Sequence of operation
CNT-SVX04A-EN
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Heating and cooling operations in the unit are determined by the space
temperature error, with the exception of the low discharge air temperature
limit. While operating to cool the zone, available heating capacity may be used
to maintain discharge air temperature, if the discharge air temperature falls
below the minimum discharge setpoint.
Heating or cooling control mode operation
There are two ways to determine the heating or cooling control mode of the
Tracer ZN.520 zone controller:
■
Communicated request (via nviHeatCool or nviApplicMode)
■
Automatically by the controller
Determining mode by communicated request
A building automation system or peer controller may communicate the heating
or cooling mode to the controller via network variables nviHeatCool and/or
nviApplicMode. Heating mode commands the controller to heat only. Cooling
mode commands the controller to cool only. The auto mode allows the
controller to automatically change from heating to cooling or cooling to
heating. See Table 70 on page 103.
Determining mode automatically by the controller
A communicated request of Auto or the controller's default operation (auto) can
place the unit into heating or cooling mode. When the controller automatically
determines the heating or cooling mode using auto mode, the unit switches to
the desired mode based on the control algorithm. When the controller switches
to the desired mode, it ramps up the heating and cooling capacity after first
determining if heating or cooling is possible.
Example:
In heating mode, if the unit is capable of heating, the controller
ramps up the heat capacity.
The 2- and 4-pipe hydronic changeover units compare the entering water
temperature to the space temperature to determine whether the unit will
provide hydronic heating or cooling. To deliver heat, the entering water
temperature must be at least five degrees above the space temperature of the
unit. To provide cooling, the entering water temperature must be at least five
degrees less than the space temperature of the unit.
If the unit has electric heat and electric heat is enabled, the controller is capable
of heating even if the entering water temperature is too cool to heat by
hydronic means.
