Desert Aire SA Series, Installation And Operation Manual

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the space. Also, if the air temperature is very low and the heater cycles off, the
supply air temperature may be lower than the space dew point. This would cause
condensation on ducts, diffusers, and surfaces.
Should there be issues with supply temperature variations, modulating heat should
be considered.
Modulating Heating
When the application has a moderate to extreme climate and outdoor air is specified,
it is recommended that auxiliary heating be modulating.
When the controls are set for Modulating heat output, both the binary heat output
contact and the modulating heat output are active. The binary heat output acts as an
enable contact that can activate a gas heater enable signal or start a hot water coil
pump system. The modulating output is intended to vary the gas valve, SCR electric
heater, or hot water coil water flow rate.
The Modulating heating sequence works based on a Zone Reset of the Supply Air
Temperature. When the zone temperature decreases to the Zone Temperature
minus the Heating Deadband, the controller enables heating mode and starts to
increase the calculated Supply Air Temperature Set Point. The heater output will
modulate to maintain the calculated Supply Air Temperature Set Point. The heating
mode is always disabled if the zone exceeds the Zone Temperature Set Point.
Resetting the supply air temperature in heating not only closely maintains the zone
temperatures, but helps ensure the supply air temperature is held to minimum values.
The default values used for the zone PID, supply air PID, and deadband are
appropriate for most applications and heater sizes. Should the commanded Supply
Air Temperature change too quickly, the PID action for the reset can be slowed by
increasing the tuning values. Should the heater not be able to closely maintain the
calculated Supply Air Temperature Set Point, the values for the heater output loop
can be adjusted.
Heater Enabled = Zone = Zone Set point – Heating Deadband
Heater Disabled = Heat Output PID Loop < 5% or Zone = Zone Set Point
Example:
- Zone Set point: 85°F
- Heating Deadband: 2°F
In this example, the binary heater contact will close at 83°F and the
analog output will start to increase. The PID loop will modulate to
maintain the zone at the 83°F temperature. Should the analog signal for
heat decrease to less than 5%, the binary contact will open. In addition,
Desert Aire - SA Manual