48
Auxiliary
Heating
Ensure that the supply air temperature sensor is downstream of any field installed
auxiliary heaters.
The pre-programmed values for the deadbands, differentials, and PID values are
expected to create a good balance between temperature control and stability in the
typical applications. Occasionally the heater may have larger or smaller capacity
relative to the space or sensor placement may be in an atypical location. If space
conditions vary greatly or frequent cycling between modes are apparent, the values
may need to be changed. Modification to the parameters and tuning should be done
with care. If it is determined that a specific application would benefit from
modification, time and patience will be required to ensure that the desired effect is
achieved.
Staged
Heating
When installed in a relatively mild climate with minimal outdoor air requirements,
there is a relatively small amount of heating capacity. In these cases, staged heating
is a cost effective and simple method.
The auxiliary heating outputs come set from the factory for control of staged control
or modulating control as specified at the time of order. When set for Staged, the dry
contact associated with the heating output will close on a drop in temperature of the
zone set point minus the deadband minus the differential. The contact will remain
closed until the temperature reaches the zone set point minus the differential.
Heater On = Zone Set point – Heating Deadband – Heating Differential
Heater Off = Zone Set point – Heating Deadband
Example:
- Zone Set point: 85°F
- Heating Deadband: 2°F
- Heating Differential: 1°F
In this example, the heater output will close at 82°F and will open at
83°F. Due to some system capacitance and sensor hysteresis, the
actual zone condition may overshoot these values slightly, but should not
overshoot these values to cause the unit to shift into a cooling mode.
Should the heater be very oversized and the overshoot significant, it may
be necessary to increase the deadband to prevent cycling between modes.
It should be noted that the nature of the staged heating will create a significant
variation in the supply air temperature as the heater is turned on and off. Heaters are
sized for extreme conditions (typically the 99th percentile heating capacity). At many
times of the year the heater is essentially oversized. Depending on the location of
the supply ducts and diffuser configurations, the variations may create drafts within
Desert Aire - SA Manual
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