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Convert Constant Volume Air Handler to VAV

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  rev. 2008-11-25        

Existing Conditions

A Constant Volume, Terminal Reheat (CVTR) system supplies conditioned air to the various zones in the building.  Each zone has a hydronic reheat coil located in the branch duct.

This type of system was popular in the '60's and '70's when the emphasis was entirely on installation cost and occupancy comfort.

Retrofit Conditions

We recommend retrofitting the CVTR system to a Variable Air Volume (VAV) system.  The retrofit would include the following:

Installation of VSD's and inverter duty motors on the supply and return fans.
Removal of reheat coils on most interior zones.
Installation of VAV box retrofit kits onto the existing ductwork


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







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