Desert Aire Aura QS, Installation And Operation Manual

Page: 75 / 94

75
Should auxiliary heating be installed, a modulating output will activate when the capacity of
the heat pump is insufficient to heat the air to the desired leaving air temperature. Use of
the modulating output is determined when the reheat valves reach 100% for a set period of
time, capability is available should there be higher airflow than anticipated.
5.4.5.2 Auxiliary Heat
In non-heat pump units, an auxiliary heating contact is energized whenever the IA is less than
68°F and a modulating output is controlled based on the supply air temperature set point.
5.4.6 SAT Control Options
Three different options exist for control of the supply air provided to the space. The most basic is the
Supply Air Temperature (SAT) control and requires a supply air temperature sensor in the supply
ductwork. SAT control varies the reheat valve position based on the SAT set point and the SAT
sensor with a PID (proportional, integral, derivative) control loop. PID control loops correct the error
between a measured process variable and the desired set point by calculating and then adjusting the
process output accordingly. The Zone Reset and the Outside Air Reset options still use this SAT PID
control loop and still require the SAT sensor. These options vary the SAT set point that is used in the
PID control loop to more closely match the requirements of the space.
The Zone Reset of the SAT uses a zone sensor to vary the SAT set point. If the zone temperature
rises, the SAT set point is lowered to provide cooler air to the space and if the zone temperature
falls, the SAT set point is raised to provide warmer air to the space. The Zone Reset is accomplished
by a second PID control loop, the output of which is the SAT set point. This control loop also allows
for setting the maximum and minimum SAT set point limits. Up to four of these sensors can be used,
providing multi-zone control. Multiple sensors may be set to either average the zone readings, or set
for High / Low zone control. Averaging would be used in large or shared spaces where the conditions
are similar from zone to zone. High / Low would be applicable to areas where there are different
conditions in the group. This would include a south facing windowed wall in the group, where the
zone may be quite a bit warmer in the summer.
The Outside Air Reset allows for four different SAT set points to be used based upon the outside air
temperature. For instance, if the outside air temperature is below 60°F, then the SAT set point will be
80°F. If the outside air temperature is between 70°F and 80°F, then the SAT set point will be 75°F.
And if the outside air temperature is above 80°F, then the SAT set point will be 65°F. All of these
set points are adjustable. Note that this option should be used with care if an energy recovery device
is in the unit, as the IA sensor is not sensing outside air.
Note: The IAQ standard controller outputs a 2 – 12 Pulse Width Modulating signal (PWM).
Field-purchased heaters must be compatible with the signal provided OR field-supplied
alternative controls must be used.
5.4.7 Compressor Rotation
For any unit with more than one compressor, the compressors rotate based upon the last
compressor to start, not the run hours. For units with 4 compressors, the refrigeration circuits
Desert Aire - QS/QV Manual