RT-SVD03G-EN 99
ReliaTel Economizer Operation
Customer Benefit
An economizer consists of a fresh air damper, a return air damper, linkage to maintain an inverse
relationship between the two, and an actuator to control the damper position. An economizer is
used to provide two unit functions: ventilation and economizer cooling. In either case, the inverse
relationship between the return and outdoor air dampers allows the unit to maintain the same
approximate total airflow regardless of economizer position. A linkage adjustment is typically
required in the field to adjust for differences in pressure drops due to different duct designs.
Economizer cooling is provided to take advantage of cooler outdoor air to satisfy a cooling load in
a conditioned space minimizing the need for mechanical cooling (with compressors). While
economizer cooling, it is necessary to limit the damper position so that the mixed air temperature
does not fall below 53°F (± 3°F) and cause excessively cool air from being discharged from the unit.
When used with a zone sensor, an economizer setpoint that is below the cooling setpoint is used
to allow sub-cooling essentially for free, further reducing the need for more expensive mechanical
cooling. To maximize the use of an economizer, mechanical cooling is delayed from running until
it has been determined that the economizer alone cannot satisfy the load.
Any time the supply fan is On and the building (unit) is occupied, the economizer damper will be
maintained at or above minimum position. The economizer damper is held closed when the supply
fan is off to prevent water from getting into the economizer section of the unit.
Compressors will be delayed from operating until the economizer has opened to 100% for 5
minutes on all unit types other than Voyager III which will have a 3-minute delay.
VAV Economizer Cooling Operation
A similar method of determining and adjusting the economizer setpoint as described in CV oper-
ation will be used in VAV Cooling operation with the additional VAV requirements below.
• Economizer Setpoint = SA Temp Control Point - 2°F.
• Economizer Cooling is only allowed when economizing is enabled and SA temp is greater than
Economizer Setpoint - 1.5°F.
• If conditions allow enabling of economizer cooling after a compressor is active, the economizer
damper is forced to 100% open until all compressors are deactivated or conditions change to
disable economizing.
• If economizer cooling is active prior to mechanical cooing, compressor operation will be
inhibited until the economizer damper reaches 100%. Once the economizer reaches 100%,
mechanical cooling will be enabled without delay.
• During VAV Unoccupied operation economizer cooling will operate as it does in CV
Unoccupied.
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