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What is a high efficiency take-off and when do you use one?
A high efficiency take-off is an angled tap that feeds a branch off a trunk line. You will only see one when the spec calls for it — they cost more and take longer to install than a standard tap. The angled side always points toward the supply.
When you actually run into one
Most commercial projects do not use HETs. They show up when an owner has prioritized efficiency and the spec section calls them out.
It is the cost, time and quality triangle, and a HET sits on one corner of it. More time to install, higher cost, better result. That is a decision somebody made on paper before you ever got to the job.
Nobody in the field decides to use one. It comes from the spec and the owner, not from code and not from you.
Sizing is the same story. The size comes from CFM requirements, and the engineer figured that out during design. You are not sizing anything — you are installing what the drawings called for.
One note if you are a homeowner reading this. HETs are not on the shelf at a supply house for residential work, and they cost considerably more than anything that is. If somebody is putting them in your house, you were probably sold on them. Owners occasionally know what they are and ask — but it is not common, and it is worth asking what problem they are solving in your particular system.
Which way it points, and my read on why
The angled portion points toward the supply. Always. If you learn one thing on this page, learn that.
As for the physics, I will be straight with you: I am a sheet metal worker, not the engineer who designed the system, and I do not know the airflow science well enough to hand it to you as fact.
Here is my read. Air moving down a trunk line hits an angled tap facing into the flow and turns less sharply to get into the branch than it would with a square tap cut straight into the side. Less turning, less restriction, more of the air the engineer intended actually going where it is supposed to.
That is speculation on my part about the mechanism. What is not speculation is what happens when you get it backwards.
What happens if you install it backwards
A backwards HET is defective. It will not do what the design intended.
The air is not pressing into the take-off the way it should. More of it passes the branch and carries on down the trunk, and the branch has to rely on system pressure to overcome an angle that is working against it instead of with it.
Here is the part that makes it hard to catch. If every other HET on that trunk is in correctly, that one branch is the one that comes up short on CFM when the system gets balanced. It does not fail loudly. It just does not get enough air, and somebody spends an afternoon chasing why one room will not balance.
If you are balancing a system and one branch is stubbornly light, go look at which way that take-off is facing.
Laying it out
Hold the HET against the trunk or branch where it is going, laid out where the branch needs to run. Point the angled portion toward the supply while you are doing it, so you are working with the real orientation and not a mirror image of it. This is also important so the branch line is laid out properly.
Trace the rectangle onto the metal with a black permanent marker. Pencils and pens do not work on duct.
Set the HET aside — on top of the duct if there is room — and adjust your line to come in one inch from the edges of what you traced. That inset is the whole job. It is what leaves metal for the flanges to sit on.
Hold the HET back up and check it. You want a good rectangle where the flanges will land, with a bearing surface between the trunk and the take-off all the way around. Reduce the rectangle further if you need to. At a minimum, the duct hole should be no smaller than the HET rectangle opening. If it is restricted, that could also limit the CFM.
Cutting it in
Here is a link to all the tools you will need as discussed below.
Set the HET off to the side again and pull out your hammer. With the pointy end, hit the duct with force inside the rectangle to open a hole to start from.
Put the hammer away, pull out your snips, and cut to the inner rectangle — the line you insetted, not the one you traced around the fitting.
Now the part people get wrong. Put the snips away and pull out your duct board knife. Insert the knife into the gap you just created and cut the insulation with the serrated edge, following your metal cut.
Do not tear the cut piece out. Ripping it will wreck the insulation around the opening and you will be patching it. Cut it free and discard it with your duct board knife (paid link).
Remember, a duct board knife cuts a variety of insulation types. A serrated edge and flat blade edge is recommended.
Setting it
Peel the protective film off the sticky border of the HET. Check the angle one more time — toward the supply.
Press it against the opening and put a screw in each corner. Add more if it needs them, usually top and bottom.
The insulation transfer nobody explains
A HET is not insulated on the inside. It gets duct wrap on the outside.
That makes the take-off the transfer point in the system — internally lined trunk on one side of it, externally insulated branch on the other. Everything downstream changes character right there.
From the HET the branch typically turns to round pipe, and then to flex duct running to the ceiling register. The flex comes insulated. The round pipe gets insulated on the job.
Mastic, outside and inside
The job is not done when the screws are in.
Apply duct seal mastic (paid link) along the seams around the outside of the rectangle and smooth it out with a 2 inch cheap paint brush. You are not keeping it after you are done. This part most people do. This linked product is a representation, make sure you match your product to the specification if they are called for.
Then seal the cut insulation on the inside of the trunk line where it was cut. That part gets skipped.
Here is why it matters. The edge of the insulation comes coated from the factory so the fibers do not blow into the airstream. The moment you cut through it, the fibers in that thickness are exposed and they are sitting directly in moving air.
The system will not complain. It will run exactly the same. There will just be more particulate in the air of that building than there should be, and nobody will ever trace it back to a take-off somebody cut in and did not seal.
The job is not done until the mastic is on the outside and on the cut insulation.
Mark the damper before you walk away
Mark the damper blade position on the drop rod so anybody looking at it later can tell which way the blade is sitting inside.
That drop rod normally extends down to the ceiling so it can be reached below the lid. Whoever balances this system is going to be standing on a ladder working that rod, and if the blade position is not marked they are guessing at what they are adjusting.
Thirty seconds now. An afternoon of somebody’s time later.
The most common mistake, and how to save it
Not accounting for the inset, and over-cutting the hole.
You trace around the fitting, you cut to the line you traced instead of the line you insetted, and now the flanges have nothing to bear on.
The fix: put a piece of patch metal over the hole and cut it out again, correctly this time. Don’t for get the mastic on all the added seams.
That works. But be ready for the other outcome — on a job where the owner is focused on quality, they may ask you to replace the piece of duct entirely. That has happened to me.
Inset your line. It is one measurement and it is the difference between a fitting that sits down tight and a morning you spend redoing a piece of trunk.
Common Questions
Which way does a high efficiency take-off point?
The angled portion points toward the supply — toward the air coming at it. Installed the other way it is defective, and the branch it feeds will come up short on CFM when somebody balances the system.
When is a HET required?
When the spec section calls for it. It is an owner and design decision, not a code requirement, and most commercial projects do not use them.
What size HET do I need?
Whatever the drawings say. Sizing comes from CFM and the engineer worked it out in design. You are not sizing it in the field.
Is a HET insulated?
Not on the inside. It gets duct wrap on the outside. The take-off is where the system transfers from internal lining to external insulation.
I cut the hole too big. Now what?
Patch metal over the hole and cut it again, this time insetting your line an inch from the traced rectangle. On a quality-focused job, expect the possibility that you will be asked to replace the piece of duct instead.
Cutting in a standard take-off instead? Here is the tool list for duct work, and if you are new to it, the hand tools worth buying first.