Mechanical Plan Symbols: Ducts, Diffusers and Equipment Tags

Why this matters

An HVAC shop moving from replacing residential equipment into light-commercial work meets the M-sheet, and most of what it carries is in tags and symbols rather than words. A square with an X and two numbers beside it is a diffuser of a specific type delivering a specific airflow. A short note on a duct is its size. A small box on a duct crossing a wall is a fire damper the inspector will ask to see. Read them right and the plan hands you a material takeoff, a balancing report target and a list of inspection items. Read them loosely and the job comes up a diffuser short, or a damper gets left out of a rated wall.

The legend slice

Every set's M-001 or general mechanical sheet carries its own legend, and it governs. A typical excerpt:

MECHANICAL LEGEND (excerpt) - this set's legend governs

[X]  S-1      SUPPLY DIFFUSER, CEILING. TAG = TYPE
     150      AIRFLOW AT THIS OUTLET, CFM
[/]  R-1      RETURN OR EXHAUST GRILLE, TYPE TAG
 14x10        RECT DUCT, FIRST NUMBER = SIDE SEEN
 10" dia      ROUND DUCT, DIAMETER IN INCHES
 ~~~~~~       FLEXIBLE DUCT TO OUTLET
 -|VD|-       MANUAL VOLUME DAMPER (BALANCING)
 -|FD|-       FIRE DAMPER AT RATED WALL OR FLOOR
 -|FSD|-      COMBINATION FIRE / SMOKE DAMPER
 (T-2)        THERMOSTAT OR SENSOR, TAGGED TO UNIT
 <RTU-1>      EQUIPMENT TAG, SEE SCHEDULE ON M-601
 <VAV-2>      TERMINAL UNIT TAG, SEE SCHEDULE

Outlets: type tag plus airflow

A ceiling diffuser or grille is drawn as a small square, commonly with diagonals for supply and a single diagonal or hatch for return or exhaust, and labeled with two things:

• A type tag (S-1, R-1, E-2, or a letter like "A") pointing to the air device schedule, which gives the face size, the neck size, the pattern (four-way, two-way, linear slot), the material and the frame for the ceiling it sits in.
• An airflow in cubic feet per minute (CFM), the amount the balancer will set that outlet to deliver.

The type tag tells you what to order. The CFM tells you what to deliver. Neither is the other: two outlets with the same type tag can carry different airflows, and a 150 CFM outlet in one firm's set may be a different device than a 150 CFM outlet in another's.

Duct sizes

Rectangular duct is labeled with two numbers, such as 14x10. On many sets the first number is the dimension you see in that view, so on a plan it is the width and the second is the depth. On others the legend states width by depth regardless of view. The difference is not trivial: a 14x10 duct read the other way round is 4 in deeper, which decides whether it fits above a ceiling. Find the rule on the legend or in the general notes before anybody fabricates.

Round duct is labeled by diameter, often with a symbol or "dia". Flexible duct is drawn as a wavy or zigzag line to the outlet, and the specification usually limits how long it may be and how it is supported; those limits come from the spec and the duct's listing, so read them there.

Sizes on the plan are commonly clear inside dimensions of the sheet metal; lined duct may be called out with its liner thickness added outside, or noted as a net free size, and external insulation adds to the outside again. The general notes say which.

Notes that ride on the duct

A duct on a plan is a route seen from above, so anything about its height or its construction travels as a note beside it:

• BOD or BOT with an elevation (bottom of duct, at a stated height above the floor) fixes the height at that point; elsewhere, the height follows the general notes, often "as high as possible" or "tight to structure."
• Rise and drop symbols. A small symbol at a point where the duct changes height, commonly with an R, a D or an arrow and the amount. A run drawn as one line can climb over a beam or drop under a pipe; the symbol is the only clue on the plan.
• Construction notes. "Lined", "lined first 10 ft", a pressure class, or a seal class from the specification. These change the fabrication and sometimes the outside size, and they are the ones a fabricator misses when the shop drawing is drawn off the plan alone.
• Up and down arrows at a shaft or equipment. A duct turning up into a rooftop unit or down to a lower floor. On a multi-story job, the matching run is on the next floor's sheet.

Dampers on the plan

• Volume damper (VD). A manual balancing damper, usually at each branch takeoff. The balancer uses it to set each outlet to its CFM. Leaving them out makes the job unbalanceable without opening the ceiling.
• Fire damper (FD). Closes on heat where a duct passes through a fire-resistance-rated wall, floor or shaft. Smoke damper (SD) closes on a signal from the fire alarm or smoke control system. Combination fire/smoke damper (FSD) does both. Where they are required is set by the mechanical code your jurisdiction has adopted (IMC Section 607 in the model code) and by the rated assemblies on the architectural life-safety plan. Each one is installed to its listing and the manufacturer's instructions, needs an access door for inspection, and is periodically tested under NFPA 80 (fire dampers) and NFPA 105 (smoke dampers) in the editions your authority having jurisdiction has adopted, an obligation that falls on the building owner. A crew never moves, deletes or substitutes one without the engineer's written direction.

Watch the letters across trades: FD is a fire damper on an M sheet and a floor drain on a P sheet, and SD is a smoke damper on one and a smoke detector on a fire alarm drawing. The sheet's own legend decides.

Equipment tags and the schedule behind them

Every piece of equipment gets a tag on the plan, such as RTU-1, AHU-2, VAV-2-3, EF-1 or UH-4, and a row in the equipment schedule, usually on a 6-series sheet. The plan says where it is and what it connects to. The schedule says what it is: capacity, airflow, static pressure, voltage and phase, minimum circuit ampacity and maximum overcurrent protection, weight, accessories. See How to Read a Schedule on a Drawing Set.

The tag is also how you find a unit's work on other sheets. RTU-1's disconnect and circuit are on the E sheets, its curb on the S sheets, its gas line on P or M, its condensate on P. And the electrical data in the mechanical schedule must agree with the panel schedule on the electrical sheets; a mismatch between a unit's scheduled maximum overcurrent protection and the breaker the electrical engineer drew is one of the most common conflicts in a set and worth checking before either trade orders. See Panel Schedules on the Drawings, Column by Column.

A worked read: one terminal unit's branch

A shop is taking off the branch served by VAV-2, a variable air volume terminal unit, in an office suite. On M-101 the plan shows:

• VAV-2 above the corridor ceiling, with a 14x10 supply duct leaving it into the office area.
• The duct crosses the corridor wall with an FSD symbol at the crossing.
• Four outlets on the branch: S-1 / 150, S-1 / 150, S-2 / 125, S-2 / 125.
• A VD at each of the four branch takeoffs and flex to each outlet.
• A sensor tagged T-2 on the office wall, tied to VAV-2.

The VAV schedule on M-601 gives VAV-2 a maximum airflow of 600 CFM and a minimum of 180 CFM (values are this set's, read from its schedule).

Takeoff. Two S-1 diffusers and two S-2 diffusers, ordered by type from the air device schedule. Four volume dampers. Four flex connections, lengths per the spec. One combination fire/smoke damper with an access door, plus coordination with the fire alarm contractor, because the smoke function needs a signal from their system. One sensor, with its wiring to VAV-2 under whichever trade the controls specification assigns.

The airflow check. Sum the outlet tags on the branch: 150 + 150 + 125 + 125 = 550 CFM. Compare with the box's scheduled maximum: 550 against 600, so the box can deliver the branch at design with 600 minus 550, or 50 CFM, to spare. The balancer will set each outlet to its tag with the box at or near 550.

When the check fails. Two months in, an ASI adds an office with one more S-2 / 125 on the same branch. The new sum is 550 + 125 = 675 CFM, which is 75 over the box's 600 maximum. The crew does not install the outlet and hope: the box cannot deliver 675 at its scheduled setting, and the balancer will report every outlet low. That goes back to the engineer as an RFI, who may reschedule the box's maximum, upsize the box, or move the outlet to another unit. See related: The Paper That Changes a Job: RFI to Change Order.

What would change the read. On a constant-volume system with no terminal units, the comparison is the sum of outlets on a zone against the supply fan's scheduled airflow, with the same logic. And if the legend says the CFM beside an outlet is a maximum, with a separate minimum scheduled, the branch sum has to be checked at both ends.

Checking your read

For every branch in your scope: the type tags are matched to the air device schedule, the outlet CFMs are summed and compared with the scheduled airflow of the unit or box that serves them, the duct size convention is confirmed from the legend, every FD, SD and FSD is listed with its access door, and every equipment tag has had its schedule row compared with the electrical sheets. Acceptance is a takeoff where every item traces to a tag and a schedule row, and a branch sum at or under its unit's capacity, or an RFI where it is not.

References

• International Mechanical Code, Section 607 (ducts and air transfer openings), as adopted by your jurisdiction
• NFPA 80 and NFPA 105, in the editions adopted by your authority having jurisdiction, for fire and smoke damper inspection and testing
• See related: How to Read a Schedule on a Drawing Set; The Symbol Classes Worth Knowing Across Trades
• See related: How to Trace a Duct Run to the Last Diffuser; Reflected Ceiling Plans and What Lives Above the Grid