All heating, ventilation and air conditioning (HVAC) duct systems have basically the same characteristics; a plenum connects the air conditioning unit to a trunk line and branch lines connect the trunk line to the diffusers in each room. On simple duct systems, the branch lines connect directly to the plenum. The branch lines, often made from flex duct, use a metal collar to connect to the trunk line. The collars make a solid airtight seal. Dampers installed in collars can balance the air flow through the duct system.
- Skill level:
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Things you need
- Duct knife
- Duct sealing mastic
- Flat screwdriver
- Tin snips
Place the collar against the trunk line or plenum. Use a marker to trace the outside of the collar.
Use a duct knife to cut the duct board along the circle. Remove the cutout circle.
Make a 1-inch slice into the exterior foil on the duct board.
Place the collar over the hole. Line up the spin-in blade with the slice made in the foil. The spin-on blade acts as the collar's locking device and looks like a flexible strip of metal.
Twist the collar clockwise until the spin-on blade has completely penetrated the interior of the duct board.
Seal the collar with duct sealing mastic. Use a brush to apply a coat completely around the outside of the connection.
Place the collar against the duct. Use a marker to trace the outside of the collar.
Place the sharp edge of a flat screwdriver against the duct 1-inch inside of the circle edge. Strike the screwdriver with a hammer until the sheet metal has a 1-inch slice. This slice must go towards the centre to avoid damaging the metal under the circle.
Place the blade of a pair of tin snips in the slice. Cut out the circle.
Make a 1/4-inch notch in the duct with tin snips.
Place the spin-on collar lip against the notch in the duct. Spin the collar clockwise one full turn.
Seal the collar to the duct with duct sealing mastic. Use a brush to apply the mastic.
Tips and warnings
- To avoid sheet metal burrs, do not completely close the tin snips when cutting.
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