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McQuay OM 751 33
Description of Operation
• Expanded (optional)—Temperature Comparison with OA Enthalpy Setpoint Economizer
(Strategy 1)
• Leading Edge (optional)—Temperature Comparison with Enthalpy Comparison Economizer
(Strategy 2)
Temperature Comparison Economizer (default)
If the default Basic economizer function is selected, the unit ventilator is provided from the
factory without the optional IA and OA humidity sensors. In this case, the UVC is factory set
for Economizer Strategy 1—the UVC automatically detects that no OA humidity sensor is
present and adjusts to use the Temperature Comparison Economizer function.
Temperature Comparison with OA Enthalpy Setpoint Economizer (optional)
If the optional Expanded economizer function is selected, the unit ventilator is provided from
the factory with the optional OA humidity sensor, which is used along with the OA
temperature sensor to calculate OA enthalpy. In this case, the UVC is factory set for
Economizer Strategy 1 and uses the Temperature Comparison with OA Enthalpy Setpoint
Economizer function.
Note – Temperature Comparison with OA Enthalpy Setpoint Economizer requires an optional OA
humidity sensor.
Temperature Comparison with Enthalpy Comparison Economizer (optional)
If the optional Leading Edge economizer function is selected, the unit ventilator is provided
from the factory with both the IA humidity and OA humidity sensors, which are used along
with the IA temperature and OA temperature sensors to calculate IA enthalpy and OA
enthalpy. In this case, the UVC is factory set for Economizer Strategy 2 and uses the
Temperature Comparison with Enthalpy Comparison Economizer function.
Note – Temperature Comparison with Enthalpy Comparison requires both an optional OA
humidity sensor and an optional IA humidity sensor.
Table 18: Economizer enable/disable tests defined
Table 19: How economizer enable/disable tests are selected
Note: The hard-wired sensor and the equivalent input must both be unreliable for the value to be considered
unreliable.
Tests
Economizer
enable/disable tests
Enable test Disable test
A OA temp set point EffectiveOATemp < (EconOATempSetpt – EconTempDiff) EffectiveOATemp >= EconOATempSetpt
B IA/OA differential temp EffectiveOATemp < (EffectiveSpaceTemp – 3.6°F – EconTempDiff) EffectiveOATemp >= (EffectiveSpaceTemp – 3.6°F)
C OA enthalpy set point
EffectiveOAEnthalpy < (EconOAEnthalpySetpt –
EconEnthalpyDiff)
EffectiveOAEnthalpy >= EconOAEnthalpySetpt
D
IA/OA differential
enthalpy
EffectiveOAEnthalpy < (EffectiveSpaceEnthalpy –
EconEnthalpyDiff)
EffectiveOAEnthalpy >= EffectiveSpaceEnthalpy
Economizer
strategy
Space temp sensor OA temp sensor
Space humidity
sensor
OA humidity sensor
Economizer enable/
disable tests
All
Unreliable Don’t care Don’t care Don’t care OA damper closed
Don’t care Unreliable Don’t care Don’t care OA damper closed
Basic
Reliable Reliable Don’t care Unreliable Test B
Reliable Reliable Don’t care Reliable Test C
Expanded Reliable Reliable Don’t care Reliable
Test C and Either
Test B or Test A
Leading Edge
Reliable Reliable Reliable Reliable Test D and Test B
Reliable Reliable Unreliable Reliable Test B
Reliable Reliable Reliable Unreliable Test B
Reliable Reliable Unreliable Unreliable Test B