McQuay MicroTech III RAH Operation And Maintenance Manual page 119

Operator's Guide
Modulating Return\Exhaust Fan Capacity Control
Three different approaches may be used to maintain the building static pressure at acceptable
levels. An analog signal is provided to the VFD to control return or exhaust fan to:
• Maintain the speed of a Return Fan based on Supply Fan speed (Fan tracking)
OR
• Maintain the building static pressure at a desired value
OR
• Maintain a fixed speed based on a signal provided by a Building Automation System via a
network
Any of the three methods may be selected for a Return fan. Either Building Static Pressure or
Speed may be selected for an Exhaust Fan.
When the unit is in an Off or Start Initial state, the variable frequency drive for the Return or
Exhaust is driven to 0% or closed.
After the Return or Exhaust Fan is started, a speed signal of 5% is sent to the variable
frequency drive for the BSPCtrlDelay (Default=30 seconds) and the VFD will increase its
speed to the larger of 5% or it's internally programmed minimum speed. Control reverts to one
of the methods below after the fan has been on for the BSPCtrlDelay. The VFD speed is not
controlled below 5% while the fan is operating.
During non-Off modes when the outdoor air damper is closed, the speed of the return Fan is
set equal to the supply fan speed. Under these conditions, the return fan is set to 100% if the
supply fan is a constant volume fan. To provide a "bumpless transfer" when the outdoor air
damper opens, the return fan will start to control building static pressure from its speed when
the damper was closed. Exhaust fans do not use this feature.
Fan Tracking Control
For units with variable frequency drives, the return fan may be controlled such that the return
fan speed tracks up and down with the supply fan speed. The user defines the position of the
return fan with respect to the supply fan for minimum and maximum positions. When the
supply fan is between the minimum and maximum positions, the fan varies proportionally
between its minimum and maximum values. There are four adjustable parameters to define
this relationship. They are:
Supply Fan (DF) Value Return Fan (RF) Value
Supply Fan Max Return Fan @ Supply Fan Max
Supply Fan Min Return Fan @ Supply Fan Min
To set the four parameters described above, the airflow and return vane positions are adjusted
until Maximum Airflow and proper indoor conditions are met. The Supply Fan is controlled
normally as the VAV boxes are adjusted to obtain maximum airflow. The Return Fan is
adjusted using the Remote RF/EF setting on the keypad. The user then uses the key to set the
Supply Fan Max values and Return Fan @ Supply Fan Max to the observed values. The
process is repeated at minimum airflow to set Supply Fan Min and Return Fan @ Supply Fan
Min values. The user may choose Outdoor Air Minimum reset to function with the building
static control. If the building static continues to drop after the variable frequency drives are at
25%, the Outdoor Air Damper minimum will be reset upwards from the initial value to a
maximum set by the user. When this method is chosen, the user may also choose to have a
Low Limit prevent the minimum from being set too high. This low limit can use the discharge
or the entering fan temperature sensor.
McQuay OM 920-1 119