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Response by Control Type—Advanced Microprocessor Controls
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5.1.3 Cooling Operation
Two-Step Cooling, Compressorized Direct Expansion (DX) Systems
The first stage of cooling activates when the temperature control calculates a requirement for cooling
of 50%. The first stage of cooling is deactivated when the cooling requirement drops below 25%. The
second stage of cooling is activated when the requirement for cooling rises to 100% and deactivated
when the requirement falls below 75%.
Four-Step Cooling, Compressorized Direct Expansion (DX) Systems)
The four stages of cooling are activated when the requirement for cooling is 25%, 50%, 75% and 100%
respectively. Each stage of cooling is deactivated when the requirement for cooling falls 12% below
the respective activation points. The four stages of cooling are accomplished in the following manner:
• 1 stage: One compressor, unloaded
• 2 stages: Both compressors, unloaded
• 3 stages: One compressor, loaded and one compressor, unloaded
• 4 stages: Both compressors, loaded
GLYCOOL
™
Cooling
When GLYCOOL
™
cooling is available, the temperature control will calculate a total cooling require-
ment of 200% rather than 100%. Assuming that full GLYCOOL capacity is available, the GLYCOOL
valve opens proportionally as the requirement for cooling rises from 0 to 100%. If more than 100%
cooling is required, then the compressors are activated at 150% and 200% respectively (125%, 150%,
175% and 200% for a four-step system). If full GLYCOOL capacity is not available, then the GLY-
COOL valve will be opened proportionally over a cooling requirement band equal to the available
GLYCOOL capacity. The compressor would be activated at a cooling requirement of 50% and 100%
above the available GLYCOOL capacity.
For example, if the GLYCOOL capacity is 60%, then the GLYCOOL valve would be full open at 60%
cooling requirement and the compressors would activate at 110% and 160% cooling requirement. In
order to reduce compressor cycling and prevent hunting, GLYCOOL capacity first becomes available
when the entering glycol temperature is at least 8° F (22% capacity) below the return air tempera-
ture, or 3°F below the return air temperature for 2 hours. GLYCOOL capacity is 100% when the gly-
col temperature is 25°F below the return air temperature. The system will continue to Econ-O-Cool as
necessary as long as the entering glycol temperature remains at least 3°F (0% capacity) below the
return air temperature. If GLYCOOL is not available, the temperature control will operate the com-
pressors in the same manner as a 2-step or 4-step system without GLYCOOL.
Dual Cooling Source
If dual cooling is available, the sensible cooling system operates in the same manner as a GLYCOOL
™
system, except that it is assumed that 100% chilled water capacity is available any time the chilled
water temperature is 3°F below the return air temperature.
Chilled Water Cooling
The chilled water control valve is adjusted proportionally as the temperature control varies the
requirement for cooling from 0% to 100%. Units with the optional variable speed drive (VSD) fan-
speed control operate in a similar manner, except that the minimum fan speed is 60% when the cool-
ing requirement is less than 60%. Also, the fan is operated at 100% on a call for any heating and/or
humidification.
5.1.4 Heating Operation
Electric Reheat
The three heat stages are activated when the temperature control calculates a requirement of 33%, 66%
and 100% respectively. The stages are deactivated when the heat requirement is 16% less than or one
half of the activation point. To help improve the life of the reheats, starts are rotated. The first reheat off
will be the last to turn on. For example, on an initial call for heating, reheat 1 will turn on. If the control
determines more heating is necessary, reheat 2 will turn on. Then, if less heating is needed, reheat 1
will turn off. If more heating is necessary, reheat 3 will turn on. If more heating is necessary, reheat 1
will turn on. If less heating is needed, reheat 2 will be the first off, then reheat 3 and so on. Reheat
starts are not rotated if 2 stages of reheat operation are selected in the SELECT OPTIONS menu.