How to Install Flex Duct Properly

We truly like flex duct. However it’s important to set up it correctly

That is the factor about flex duct. When HVAC contractors use it the way in which it is supposed for use, it is high quality. Nevertheless, in lots of houses, flex duct is overused as a result of it is comparatively straightforward to put in in comparison with onerous pipe duct.

Along with being overused, it is incessantly put in sloppily. Over time, it sags or will get bent and inhibits treasured airflow by your system.

This does not need to occur. HVAC installers can use flex duct and keep away from these issues. They only want to put in it the correct means

Talking of, the next steps cowl the correct strategy to take when putting in flex duct:

Seal the ducts with mastic

All ducts needs to be sealed and situated inside the conditioned area (dropped ceiling, soffit, inside a ground system, or in a conditioned crawlspace or attic). It could be good if this had been at all times true, however the truth is that that is hardly ever the case. When ducts are exterior the conditioned area, they need to be closely insulated along with being sealed.

Tip: All the time use UL-181 rated duct mastic and tie wraps to make the seal between the ducts and different connectors.



Slicing holes for the registers

You may start the set up by chopping the holes within the ceiling for the registers. (We suggest utilizing MV4 registers. They’re a plastic register with inbuilt collar.) They arrive with a template that’s traced onto the ceiling. Then the drywall is reduce out. The registers needs to be centered in the rafter area and stored roughly two toes from the wall. For the commonest kind of system, there’s a widespread return grille that’s often put in within the ceiling of a central hallway within the dwelling. That is known as a return air filter grille as a result of it additionally homes the air filter for the system. That is really helpful to keep away from going into the attic to interchange the filter. Find the register between the rafters within the corridor and reduce the drywall out for the right dimension filter grille. Then, 2×4 blocks needs to be nailed between the rafters to make a field and a bit of plywood will be nailed over the highest to completely enclose it. Wooden is really helpful to keep away from condensation.

3. Use flex duct for straight runs

You will know you should utilize flex duct in case your design comprises straight runs (i.e. lengthy, straight sections of duct). Since flex duct sometimes underperforms because of sagging and kinking, straight runs just about get rid of this.

Sure, the duct can nonetheless sag on a straight run. But it surely will not sag in case your contractor helps it correctly.

Something duct sections that are not straight? Use onerous pipe! This contains bends, elbows, and something that is coming proper off the plenum. Bent flex duct won’t enable air to stream correctly, however onerous pipe duct will.

The one time it is likely to be okay to make use of flex duct for a bend is on a really lengthy one with an enormous radius. However even then, onerous pipe might be higher.


Assist horizontal duct runs with 1-1/2-inch-minimum-width hanger/saddle materials. Assist intervals needs to be in accordance with the producer’s suggestions, or as your native code specifies, however at no higher a distance than 5 toes.

Lengthy duct runs with sharp bends could require extra helps earlier than and after the bends.

The utmost permissible sag is 1/2 inch per foot of size between helps. Ceiling joists supply an alternative choice to suspension straps for supporting ducts in attic areas.

To stop harm to the duct by the sting of the metallic collar, the duct ought to have an extra help between the metallic connection and the bend. This permits the duct to increase straight for a couple of inches earlier than making the bend.

Some native codes could preclude any vertical use of versatile air duct. The place it’s allowed, ADC pointers say that factory-made flex air ducts might not be used for vertical risers in air duct techniques serving greater than two tales. Vertically put in ducts additionally needs to be stabilized by help straps each 6 toes.

Putting in return air ducts

The versatile duct set up continues with the return air duct. The duct can be hooked up to the return air plenum utilizing a collar. Then stretch the duct to the return air field that was beforehand constructed. Reduce a gap within the prime of the field and insert the collar and fix the duct.

Sidebar: Contractors Communicate Out On The Deserves Of Flex Duct

Bucky Holland mentioned that he installs versatile duct “as usually as potential.” The proprietor of Holland Heating and Air, Macon, Ga., mentioned that 99 % of his installations embrace flex duct.

“There are a lot of causes for that,” he mentioned. “No. 1 is packaging. Additionally it is faster and simpler to put in.”

Holland mentioned the truth that his enterprise is a labor-driven firm makes it a necessity to save lots of as a lot time as potential. He mentioned that using flex duct in lots of instances can scale back the labor prices of an set up.

His firm makes use of flex duct primarily for turns and run-outs, that are tied into the primary metallic duct traces. “If used correctly, flex duct makes a neat job,” he acknowledged. “It’s abused each day, but when it is put in accurately, it should do nearly as good a job or higher than onerous metallic ductwork.”

A Knoxville, Tenn., contractor pointed to the acoustic properties of flex duct as one other benefit. “When put in correctly, flex duct has a sound-deadening impact with out the restrictions of different set up strategies,” mentioned Mike Davis, president of Metropolis Warmth & Air Conditioning. “Additionally it is more economical to put in than onerous pipe with 2-inch ductwrap.”

Michael Hays, basic supervisor for Customized Air Ideas, Winder, Ga., mentioned {that a} substantial variety of installations within the Atlanta market embrace flex duct. He mentioned his firm presents prospects a selection of sheet metallic, ductboard, or flex duct merchandise.

“Ease of use and velocity of set up are the plain benefits of flex duct,” Hays added. “There are additionally fewer seams in flex duct, which suggests much less potential for air leakage.”

He famous that techniques needs to be designed utilizing flex duct specs, as flex duct is extra restrictive to airflow than another techniques. “However when you design it correctly, you can arguably make a greater system,” he mentioned.

Hays emphasised that correct set up is important. “It is simple to abuse as a result of somebody could resolve to attach 5-foot runs with 25-foot lengths of flex duct,” he mentioned. “The product is okay – it is the set up that must be addressed. And flex duct producers do an excellent job of offering set up data.”

A Michigan contractor lists yet one more use for flex duct – shifting make-up air. “We do not use flex duct a lot apart from piping in make-up air,” mentioned Larry Paquette of Hutchison Mechanical Co. Inc., Macomb, Mich. “I purchase numerous 6- and 4-inch [flex duct] for make-up air. Make-up air needs to be insulated. Drawing in chilly air causes condensate, which does not occur with flex duct.”

– John R. Corridor

Publication date: 10/25/2004

4. Assist the Ducts

Typically the most effective route for a versatile duct run is up and over an obstruction. However with out correct help, ducts can develops sags or kinks that scale back airflow [4A, 4B].

To stop issues, use saddles or straps to help at common intervals [4C, 4D]. Comply with the producer’s directions, however in case they “get misplaced,” here’s a listing of finest practices for supporting ducts: 

  • House helps not more than 4 toes aside (a connection to a inflexible duct or to gear counts as a help level).
  • Helps needs to be at the least 1½ inches large.
  • In lengthy horizontal runs with sharp bends, use further helps earlier than and after the bends.
  • Don’t let the ducts sag greater than ½ inch per foot between helps; most sag shouldn’t exceed 2½ inches [4E].
  • Assist straps mustn’t compress the interior core or constrict airflow.
  • Helps mustn’t constrict the insulation as a result of that may trigger chilly spots and condensation, which may result in mould development. 

When ducts must be routed up and over an obstructi

When ducts have to be routed up and over an obstruction, too few helps may cause kinks [4A]. The answer is to create a gradual bend utilizing helps at frequent intervals [4B]. Supporting materials needs to be large sufficient to keep away from compressing the duct.

A good rule of thumb is to provide a duct support

A great rule of thumb is to supply a duct help at least each 4 toes, and extra incessantly when utilizing large-diameter versatile duct [4C], even when ducts are partially supported by framing members [4D]. Ideally, ducts shouldn’t sag greater than ½ inch per foot between helps, and most sag mustn’t exceed 2½ inches [4E]. 

Illustration: courtesy U.S. Dept. of Energy

Illustration: courtesy U.S. Dept. of Power

This text was tailored from a video sequence at, which relies on materials from the Building America Solution Center (BASC), a web based software from the U.S. Division of Power that collects best-practices suggestions from the nation’s prime constructing science and residential constructing consultants.

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Assist your ducts

Use saddles or help straps to carry ducts up, and use sufficient of them to verify they do not trigger kinks (there are these kinks once more) within the flex duct.

Comply with the producer’s directions, however in case they “get misplaced” … 

  1. House the helps not more than 4 toes aside.
  2. Do not let the ducts sag greater than 1/2 inch between helps.
  3. Any connection to a inflexible duct or to gear is taken into account a help joint.
  4. Lengthy horizontal runs with sharp bends ought to have further helps earlier than and after the bends.
  5. Helps needs to be at the least 1-1/2 inches large.
  6. Straps mustn’t assemble the interior core or scale back airflow.
  7. Helps mustn’t constrict the insulation as a result of that may trigger chilly spots, which may trigger condensation contained in the ducts, which can result in mould development.




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— Building America Solution Center is a web based software that collects finest observe suggestions from the nation’s prime constructing science and residential constructing consultants to assist builders and remodelers get to net-zero ENERGY READY houses.