Asked by: Cel Shadhan
home and garden indoor environmental quality

How much flex duct should be used?

Last Updated: 24th May, 2020

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The official recommendation from the ADC is that flex ducts should sag no more than 1/2" per foot of length between supports. The supports need to be at the manufacturer's recommended spacing, but no longer than 4' maximum. The straps that hold up the flex duct need to be no less than 1.5" wide.

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Also, is flex duct any good?

Flexibility. Flex ducts are better for existing trunk-and-branch heating and cooling systems. This is because they're more versatile and flexible. Metal ducts are more rigid due to the nature of steel, making them ideal to build an entire HVAC system.

One may also ask, how do I know what size flex duct I need? How to Size a Flex Duct

  1. Measure the length and width of the room where the flex duct will be supplying the air. Multiply the length and width together to get the square footage of the room.
  2. Figure out the amount of CFM that is required for the room.
  3. Use the online duct calculator (see References) and calculate the needed flex duct size.

Beside above, how far can you run flex duct?

E. Flexible duct shall be supported at manufacturer's recommended intervals, but at no greater distance than four feet. Maximum permissible sag is 1/2 inch per foot of spacing between supports.

What is the best ductwork?

Flex ducts are good for air quality because you are not breathing in fiberglass insulation lined ductwork like the Fiber Air Ducts and they are mold and rust resistant unlike galvanized sheet metal ductwork.

Related Question Answers

Abdelhadi Watzold

Professional

Does Flex duct reduce airflow?

Friction is the enemy of airflow. Unlike rigid ducts, which are cut to length within a tolerance of 1 inch or less, it's easy to cut a length of flexible duct that's several feet longer than is needed to get from point A to point B [2A]. That creates slack in the duct, which reduces airflow for two reasons.

Harold Raffauf

Professional

Can you use flex duct return air?

Re: Flex Duct on Return Air Diffusers - Yes or No? Flex duct effectively creates a tight radius elbow where it turns into the top of a diffuser. The high pressure loss is further increased because the flex fabric gathers at the inside of the throat and reduces the effective diameter.

Derlis Izarondo

Professional

Should I replace metal duct with flexible?

Metal duct can still be found in many older homes, but flexible duct is more prevalent in newer homes. Both work well. Still, should you decide to change from metal to flexible duct, don't waste money on an HVAC contractor. It's a time-consuming job, but definitely one you can do yourself.

Hunter Ruberto

Explainer

What is the R value of flex duct?

For example, a six-inch diameter, nominal R-8 flex duct has an actual R-value of only 5.62 not counting surface films. With the film resistances the total R-value of 6.45 is 19% less than the nominal value. For R-11 ducts (six-inch diameter) the R-value with films is only 7.81, 29% less than the nominal value.

Laquita Oldenkotte

Explainer

Should ductwork be replaced after 20 years?

“If your ductwork is over 15 years old, you probably should replace it. Ductwork has a maximum lifespan of 20-25 years. By 15 years, however, it begins to deteriorate, significantly reducing your HVAC system's efficiency, so replacement is the prudent option.”

Wonda Sandfoss

Explainer

Can you cut Flexible ducting?

Installation Tips & Tricks — To cut flexible duct, use a steak knife from your kitchen drawer to cut through the outside jacket, fiberglass insulation and inner plastic core. Then cut the wire with a pair of wire cutters.

Yedra Agosti

Pundit

Can I replace ductwork myself?

Yes, having punctures in your ductwork due to pests is something that necessitates getting those ducts replaced, or at least repaired, and, yes, it is also a really tough job for you to take on yourself.

Sanford Corbaton

Pundit

Should you insulate your ductwork?

Cool air passing through warm areas in your home can cause condensation in the ductwork. Proper insulation levels can actually prevent condensation form occurring in your ductwork. Seal Ducts Before Insulating. Before adding ductwork insulation, it is highly recommended to first seal leaks in the ductwork.

Hannibal Yakushkin

Pundit

How do you install a flexible air duct?

How to Install Insulated Flex Duct
  1. Measure the distance between the plenums or collars where the flexible duct installs.
  2. Roll back the plastic or foil jacket and the fiberglass lining approximately 8 to 12 inches from the inner core.
  3. Spread a thin layer of mastic glue onto the beaded end of the plenum or metal collar.

Zarko Porteiro

Teacher

How can I help my flexible duct?

  1. Support flex duct using straps or saddle supports or as specified by the manufacturer.
  2. Space supports at 4-foot intervals or closer to ensure that the flexible duct does not sag more than 1/2 inch per lineal foot between the supports.
  3. Place supports at each connection between flex duct and rigid metal duct.

Karle Yams

Teacher

Is flexible ductwork vs rigid ductwork?

A: Most of what's being installed in new houses now is flexible ducts because they cost so much less than rigid, sheet-metal ductwork. The installation of hard ducts is also more difficult because they have to be wrapped in insulation and more transition points have to be made.

Dobrita Oldorp

Teacher

How do you seal a flex duct?

Seal the duct-collar junctions with mastic. Pull insulation wrapped in vapor barrier over the connections and seal seams with both UL-181-approved tape and mastic. Do not compress the insulation.

Neculina Arraitagoien

Reviewer

What is a Class 1 duct?

To be identified as an official Class 1 duct, the duct and connector materials must have a flame spread rating of no greater than 25 with no evidence of continued progressive combustion, and a smoke developed rating of no more than 50.

Vitalina Tannenbaum

Reviewer

How do you install a return air duct?

How to Install a Return Air Duct
  1. Locate where you want your return air ducts.
  2. Go into the basement and locate the holes.
  3. Install the square return air duct perpendicular to the joists.
  4. Cut a hole in the bottom of the return duct next to where the furnace is.
  5. Install your HVAC filter into your filter rack.

Consolacion Pie

Reviewer

How do you measure duct size for a room?

How to Determine Duct Size Per Room
  1. Look at the plate on your furnace to figure out how many BTUs it produces.
  2. Multiply that number by the amount of cubic feet of airflow per minute (CFM) your type of furnace produces.
  3. Divide your number by 10.
  4. Calculate the size of your ducts vents by multiplying the length by the width.

Aylin Rosenau

Reviewer

What is the best material for ductwork?

Galvanized steel and aluminum are the most common materials for sheet metal ducts. Aluminum in particular is relatively light and easy to install. They are also the least likely to harbor dangerous molds or growths because they have non-porous surfaces. Fiberglass lined ducts.

Rangel Ruiz Portal

Supporter

How do I create a ductwork layout?

6 Tips for Good Ductwork Design
  1. Choose the best location for HVAC equipment and ducts.
  2. A detailed load calculation.
  3. Consider your equipment type and supporting systems.
  4. Use the right materials, fittings and supports.
  5. Choose the right duct size and layout.
  6. Ensure proper ductwork sealing.

Gearldine Bakhtchivandzhi

Supporter

How do you size a plenum?

width is 2.5 times the width of the connecting supply trunk or simply the width (W) of the entrance opening. Box plenum width may vary from 2 to 3 times the opening width. The preferred box plenum length is two times the plenum width, but may vary from 1 to 3 times the plenum width.

Waclaw Dalacker

Beginner