Instruction manual

McQuay OM 920 77
Operator’s Guide
OA/EWT Lockout
Heating is disabled whenever the outdoor air temperature is greater than the Outdoor Air
Ambient Heating Lockout Set Point. When the outdoor air temperature drops below the
Outdoor Air Ambient Heating Lockout Set Point by more than the Heating Lockout
Differential, heating operation is re-enabled. Cooling is disabled if outdoor air temperature or
entering water temperature is too low for operation. The outdoor air temperature becomes too
low for operation when it drops below the Outdoor Air Ambient Cooling Lockout Set Point.
Outdoor air temperature becomes high enough for operation when it rises above the Outdoor
Air Ambient Cooling Lockout Set Point by more than the Cooling Lockout Differential. The
entering water temperature becomes too low for operation when it drops below the Minimum
Entering Water Temperature Setpoint. Entering water temperature becomes high enough for
operation when it rises above the Minimum Entering Water Temperature Setpoint by more the
Cooling Lockout Differential. The parameter for setting the OA Clg Lock is located via the
path Extended Menus/Compressor Setup/OAT Clg lock (or Min EWT). The parameter for
setting the OAT Htg Lock is located via the path Extended Menus/Heating Setup/OAT Htg
Lock.
Tenant Override
The tenant-override button provided with the optional zone temperature sensor packages is
used to override unoccupied operation for a pre programmed time period. This time period is
set with the Tenant Override Time Increment. This value can be adjusted from 0 to 300
minutes (default is 120 minutes). When an occupant presses and releases the tenant override
button on the zone temperature sensor (ZNT1), the Tenant Override Timer is set equal to the
Tenant Override Time Increment. (The button must be held for at least 1 second but not more
than 30 seconds.) The unit then starts and runs in the tenant override mode which is the same
as occupied mode except that it is temporary.1 The Tenant Override Timer begins timing out
and the unit runs until the timer expires. If the tenant override button is pressed again while the
unit is operating in tenant override mode, the Tenant Override Timer is reset to the Tenant
Override Time Increment and the unit continues to operate. For example, assume that the
Tenant Override Time Increment is 120 minutes. One press of the override button provides at
least 120 minutes of unit operation. If the button is pressed again 60 minutes later, the Tenant
Override Timer is reset to 120 minutes, and a total of 180 minutes of uninterrupted operation
results.
Note – The same operation occurs if, instead of pressing the override button on a zone
temperature sensor, the Occupancy Mode is set to “Tnt Ovrd.” Once set to “Tnt Ovrd”, the
Occupancy Mode automatically reverts to the “Auto” setting once the Tnt Ovrd Timer is set
to the Tnt Ovrd Time Increment. The same operation will also occur if the network
occupancy manual command it set to bypass
Zero OA Time (Morning Warm-up)
A separate Morning Warm-up State is not provided, but an edited ZeroOATime is used to keep
the Outside Air damper closed when the unit first starts. The Minimum OA Position is set to
zero as long as the as the fan has been on for less than the ZeroOATime. This allows the
Return Air type units to cool down the space with mechanical cooling or to warm up the space
with the dampers closed. If the ZeroOATime is set correctly, the OA dampers will be open
only during occupied periods. When Optimum Start is used ZeroOATime is set equal to the
time to occupancy when the unit starts so that the OA dampers will open at occupancy time.
The parameter is adjustable via the following path: Extended Menus/Timer Settings/Zero OA
Time.