3.1602.8 Supply Plenum (Furnace to Trunk Duct Connection) in Both Upflow and Downflow Air Handler Configurations

Duct Sealing
Desired Outcome: 
Deliver all air from air handler to the trunk duct without leakage or restriction
Work assessment

Installer prework assessment will be conducted to determine:

  • Size of plenum
  • Alignment
  • Connection method
  • Existing sealing

Ensure an efficient and effective way to accomplish work

Verify scope of work


Debris will be removed

Surface will be prepared for work (e.g., remove tape, oil)

Floor will be prepared to receive the appropriately sized plenum

Provide unobstructed path for work access and air flow

Ensure adhesion of materials to be installed

Provide a properly sized plenum to maximize distribution of air flow (equal to the furnace discharge)

Plenum rebuild or repair

Plenum will be rebuilt or repaired using compatible materials and will be:

  • Mechanically fastened
  • Sealed
  • Durable
  • Structurally sound
  • Insulated
  • Equipped with a vapor retarder Vapor retarder A material that slows the passage of water vapor and contains a perm rating above 1 where climate appropriate

If possible, flow diverter or turning vanes will be installed for air flow and/or balancing (e.g., bullhead Ts, offset air handler)

Minimize restrictions

Maximize air flow and air distribution

Minimize moisture issues

Prevent condensation on plenum

Repair work access

Point of access options include:

Option 1: Through the trunk duct

  • Repair and seal access hole in the trunk duct
  • Install insulation
  • Repair belly/bottom liner

Option 2: Remove crossover duct

  • Reattach crossover duct
  • Seal and insulate crossover duct
  • Repair belly/bottom liner

Option 3: Remove air handler

  • Install new gasket, if necessary
  • Mechanically attach furnace to the structure
  • Reconnect utilities
  • Replace and seal panels

Option 4: Through the furnace panel

  • Replace and seal panels

Repair work access

Prevent condensation

Minimize heat loss and heat gain from plenum

Safety testing

Equipment will be cycled

Combustion Appliance Zone ( CAZ CAZ Combustion appliance zone ) test will be performed where combustion appliances are utilized

Verify operation

Identify unsafe equipment operating conditions

Performance testing

Pre- and post-retrofit duct leakage will be performance tested using a duct blaster or pressure pan, and results will be documented and reported to the homeowner and/or program

Document post-retrofit duct leakage test has been performed