How to Install AC Ductwork

Perform the load calculation. This consists of determining the heat gain and loss for each room in the house, and can be done with an online load calculation calculator (see Resources section). Next, use this to measure and plan the size and location of each duct using the tape measure. For further design and sizing specifications, see the ACCA Manual J, ACCA Manual D and ACCA Manual S.

Plan where to place various parts of the ductwork system. This consists of finding the most appropriate location for the fan coil unit, followed by the return air duct and air filter. The fan coil unit may be placed in the attic, garage or crawl space, as well as a closet, utility room or other enclosed space. The return air duct should not be located in the dining room, living room or kitchen.

Put on the goggles, gloves, helmet and respirator.

Cut holes for the return air duct opening. Pass the fan coil through this opening.

Build a structure to secure the fan coil unit. This can be done by constructing a simple wood platform to attach the fan coil unit, or by building a suspension platform using ¼ inch threaded rods. See the instruction manual which comes with the fan coil unit for specific instructions for this step.

Attach the plenum ring gasket to the fan coil cabinet. Next, connect the supply plenum ring to the platform using four screws screwed onto the square opening in the platform; this should be at least 4 inches lower than the fan coil unit.

Attach the plenum duct to the air conditioning unit. Cut the male end off of the first section, and insert the plenum duct section into the receiving collar onto the fan coil to form a snug joint; next, flatten the sheet metal tabs against the plenum duct unit and insert the flat head pins. Assemble the rest of the "shiplap" plenum duct joints using the above steps. Place the plenum end cap over the plenum run above; this is done by pinning the sheet metal tabs at the end of the cap with flat head pins. Tape this end cap with a cross-tape section.

Cut the supply ducts according to the size determined in the layout plan above; next, pull back the jacket and insulation from core. Slide at least 1 inch of core over the collar; tape core with at least two layers of duct tape. Secure with clamp. Install a connector into the remaining open end of the supply ducts by threading it into the end of the duct following the previous steps above. Connect the ducts to the connector using a twisting action; next, connect and join the ducts themselves following the above planned layout using the metal tees and collars; attach the ducts to the joists/ceiling with the metal tees (if attaching to joists) or saddles and/or hangers (if attaching to ceiling) at intervals no greater than 5 feet.

Cut a 2-inch hole in the plenum at a 20-degree angle facing downward. Attach plenum takeoff gaskets around this hole; place the plenum takeoff connector in this hole. Insert the plenum fasteners with the pliers.

Attach the return air box into the return air opening cut above. Using the four screws, attach the frame on the long side of the return air box. Next, insert the return air grille in the box with the four screws listed for this purpose. Insert the filter into the grille.

Cut the return air ducts following the instructions in Step 7. Connect one end of the return ducts to the elliptical end of the fan coil and the other end to the return air box connectors with the clamp bands.

Test that the duct installation is correct by running the air conditioning system and checking for duct leakage, desired temperature levels reached, and appropriate air pressure levels. Check with your state's government website further for required testing procedures, and ensure that your system passes these satisfactorily.