A first year hands you a plan sheet and asks where the duct is. It is a fair question. The sheet is covered in lines, and most of them are not yours.
This is how to find your work on a mechanical plan, follow it from the equipment to the last register, and read what the drawing is telling you about size and direction. It is part of how to read HVAC drawings on a commercial job, which covers the rest of the set.
How do you find a duct run on a mechanical plan?
Duct gets labeled one of two ways. Round shows a number with a diameter symbol. Rectangular shows a value by a value.
That label is your entry point. Once you find one size callout, you have found the run, and you can walk it in both directions.
How do you follow duct from the equipment to the register?
Every run goes somewhere. That sounds obvious and it is the most useful thing on this page, because it means you can always finish the trace.
Trunk lines are larger. Branch lines are smaller and break off of them. A branch terminates at a register or at a piece of equipment. Follow the size down and you are heading toward a termination. Follow it up and you are heading back toward the unit.
When a duct goes vertical it will be sized and noted in wall, or drawn with a hash through it instead of a solid line. That is the drawing telling you the run left the ceiling and went up or down inside construction. Do not lose it there. Pick it up on the sheet for the next floor.
How do you tell supply from return, outside and exhaust?
Four labels do most of it.
| Label | What it is |
|---|---|
| SA | Supply air. Conditioned air heading out to the spaces |
| RA | Return air. Air heading back to the fan |
| OA | Outside air. Intake or make up air coming into the system |
| EA | Exhaust air. Air leaving the building |
If the run itself is not labeled, do not guess. The label is usually waiting at the grille. EG is an exhaust grille. Find that, then follow the run backward, and now you know what the whole run is.
That trick works because the drawing only has to say it once. The engineer labeled it where it was clearest, not where it was most convenient for you.
How do you recognize a branch?
A branch is a take off from a trunk line. That is the whole definition.
What the drawing usually will not tell you is which kind of take off it wants. That comes from the details, the specification, and what the shop builds. We went through the options in duct take off types.
How do you read airflow arrows?
Airflow arrows usually live on the reflected ceiling plan, at the register locations. The arrow says which direction the air gets thrown into the room.
That is not a suggestion. Directional blades get ordered to match the arrow. If you ignore it, the register is wrong before it ever ships, and nobody catches it until somebody is standing in the room wondering why the air is blowing at a wall.
How do you recognize fittings and transitions?
Here is the one that catches people. Transitions are usually not labeled at all.
Any time two ducts are labeled at different sizes, there has to be a transition between them, whether or not the drawing drew you one. You determine it from the two ducts that feed either side. The plan gives you the before and the after. The piece in the middle is yours to work out.
How are duct sizes written?
Rectangular duct is written as an interior dimension. Not the outside of the metal. The clear open space inside it.
That matters more than it sounds like it does. If a duct gets built to the outside dimension and then internal insulation eats into the clear area, you no longer have the open space the design asked for. The number on the plan is the air’s space, not the sheet metal’s.
Round duct is written as a number with a diameter symbol. Always.
As for which size belongs to which section, the callout sits on the trunk or the branch it applies to, or an arrow points at it. When a run has several sizes along it, each one owns the stretch it is sitting on until the next callout takes over.
Why do duct sizes change along a run?
Because the air keeps leaving.
Every branch off a trunk takes some of the air away with it. Past each distribution point there is less air left to move. So the duct steps down to what is actually needed beyond that transition, and the system holds its pressure instead of losing it into oversized pipe.
Once you see it that way, the sizes stop looking arbitrary. A run that steps 20 by 12 down to 14 by 12 down to 10 by 8 is a run that dropped air off three times.
What the size does not tell you
The size gives you the clear open space the design requires. It does not tell you whether that duct needs internal insulation, what gauge to build it out of, or how it gets hung. All of that lives somewhere else, in the details, the schedules and the specification.
A number on a plan is a requirement, not a set of instructions.
Following duct through a change in direction
Follow a noted elevation as strictly as you can. It is rare for mechanical drawings to note one, but when they do it is there for a reason, and something else in the building is usually depending on it.
Most owners want duct as high as possible. Everybody is working in the same ceiling, so there is give and take with electrical, plumbing and fire suppression. What does not show up on the drawings at all is the offsets and special pieces you end up building around work another trade already installed. That is not a drawing failure. That is the job.
The short version
- Find a size callout. That is your entry point to the run.
- Bigger is trunk, smaller is branch, and every run terminates somewhere.
- If the run is not labeled SA, RA, OA or EA, go find the grille and work backward.
- Two different sizes means a transition, whether it is drawn or not.
- Rectangular sizes are inside dimensions. That is the air’s space.
- Sizes step down because air keeps getting dropped off along the way.
- The arrow at the register decides which blades get ordered.
The rest of the set, the sheet numbering, the schedules, the legend and what to do when two sheets disagree, is in how to read HVAC drawings on a commercial job.