Here is a question worth two inches.
A duct is called out at an elevation and that duct is going to have insulation. Is the number to the metal, or to the outside of the insulation?
It is to the metal. Always, regardless of where the insulation goes.
If you have never had to ask that out loud, good. Plenty of people have guessed wrong, and the CAD software has a setting for it, which tells you how settled the question is on the design side.
This is part of how to read HVAC drawings on a commercial job. That also means you have to do some extra math to get to the right elevation. Duct size is not always metal size.
The four abbreviations
| Abbreviation | What it is |
|---|---|
| AFF | Above Finished Floor |
| BOD | Bottom of Duct |
| TOD | Top of Duct, or Top of Deck |
| TOS | Top of Slab, or Top of Structure |
Two of those four have two meanings, and they are not close to each other. Get one wrong and everything downstream of it is wrong by whatever the difference was.
TOS is top of slab or top of structure, and the sheet you are reading decides. A structural sheet, the S pages, is going to mean top of structure.
TOD is top of duct or top of deck, and it depends which drawings it is on. The legend will state which one is true for that set.
That is the whole argument for reading M000 before you read anything else.
What the number includes, and what it does not
Insulation is not in it. An elevation is to the metal whether that duct is bare, wrapped or lined. The wrap goes on after and it does not move the callout.
Lined duct is fabricated to the clear inside dimension, so the metal is bigger than the callout by twice the liner. Set your BOD off the metal you’re actually hanging, not the number on the sheet.
And BOD is the bottom of the duct. Not the bottom of the strap. Not the bottom of the tab, the insulation, the screw, the trapeze or the Unistrut.
Everything hanging below that duct is yours to keep out of somebody else’s way, and none of it is what the number is describing. If a light fixture has to clear your work, it has to clear the strap too, and the drawing never said a word about the strap.
Where the number comes from when the drawing does not have one
Most of the time, mechanical drawings do not note an elevation at all. So somebody decides it, and that somebody is the field.
It comes out of coordination. You are getting past plumbing, around large conduit banks and cable trays, and staying out of the way of the fire suppression drops. The elevation is whatever survives all of that. There is more on why that fight goes the way it does in why duct loses the fight for space in the ceiling.
What “as high as possible” actually means
Most owners want the duct high and out of the way, and most of the time you will be told exactly that and nothing more.
In practice it means tight to the bottom of the joists while you keep your clearance, or up in the joist cavity if you are running parallel to them.
And that inch of clearance is not only for you. Conduit gets mounted to the bottom of a joist and it has to sit there without touching your duct. Fill that gap and you have taken somebody else’s space, not just your own margin. We went through that clearance and the rest of the structure rules in hanging duct from steel joists.
The caveat nobody tells a first year
In a big building, as high as possible is the wrong answer.
Say the deck is at twenty feet and the ceiling is going in at ten. You do not want the duct at twenty. Put it up there and nobody will ever touch it again.
Think about what servicing it would take. The ladder to reach twenty feet will not fit in the space you would have to set it up in. A lift will not get you above a ceiling at that height either, because the grid is in the way. You have built something that works perfectly and is unreachable.
So in a large building it is common to deliberately drop duct down so it sits at a height where somebody can get to it above the ceiling. That is not sloppy. That is the right answer.
Word of the day: accessible. Not whether you can install it. Whether anybody can ever reach it again. It is the one thing that overrules “as high as possible,” and it is the thing a first year never thinks about because he is not the one coming back in four years.
Exposed duct in an open ceiling
Here “as high as possible” comes back, and now it is about how the room feels rather than access.
Duct up and out of the way means the walls can be painted and decorated without metal hanging into them. You see this in restaurants, gymnasiums, auditoriums, and warehouses that have no built out offices with low ceilings.
Somebody is going to look at that run every day for the life of the building, and they are going to look at it from the floor. That changes what “good enough” means.
How you actually set it
A laser level, run off the building benchmark, is the best practice. That is the short version and it is what you should be doing.
We did not have a laser for a long time, so we measured off the floor. It works and plenty of good duct got hung that way, but it carries a risk, because the floor is not the same thing as the building’s reference. That is why you never set duct height off the floor.
Then a torpedo level on it to check after strapping.
The sequence with a crew
- Measure out your straps, give them a bite, bend them.
- Assemble twelve to sixteen feet of duct on the ground.
- Three guys take it up the ladders, everybody holding a part of it.
- A fourth positions and fastens the straps.
Short on people, you do it in pairs. Shorter than that, you work it solo, and it is a lot slower. That is its own subject and it is covered in hanging duct by yourself.
Who checks it
Realistically, your foreman, and it is a visual inspection.
No inspector is checking your duct elevations. The GC superintendent might say something if it is in the way of somebody else or if the coordination did not hold, but in my experience that was rare.
Which means the check that matters is the one you do before you let go of it.
The shop does not care
Elevation is not fabrication information and it does not need to get to the shop.
The elevations are set on the drawings. The duct gets built to size and then it hangs at the elevation in the building. Nobody in the shop needs the number to make the piece.
The short version
- Elevations are to the metal. Insulation is never in the number.
- BOD is the bottom of the duct, not the strap, insulation, the tab, the trapeze or the Unistrut.
- TOS and TOD each mean two different things. The sheet and the legend decide, not you.
- Most mechanical drawings do not note an elevation. It comes out of coordination in the field.
- “As high as possible” means tight to the joists and keep your inch, and that inch is conduit’s room too.
- Unless the building is tall. Then high enough is the height somebody can still reach.
- Laser off the building benchmark. The floor is not the benchmark.
- Your foreman is the only one checking. Nobody else is coming.
I am not an engineer. Any sizing or heights noted here are for representation only, and the design and specification on your job govern.