HVAC For Beginners

Flexible Duct Installation

A Step By Step Guide

With a little help you can do your own flexible duct installation. The typical rate for an hvac dealer to do the work would be about $50/hour. By providing the labor, you can save around $1000 on the typical duct system.

**NOTE** Only you can assess your ability to perform this task. This is a guide and cannot provide all of the details for every situation.

You will need some basic tools to perform the job but they are not very expensive. We will look at installing an extended plenum system in an attic. After you understand the process, the methods can be used for other applications.

The Plan

The flexible duct installation process begins with a load calculation. This is a computerized process of determining the heat gain/loss for each room in the home. This information is then used to design a properly balanced duct system. The result is a duct layout plan which shows the size and location of the air ducts and registers in the home.

Cutting holes for the registers

You can begin the installation by cutting the holes in the ceiling for the registers. (We recommend using MV4 registers. They are a plastic register with built in collar.) They come with a template that is traced onto the ceiling. Then the drywall is cut out. The registers should be centered in the rafter space and kept approximately two feet from the wall. For the most common type of system, there is a common return grille that is usually installed in the ceiling of a central hallway in the home. This is called a return air filter grille because it also houses the air filter for the system. This is recommended to avoid going into the attic to replace the filter. Locate the register between the rafters in the hall and cut the drywall out for the proper size filter grille. Then, 2x4 blocks should be nailed between the rafters to make a box and a piece of plywood can be nailed over the top to fully enclose it. Wood is recommended to avoid condensation.

Installing the plenums

After the holes are cut, the furnace/air handler and evaporator coil should be set in place. You can use our furnace installation guide to show you how. Your flexible duct installation continues by attaching plenums to the unit with sheet metal screws. The plenum is a metal box with one end open and one end closed. They are usually shipped in pieces and require assembly. One plenum is attached to the outlet of the evaporator coil or the furnace if you do not have a central ac system. This is called the supply plenum. The other plenum is attached to the bottom of the unit and is called a return air plenum.

Flexible Duct Installation, Installing the supply ducts

Once the plenums are installed, round metal start collars are installed in the plenums. These collars allow the flexible duct to be attached to the plenums. Holes are cut in the plenum and the collars are inserted. The metal tabs are bent over using your fingers. The supply duct starts at the plenum and metal tees are installed with reducers as necessary to allow the branch ducts to be attached. The supply duct will get smaller as more branch ducts are attached. The final piece of supply duct will be used as a branch duct to supply the register that is furthest from the unit. The flex ducts are attached by pushing them onto the collar or tee and wrapping the end with a foil tape. Then, a plastic duct strap is placed around the duct and tightened by pulling on it with pliers. The excess strap is cut off and the insulation is pulled over the connection and secured with another strap. The insulation should cover all metal to prevent sweating and condensation. The duct is cut as required using scissors to cut the plastic and insulation. Then, wire cutters are used to cut the wire reinforcement on the duct. The next step in flexible duct installation is the branch ducts. The flexible ducts are attached to the tees and stretched to the holes that were cut into the ceiling. About two feet extra is allowed then the duct is cut to size. The end of the duct should then be fed through the hole in the ceiling and stick out in the room below. The flexible ducts should lay on the rafters for proper support. In any case, they should be supported at least every five feet. All metal such as the tees and plenums should be insulated.

Installing return air ducts

The flexible duct installation continues with the return air duct. The duct will be attached to the return air plenum using a collar. Then stretch the duct to the return air box that was previously built. Cut a hole in the top of the box and insert the collar and attach the duct.

Installing the registers and grilles

The flexible duct installation continues by installing the registers in the holes that were cut in the ceiling. The ducts are attached to the registers with tape and straps. Then, the registers are shoved up into the hole. The registers are attached by tightening the screws per the instructions. The return air filter grille is installed with screws or nails and the filter is placed in the unit.