85
VCCX2 Controller Technical Guide
VAV Operation with Supply Air
Tempering (VAV Operation with
Outdoor Air Temperature Control)
On a VAV unit that may need daytime heating in order to
maintain the Cooling Supply Air Setpoint, previous controllers
used a Supply Air Tempering sequence with the Supply Air
Temperature Sensor configured as the controlling sensor. The
VCCX2 Controller accomplishes the same result using the Hood-
On Outdoor Air (OA) Setpoints to initiate Cooling and Heating.
To utilize this sequence, the HVAC Mode Enable Source must
be configured as Supply Air Tempering. Then, configure the
Hood On HVAC Setpoints for the OA temperature values that
will enable Cooling and Heating. The Hood On Heating Setpoint
should be set at or above the OA Temperature, that when mixed
with the Return Air (with the Economizer at its minimum
position), will require Heating in order to achieve the Heating
Supply Air Setpoint. The Hood On Cooling Setpoint would be
set above that, which will allow a Vent Mode in between.
Then, configure the Cooling and Heating Supply Air Setpoints.
While not set at the same value, those would normally both be set
at or near 55°F to allow the box heat to keep spaces comfortable.
With this configuration, as the OA Temperature rises above the
Hood On Cooling Setpoint, the unit will be in Cooling Mode,
controlling to the Cooling Supply Air Setpoint. The Economizer
can operate as normal for free cooling to maintain the Cooling
Supply Air Setpoint.
During the Vent Mode, when the OA Temperature is between
the Hood On Cooling and Heating Setpoints, the economizer
can modulate if necessary to maintain the Cooling Supply Air
Setpoint.
Whenever the OA Temperature falls below the Hood On Heating
Setpoint, the unit will be in Heating Mode controlling to the
Heating Supply Air Setpoint. The Economizer can still modulate
during Heating Mode if the Supply Air Temperature is too warm,
and it will control to a setpoint calculated to be 2°F above the
Heating Supply Air Setpoint. In this way, the Economizer will
have time to close to its minimum position before the Supply
Air Temperature falls below the Heating Supply Air Setpoint
and Heating is initiated.
During Morning Warm-Up, heating will be controlled to the
Morning Warm-Up Supply Air Setpoint.
In this operation, if Night Setback operation will be initiated
by a space sensor connected to the VCCX2 Controller, then the
Night Setback Cooling and Heating Offsets will be applied to
the normal Occupied HVAC Mode Enable Setpoints (not the
Hood On Setpoints). During Night Setback operation, Heating
will be controlled to the Morning Warm-Up Supply Air Setpoint.
Finally, configure the VCCX2 for Duct Static Pressure Control
(see the Duct Static Pressure Control section for complete
details).
Air to Air Heat Pump Operation
Cooling Mode will operate in the same manner as described in
the Cooling section.
A reversing valve relay output can be configured to activate with
the first compressor stage in the Heating Mode or the Cooling
Mode of operation.
In the Heating Mode, Compressor Heat, Auxiliary Heat, and
Emergency Heat can be used to achieve the Active Supply Air
Heating Setpoint. Auxiliary Heat can be either a modulating
or staged form of heat, or it can be a modulating form of heat
followed by staged heat.
When Auxiliary Heat comes on in conjunction with a digital
compressor heat stage, the digital compressor will be locked at
100% until the Supply Air Temperature (SAT) rises above the
SAT Heating Setpoint plus the Heat Staging Window. At that
point, the Auxiliary Heat will stage off (after a stage-down delay)
and the digital compressor heat will be allowed to modulate.
When the Outdoor Air Temperature (OAT) is below the Heating
Lockout but above the OAT Compressor Heating Lockout,
Compressor Heat will be used and can be supplemented by
Auxiliary Heat.
When the OAT is below the OAT Compressor Heating Lockout,
Compressor Heat is locked out. Auxiliary Heat will then be
the primary heat and can be supplemented with stage(s) of
Emergency Heat (if available). Emergency heat is only available
when the OAT is below the OAT Compressor Lockout.
Heat Pump Standard Defrost
Operation
If using the VCCX2 Controller with an installed Defrost Coil
Temperature Switch, a Defrost Cycle is available.
If the compressor(s) are operating in the Heating Mode and the
Defrost Coil Temperature Switch closes, the unit will enter the
Defrost Mode, provided the user-adjustable Defrost Interval
Timer has elapsed since the last Defrost Cycle.
SEQUENCE OF OPERATIONS
VAV w/SA Tempering, Air to Air Heat Pump and Standard Defrost
