How to Trace a Duct Run to the Last Diffuser
Why this matters
A mechanical plan for one tenant suite can carry a dozen diffusers, three branch sizes, a fire damper and a match line, and the installer, the estimator and the balancer all need the same picture of it: where the air goes, how much goes to each outlet, and whether the outlets add up to what the unit makes. When the airflow on the diffusers does not total the unit's supply, something is missing from your count or from the drawing, and you want to know which before the duct is fabricated. When the run crosses a rated wall, there is a damper there that somebody has to buy, install with an access door, and leave inspectable.
Symbol meanings are covered in Mechanical Plan Symbols: Ducts, Diffusers and Equipment Tags. This card is the walk from the equipment tag to the last diffuser.
The example zone
A single-zone rooftop unit, RTU-1, serves a suite: an open office, conference room 205, copy room 207, and a stretch of corridor on the far side of a 1-hour rated corridor wall. The suite is on sheet M-201A, and the corridor continues on M-201B across a match line.
RTU-1 supply 1,600 CFM (unit schedule)
|
| 24x14 main
+----VD--> 4 x S-1 @ 200 = 800 OPEN OFFICE
|
+----VD--> 3 x S-2 @ 150 = 450 CONF 205
|
| 16x12
+----VD--> 1 x S-3 @ 200 = 200 COPY 207
|
--------FD-1-------- 1 HR corridor wall, access door
| MATCH LINE, continued on M-201B
+----VD--> 1 x S-2 @ 150 = 150 CORRIDOR
The drawing above is the path, straightened out. On the real plan the main wanders around beams and the branches leave at angles, but the sequence of takeoffs, dampers and outlets is what you are reading for.
Step 1: Start at the unit schedule, not the plan
Find RTU-1 on the mechanical equipment schedule and write down the supply airflow (1,600 CFM), the outside air airflow, and the external static pressure the unit is selected for. The supply airflow is the number every diffuser on the zone has to add up to.
Why first: the schedule is the designer's statement of intent for the whole zone. The plan is that intent broken into pieces, and pieces get lost. If you total the plan first, you have nothing to check it against.
What breaks if you skip it: a 150 CFM omission reads as normal, because the plan by itself always adds up to itself. See related: How to Read a Schedule on a Drawing Set.
Step 2: Find where the air enters the building
On a rooftop unit, the supply drop is where the duct leaves the curb and enters the ceiling space. Find it on the roof plan and the floor plan, and confirm the tag matches: RTU-1 on the roof, a supply drop labeled RTU-1 below. Note the main's size as it leaves the drop (24x14 here, width by depth in inches, and commonly the first number is the side you see in plan, though the legend governs).
On a split system or an air handler in a mechanical room, the start is the supply plenum on the unit. On a VAV system, each terminal box is its own small version of this exercise: start at the box schedule's maximum airflow and trace that box's outlets.
What breaks if you skip it: on a building with several units, a main that looks continuous on the plan may be two units' ducts that run side by side, and you trace the wrong one.
Step 3: Walk the main and log every takeoff
Follow the main from the drop and stop at every branch. At each one, log four things: the branch size, the balancing damper (VD, a volume damper, which the balancer uses to set each branch's airflow), the outlet tag, and the outlet's airflow. Outlet tags commonly read like "S-1 / 200," meaning supply type S-1 from the diffuser schedule at 200 CFM, sometimes with a neck size; the air distribution schedule and legend govern. Note every size change on the main as well, because each one is a transition fitting someone has to fabricate.
On the example, sheet M-201A:
| Takeoff | Outlets | Each | Subtotal |
|---|---|---|---|
| Open office | 4 x S-1 | 200 CFM | 800 CFM |
| Conf 205 | 3 x S-2 | 150 CFM | 450 CFM |
| Copy 207 | 1 x S-3 | 200 CFM | 200 CFM |
The main reduces from 24x14 to 16x12 after the conference takeoff, because less air remains.
What breaks if you skip it: a takeoff with no damper drawn is a takeoff the balancer cannot adjust. If the spec section requires a damper on every branch and the plan omits one, that is a question, not an install-as-drawn.
Step 4: Total the outlets against the schedule
Add the subtotals: 800 + 450 + 200 = 1,450 CFM. The schedule says 1,600 CFM. The difference is 1,600 - 1,450 = 150 CFM, which is 9.4 percent of the scheduled 1,600.
Do not explain a gap like that away as leakage or a rounding allowance. Designers commonly make the outlets on a zone total the unit's supply, so a gap usually means an outlet you have not found. Look in this order:
- Across a match line. The main runs off the edge of M-201A. Following it onto M-201B finds one more S-2 at 150 CFM in the corridor. New total: 1,450 + 150 = 1,600 CFM, matching the schedule.
- On an enlarged plan. Toilet rooms and small spaces often show outlets only at a larger scale.
- In a note. "Provide 2 additional S-2 per enlarged plan" is easy to miss.
- In the drawing. If none of those finds it, the plan and schedule disagree, and that goes to an RFI with both figures. See related: How to Write an RFI That Gets a Usable Answer.
A gap in the other direction, outlets totaling more than the unit, is equally a question: either the schedule is stale after a unit change or an outlet was copied twice.
What breaks if you skip it: the corridor diffuser does not get fabricated or ordered, and the balancer finds 150 CFM nowhere to go at the end of the job.
Step 5: Find every damper at a rated wall, and its access
Lay the duct route over the life safety plan (commonly a G or LS sheet, or rated walls shown on the architectural plans with a hatch or line pattern the legend defines). Every place the duct crosses a rated assembly, look for a damper symbol: fire damper (FD), smoke damper (SD), or combination fire/smoke damper (FSD).
On the example, FD-1 sits where the main crosses the 1-hour corridor wall, right before the match line.
Whether a particular rated wall needs a fire damper, a smoke damper, both, or neither depends on the assembly type, its rating and the exceptions in the building code as adopted (IBC Section 717 in the editions most jurisdictions use). That determination is the designer's, and the drawing shows the result. Your job is to find every damper that is drawn and flag every rated crossing where none is drawn, as a question.
Then find the access. A damper has to be reachable for inspection and resetting, and the mechanical code as adopted requires access and identification (IMC Section 607). Check the reflected ceiling plan at FD-1: in a lay-in grid ceiling, a tile below the damper serves; in a hard drywall ceiling, there has to be an access panel, and somebody has to provide it. If the RCP shows a hard ceiling with no access panel near FD-1, that is a coordination item between you and the drywall trade, and it is cheaper before the ceiling closes.
What breaks if you skip it: a damper installed in a closed ceiling with no access, which the inspector or the facility's damper inspector finds later, at a cost that lands on whoever installed it.
Step 6: Mark every crossing with another trade
Walk the route once more on the reflected ceiling plan and the structural framing plan. Note every place the duct meets:
- Beams and joists. A 14 in deep main plus insulation under a beam has to fit between the beam and the ceiling. Compare the bottom of steel on the structural sheets with the ceiling height on the RCP.
- Sprinkler mains and branches, which on many projects get priority in coordination because their layout is set by spacing and coverage rules.
- Light fixtures and diffusers on the RCP. A recessed fixture and a diffuser drawn on the same ceiling tile cannot both be there.
- Plumbing with slope. A drain line cannot rise to clear a duct; the duct usually moves.
Where the project uses coordination drawings, those govern who gets which elevation. See related: Coordination Drawings and Who Gets the Ceiling Space.
What breaks if you skip it: the duct is hung first and the conflict becomes another trade's field problem, or yours, after the materials are cut.
What changes the method
- Ducted versus plenum return. A ducted return gets the same trace, outlet by outlet, against the return airflow. A plenum return has no duct to trace, but it changes what may live above the ceiling (plenum-rated cable and materials), which the electrical and low-voltage trades care about.
- VAV systems. Each terminal box has its own maximum and minimum airflow on the box schedule. Trace and total each box's outlets against its own maximum; the unit's total is a diversified figure and the outlets will not add up to it.
- Transfer air. A transfer grille moves air between rooms without a duct from the unit. It carries a CFM tag but is not supply, and counting it in your supply total double-counts air.
How to check your trace
Total the outlets on every sheet the run touches and confirm the figure matches the unit schedule, or write down the difference and which document is wrong. List every rated crossing with its damper tag and the access method shown on the RCP. List every crossing with structure or another trade with the clearance you found. If a balancer could take your log, set every damper, and not find an outlet you did not list, the trace is complete.
References
- International Building Code Section 717 and International Mechanical Code Section 607, as adopted by your jurisdiction, for duct penetrations of rated assemblies and damper access
- See related: Mechanical Plan Symbols: Ducts, Diffusers and Equipment Tags
- See related: Reflected Ceiling Plans and What Lives Above the Grid
- See related: Coordination Drawings and Who Gets the Ceiling Space
- See related: How to Find Your Place by Column Grid and Key Plan