Instruction manual

McQuay OM 920 87
Operator’s Guide
Slave Control
Slave control can not be selected unless a water economizer is installed. When the unit is
configured for slave control, the Bypass Valve position is set to 100% minus the Economizer
Valve Position except in the Manual Mode. This provides nearly constant flow of water
through the unit regardless of the requirements of the economizer. When the unit is in Manual
Control mode, the Bypass Valve and Economizer Valve are controlled independently.
Bypass Control
When the unit is configured for Bypass control, the Bypass Valve operates independently of
the waterside Economizer Valve. When it is closed, no water flows through the unit if there is
no economizer or if an airside economizer is used. When it is closed in Waterside Economizer
applications, all water flowing through the unit flows through the economizer before flowing
through the condenser.
The Bypass Valve is open for units with compressors whenever the unit is in dehumidification.
In normal operation of units with compressors, the Bypass Valve may be open or closed as
described below in the Fan Only, Mechanical Cooling, and Economizer states. It is closed in
all other states.
In the Fan Only state, the Bypass Valve is opened when cooling is required so that the
controller can determine an accurate entering water temperature and whether water flow is
established. The Bypass Valve is opened in the Fan Only state whenever:
• Waterside economizer is installed AND
• EconStatus=Enabled or Off Ambient AND
• Control Temperature is greater than the Zone Cooling Setpoint by more than ½ the Zone
Cooling deadband
OR
• Waterside economizer is installed AND
• EconStatus = Enabled or Off Ambient AND
• ClgStatus= Enabled or Off Ambient
• Control Temperature is greater than the Zone Cooling Setpoint by more than ½ the Zone
Cooling deadband
OR
• Waterside economizer is NOT installed AND
• EconStatus = Enabled AND
• ClgStatus= Enabled or Off Ambient
• Control Temperature is greater than the Control Temperature is greater than the Zone
Cooling Setpoint by more than ½ the Zone Cooling deadband
When the Bypass Valve is opened in the Fan Only state, a Bypass Valve Timer is set to an
adjustable value (Default of Five Minutes). The parameter is adjustable via the following path:
Extended Menus/Timer Setting/Bypass Valve. The unit remains in the Fan Only state at least
until the Bypass Valve Timer expires so that Water Flow and Entering Water Temperature can
be checked before the unit enters a cooling state. The Waterside Economizer and Mechanical
Cooling operation are disabled until the Bypass Valve Timer expires.
In the Fan Only state, the Bypass Valve is closed if:
• All Cooling is disabled (other than due to Off-Ambient or Off No Flow)
OR
• Control Temperature less than or equal to the Zone Cooling Setpoint by more than ½ the
Zone Cooling deadband
In Mechanical Cooling states, the Bypass Valve is closed when: