What Is a Mechanical (HVAC) Plan?

Each supply diffuser, return grille, and exhaust register carries a tag with its device type and airflow, such as a 12 by 12 diffuser at 250 CFM. Those numbers add up to the capacity of the air handler or rooftop unit on the schedule, and they come from the load calculation and the ventilation calculation for each space.

When a room changes use, its airflow changes too. A storage room turned into a conference room needs more outside air under IMC Section 403 and ASHRAE 62.1, plus more cooling, so the diffuser count, the branch duct size, and sometimes the unit selection all move.

The main trunk leaving the unit is the largest duct on the plan, and each takeoff lowers the airflow downstream, so the duct steps down in size. Rectangular duct is called out width by depth, with the first number being the side seen in plan, and round duct by diameter.

Depth is the constraint that matters most. A 24 by 16 duct may fit under a beam where a 20 by 20 duct carrying the same air will not, so the drafter checks the ceiling height on the RCP, the structural framing, and the sprinkler mains before settling on a size.

Where a duct passes through a fire-rated wall, a fire barrier, a shaft enclosure, or a smoke barrier, IMC Section 607 may require a fire damper, a smoke damper, or a combination fire smoke damper. The mechanical plan shows each one, and where they go depends on the architect's life safety plan and wall types.

Dampers need access for inspection and testing, so each one implies an access door in the duct and often an access panel in a hard ceiling. A rated corridor wall that moves on the architectural background can add or remove a dozen dampers at once.

Tags such as RTU-1, AHU-2, VAV-3-12, and EF-4 point to the equipment schedules, which list capacity, airflow, electrical characteristics, operating weight, and the basis of design. The electrical engineer circuits from those values, the structural engineer checks the weight on the roof, and the plumbing engineer picks up the condensate.

A late substitution with a different voltage, weight, or curb size is a change on four sets of drawings. Keeping the schedule, the plan tag, and the electrical connection in sync is a large share of the revision work on a mechanical set.

Supply, return, and exhaust duct sit on separate layers within the M-HVAC group of the US National CAD Standard, so each system can be colored, frozen, or plotted at its own weight. Equipment, piping, and annotation each get their own layers too.

Overlaying the mechanical plan on the RCP and the sprinkler plan is how teams find diffusers landing on light fixtures and sprinkler heads trapped above wide duct. When a duct widens in a revision, the fire protection designer may have to add heads below it.