressure in the room causes exfiltration to the outside of the building envelope; 3) reduced air flow to the return side of the handler causes infiltration of outside air; and 4) system static pressures will rise to unacceptable levels, reducing system efficiency.
