Attic Air Conditioning Installation

Bolt or screw a platform to the attic joists to minimize air handler unit vibration. Use plywood or other platform materials recommended by the manufacturer.

Lay the large secondary drain pan on the platform. Position it under the openings on the low end of the air handler. Pop out the opening on the side and attach a 1/2-inch drain hose to form a drain from the attic to an obvious location near the compressor unit. Water running from this back-up drain alerts you when that the primary drain needs cleaning or repair.

Assemble the air handler -- perhaps labeled as the “plenum” on a complete furnace unit -- according to unit instructions. On an attic unit, the air handler will include an evaporator coil, either A-shaped or flat, a blower on the “negative,” or incoming, side of the coil and a condensate pump or drain connection.

Connect the condensate pump drain. This primary drain carries water that condenses on the evaporator coils out of the attic -- typically through a PVC sleeve. Use the same 1/2-inch tubing as on the secondary drain pan in step 2.

Attach the starter, also called the plenum extension, to the end of the air handler; insert a takeoff, or start collar, into the opening of the air handler and fold its tabs back against the interior of the opening. Drill three or four pilot holes through the air handler and takeoff tabs and secure them with sheet metal screws. Screws in kits may have hex-or slot-heads.

Lay a bead of mastic around the joint formed by the air handler and takeoff, covering the screw heads. Cut halfway through a strip of fiberglass tape at 1-inch intervals along its length and wrap it around the takeoff, easing the tabs against the air handler end.

Add ducting to the takeoff, one section at a time, working around the perimeter of the attic. Secure sections with screws and mastic. Install ducts between floor joists and under truss bases.

Locate ceiling vent outlets between or alongside joists, marking them by drilling guide holes at each corner from above and sawing out openings from below.

Cut holes in duct sections when you get to each vent. Snip tabs into the ends of the register boots, fold them over and attach them to the ducts with screws, mastic and tape.

Screw the flaps of the register boots into the ceiling and screw the vent, or diffuser, into the ceiling.

Cap the end of the supply duct or add a turn and drop it down to the first floor between studs using a “wall stack” of attached sections.

Insulate around and over the duct work. Building codes often recommend up to R-30 blown or blanket insulation.