Technicians Guide Workbook for Duct Diagnostics and Repair

Technician’s Guide & Workbook for Duct Diagnostics and Repair

Part 6 Section 2. 3: Special Repair Tips

Wall Cavities Used As Ducting Simply stated try to replace them

Sheet Metal Ducts When sealing sheet metal ductwork, the places to emphasize are: • Joints between trunks and run-outs. These are usually fairly easy to get at because insulation, if present, usually has to be joined here also. • Leaks around register boots, both at the interface between the boot and the floor or wall and at the joint between the boot and the run-out. These leakage zones are best reached by removing the register grille and working from the inside.

Tips By Duct Type Sheet Metal Ducts: Sheet metal ducts should be sealed with an approved mastic along the seams and where connecting fittings meet.

Sheet Metal ducts • Transverse seams in trunks and run-outs. These may be more difficult to reach if covered by insulation. Do the best you can here and hope that in this system transverse seam leaks are minor. Fortunately this is often the case.

Sheet Metal ducts When sealing or making repairs, using the right sealing materials correctly is important: • In order to fill the small space between overlapping pieces of metal, liquid sealants are recommended for slip-type joints. • For openings greater than 1/16 of an inch, several applications may be needed with the manufacturer’s recommended drying time being allowed in between applications.

Sheet Metal ducts • Mastics are generally used for grooves or between flanges. • Gaskets may also be used for flange joints. They generally have an adhesive backing and are made of a soft elastomer Butyl or Extruded form. • All sealants have a shelf life and the technician should check the label to make sure the product is still usable.

Installing New Sheet Metal ducts • Follow SMACNA HVAC Duct Construction Standards-Metal Flexible requirements; • Fabricate the duct sections carefully, so that they will fit together tightly; • Make sure slips fit properly; • Make sure mechanical fasteners are properly installed; • Make surfaces are clean before applying closures;

Installing New Sheet Metal ducts • Make sure seams and joints are properly sealed; • Support the duct sections properly. • Insulate the duct internally or externally, and make sure the insulation is properly sealed.

Flexible Ducts Repair of flexible ducts generally involves the following activities: • Removing excessive lengths of duct; • Correcting drooping sections; • Improper support systems;

Flexible Ducts • Replacing damaged/torn lengths of flex duct; and • Securing and sealing end clamps. • Mastics and tapes used must be approved for the usage.

Flexible Ducts

Installing New Flexible Ducts The greatest advantage of flexible ducting (flexduct), as far as efficiency is concerned, is that they don’t leak between ends unless torn. Suggestions for working with flexible ducts would include: • Follow SMACNA HVAC Duct Construction Standards-Metal Flexible requirements or Air Diffusion Council, and manufacturer directions. • Support the sections properly. Leaving too much space between supports will result in excess sag, which increases pressure drops and cuts air flow by increasing the total effective/equivalent length of the system.

Installing New Flexible Ducts • Cut the duct sections to the proper length before installing. Serpentine bends in excess length has the same effect in the horizontal dimension that poor supporting practice does in the vertical. • Make sure all end connections are tight and sealed with mastic, to ensure long-lived, leakfree joints. • The use of metal Ts, and elbows can reduce the friction loss where bends and takeoffs are required.

Duct Board (Fibrous Glass Ducts) Repair of fibrous glass duct-board systems should generally follow the same procedures as installation of a new system. For existing system repairs, the system design is already laid out. Thus, all that needs to be done is to reattach/reseal the seams and joints. For any sections where the duct-board itself is severely damaged, a new section should be fabricated to replace it.

Duct Board • • Use proper sized duct cutting tool Staple flaps Use UL 181 tapes Use mastic

Installing New Duct Board Fibrous glass duct-board has the advantage of always being insulated. Some suggestions for working with this material are: • Follow SMACNA Fibrous Glass Duct Construction Standards or NAIMA, and manufacturer directions; • Fabricate the duct sections carefully, use a hand tool specifically designed for use with duct board so that they will fit together tightly; • Make sure the duct board tool is sized for the duct-board’s thickness;

Installing New Duct Board • Make surfaces are clean before applying closures. Only use UL 181 rated tape that is specified for use with duct-board. When pressure-sensitive tape is used rub firmly with a plastic squeegee. Heat-activated tape must be stored at 70 OF before usage and the seam should be sealed with a heating iron set at 400 OF +25 OF. • Support the duct sections properly. Fibrous glass ductboard is not a structural or load-bearing material!

PVC Duct Repair of PLASTIC/PVC/CPVC ducts generally involves the following activities: • Removing excessive lengths of duct; • Replacing cracked lengths or fittings; and • Securing and sealing joints. • Glue and mastic used must be approved for the usage. PVC

Installing New PVC Duct Plastic/PVC/CPVC duct has the advantage of durability and will not rust in harsh environments. Thus, it is often used for direct ground contact, and slab applications. Suggestions for working with Plastic/PVC/CPVC ducting would include: • Follow SMACNA and/or OEM installation directions. • Support the sections properly. • Follow OEM directions for gluing, welding, and fastening/connecting sections. • Insulate all duct not in conditioned space. PVC

Lessons Learned You should now be able to explain what types of tape and mastic is required. You should now be able to explain basic steps for repairing different duct types. You should now be able to identify different types of duct materials. You should be able to explain why replacing and sealing insulation is important.