Air Handling > Convert Dual Duct Air Handler to VAV

Existing Conditions

Assuming some form of efficient perimeter heating exists and can be extended to areas formerly heated by only the dual duct air handler. A requirement to install reheat coils for perimeter heat would undermine the viability of this opportunity.

Dual duct air handler xxx operates by delivering heated air and cooled air through parallel distribution ducts. Ceiling-mounted constant-volume boxes provide a controlled mixing of air from the hot and cold ducts in the proportions required to satisfy zone thermostats. These air handlers are a high velocity, high pressure design, and therefore require a lot of fan power to provide for the variation in air flow in each duct system. For example, when the primary demand is for cooling, the system must be able to deliver the necessary ventilation volume through the cold deck only.

Retrofit Conditions

Conversion to a cooling-only variable air volume system will allow the air handlers to make use of all installed ductwork all of the time. By effectively doubling the available ducting under high load, fan electrical requirements would be drastically reduced. The zone thermostats would control temperature by throttling air flow in each zone to modulate cooling capacity. Energy savings would come from reducing air velocity and pressure, resulting in lower fan power requirements. In addition, simultaneous heating and cooling at the air handler would be eliminated

Further Benefits

Application Details

The installation involves:

•	Removal of the existing heating and cooling coils.

•	A new, larger cooling coil installed in the plenum prior to the separation of the two discharge ducts.

•	A variable frequency motor drive, with proper electrical protection, for the supply and return fans.

•	Replacing all dual duct mixing boxes with variable air volume boxes.

•	Digital air handler controls.

•	Confirmation of the adequacy of perimeter heating in each zone, and modification of heating distribution as needed.

Convert the system to variable-air-volume (VAV) operation.

This is the most efficient configuration for dual-duct systems. Achieving both comfort and high efficiency is challenging.

Issues and Concerns

References

Analysis

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