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There are two options for the return fan assembly:
• Return fan discharge pressure
• Supply fan airflow tracking
A return fan assembly is always controlled by a VFD.
Both return fan assembly sequences have an exhaust
air damper installed. The exhaust damper is controlled
by the building pressure as explained above in Exhaust
System on page 60.
Return fan setpoints can be found in the Views > De-
tails > Return Fan > Setup menu.
Return Fan Discharge Pressure
With discharge fan pressure tracking, the return fan
speed is controlled to maintain the return fan pressure
setpoint. This is an adjustable value.
• As the supply fan speed increases or the return/
exhaust dampers open, the return fan discharge
pressure drops to increase the return fan speed.
• As the supply fan speed decreases or the return/
exhaust dampers close, the return fan discharge
pressure rises to decrease the return fan speed.
The return fan pressure setpoint is reset by the outside
air damper position and the supply fan speed.
Supply Fan Airflow Tracking
When the rooftop unit is ordered with the optional sup-
ply and return fan AMSs, the return fan can be con-
trolled by this sequence.
• With supply fan airflow tracking, the unit controller
regulates the speed of the return fan so the return
airflow matches the supply airflow minus the re-
turn airflow differential
• Both AMSs perform a fan differential pressure
auto-zero every 10 hours of operating time
Energy Recovery Wheel
The rooftop unit can have an optional factory installed en-
ergy recovery wheel (ERW). The ERW uses the exhaust
air to pre-heat or pre-cool the outside air brought into the
unit. This increases the overall efficiency of the unit.
The ERW can operate in both cooling and heating modes.
• When the economizer is active in cooling mode,
the ERW is disabled and the bypass damper is
fully opened
• The ERW is enabled when the difference between
the return air and the outside air is greater than
the energy recovery setpoint.
YORK
Start-Up & Operation Guide | 5587001-YSG-A-520
The energy recovery setpoint is the effective DAT heat-
ing setpoint
The unit controller uses enthalpy values to compare
the return air and the outside air as long as both %RH
values are valid. When the %RH values are not valid,
the unit controller uses dry bulb temperatures for the
comparison.
When the ERW is stopped for more than 1 hour, it is
energized for 1 minute every hour for cleaning and
blocking protection.
There are two types of ERWs:
• Single speed energy recovery wheel
• Variable speed energy recovery wheel
Single Speed Energy Recovery Wheel
The single speed ERW has an ON/OFF ERW and an
OPEN/CLOSED bypass damper.
When enabled, the ERW runs at full speed and the
ERW bypass damper is fully closed.
When the exhaust air temperature drops below the
energy recovery low limit setpoint, the bypass damper
fully opens to bypass the ERW.
Variable Speed Energy Recovery Wheel
When enabled, the ERW runs at full speed and the
ERW bypass damper is fully closed.
When the exhaust air temperature drops below the en-
ergy recovery low limit setpoint, the ERW speed de-
creases to maintain a minimum ERW temperature to
prevent freezing.
When the minimum speed is not able to maintain the
temperature to prevent freezing, the bypass damper is
fully opened to bypass the ERW.
Ultraviolet Lights
The rooftop unit has the option for factory installed ultra-
violet (UV) lights, which help to eliminate the growth of
mold and mildew in the evaporator section of the unit.
The UV lights are located in the evaporator section of the
unit. There is a safety switch attached to the door that
removes power to the UV lights when the door opens.
The UV lights are controlled by their own, separate
controller instead of by the unit controller, though the
unit controller tracks unit runtime and issues an alarm
when the lights should be replaced (once a year).
UV lights are energized any time the unit power is ON.
61
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