Indoor Air Fan - On/Off Control - McQuay MicroTech III RAH Operation And Maintenance Manual

Operator's Guide

Indoor Air Fan - On/Off Control

A Supply fan is provided on every unit. That may be the only fan, but either a Return Fan or
an Exhaust Fan can be provided also. The start/stop signal and the speed signal for fans that
are controlled by variable frequency drives are provided via an internal ModBus network.
Constant volume Supply and Return fans are started and stopped through a digital output.
Exhaust Fans are always controlled by variable frequency drives.
Supply Fan
The Supply Fan is turned on when the unit enters the Recirculation state. The Supply Fan is
turned off when the unit transitions to the Off state, but it stays on for a OffHtClDelayTime
(Default- 120 seconds) if the unit is turned off while DX cooling or Staged heating is active.
The OffHtClDelayTime function is overridden when and Emergency Off or Duct High Limit
fault is active.
Return Fan
A Return fan driven by a variable frequency drive is started four seconds after the Supply Fan
is started to reduce the amp draw peak on startup. A constant volume Return Fan is turned on
through the same output as the Supply Fan. An external Fan Delay Relay is used to provide a
delay between startups if required.
Supply Fan Capacity Control (VAV)
The speed of a modulating Supply Fan is controlled by an analog signal provided to the VFD.
Supply Fan Capacity Control for a modulating fan is controlled to either:
• Maintain the duct static pressure at a desired value
OR
• Maintain a fixed speed based on a signal provided via a network.
The choice of control method, SF Cap Ctrl, may be set to Duct Press or Speed via the keypad.
After the Supply Fan is started, a speed signal of 25% is sent to the variable frequency drive
for the DSPCtrlDelay (Default=30 seconds) and the VFD will increase its speed to the larger
of 25% or it's internally programmed minimum speed. Control reverts to either duct pressure
or speed after the fan has been on for the duration of the DSPCtrlDelay time. The VFD speed
is not controlled below the minimum SAF speed setting (default 25%) while the fan is
operating.
Duct Static Pressure Control
The supply air fan speed is controlled by a VFD. The control parameter for the fan speed is the
duct static pressure setpoint. If the duct static pressure is below the duct static pressure
setpoint by more than ½ the deadband, the fan speed will increase. Likewise if the duct static
pressure is above the duct static pressure setpoint by more than ½ the deadband the fan speed
will decrease. Example - if the duct static pressure setpoint is 1.2" and the deadband is 0.1",
the duct static pressure must reach 1.14 before the fan will increase in speed. The Duct Static
Pressure setpoint may be set through the keypad or via a network signal. The active setpoint is
changed whenever either of these values changes so it equals whichever value was changed
most recently.
Speed Control
When speed control is selected, the fan operates at the larger of its minimum speed or a value
provided via a connected network or the keypad/display. This value reverts to zero when the
unit is off and remains at that value after startup until a non zero value is written via the
network or the keypad/display.
McQuay OM 920-1 117