Desert Aire SelectAire Plus SP Series, Installation And Operation Manual, Page: 90 / 110

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This will energize the Circuit A compressor(s) and open the cool exhaust damper. This energy
recovery can be considered the first stage of heating and will occur before the auxiliary
heating is activated.
During the daytime the unit may occasionally be in dehumidification mode. Energy recovery
will index on as required and auxiliary heating will become active as required to maintain
space conditions.
Note that it is not uncommon for the unit to occasionally shift into cooling mode in the
wintertime when the solar load is high and the envelope contains a high number of south
facing windows. This can occur down to 30°F or even lower in some areas.
Note that in extreme cool and dry conditions when in occupied mode the outdoor air may dry
the space significantly. When this is the case, the exhaust air may no longer have the energy
to recover. The unit controls will disable the energy recovery mode when this occurs if the
unit is equipped with this option.
5.5.8 Heating Operation
Auxiliary Heating
The SP offers three types of auxiliary heaters: Gas, Electric and Hot Water Heaters.
Gas Heaters use the combustion of natural gas or liquid petroleum (lp) gas to create a heat
source over tubes. Air passes over these tubes and the warmth from the tubes is exchanged
into the air. Electric heaters use the same method but instead of using the combustion
process electrically heated elements make contact with the air to warm it. Lastly, hot water
coils use a tube and fin arrangement. Supplied hot water is pumped through the tubes. The
tubes transfer heat to the perpendicular fins and the air passing through absorbs the heat
from both the tubes and fins.
Ensure that the supply air temperature sensor is downstream of any fluid 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 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.
Desert Aire - SP Manual