46 WGS 130A to 190A OM WGS-5
Start Reset Delta T
LWT set Point+Max Reset
(54)
LWT Set Point
(44)
Return Reset
0
Max Reset
(10)
Evap Delta T (
o
F)
Active
LWT
(
o
F)
The active setpoint is reset using the following parameters:
1. Cool LWT setpoint
2. Max Reset setpoint
3. Start Reset Delta T setpoint
4. Evap Delta T
Reset is accomplished by changing the Active Leaving Water variable from the Cool LWT
setpoint to the Cool LWT set-point + Max Reset setpoint as the Evaporator EWT – LWT
(Evap Delta T) varies from the Start Reset Delta T setpoint to 0.
Referring to the above figure as an example, the LWT is 44F and a 10-degree maximum
reset value was selected. The Active LWT setpoint would range from the normal 44F
setting up to 54F depending on the Evap Delta-T. The amount of reset would be at the
maximum value (10 degrees) when the Evap Delta-T is zero and at the minimum value
when the Evap Delta-T is at the Start Reset Delta T value and is proportional in between.
The Start Reset Delta T function is available so that the start of reset can be adjusted. For
example, on a system with a 10-degree Delta-T, it may be desirable to not start the resetting
until the evaporator Delta-T goes down to eight degrees (80% load), or some other value.
Reset Type – 4-20 mA
The Active Leaving Water setpoint is adjusted by the 4 to 20 mA reset analog input, using:
1. Cool LWT setpoint
2. Max Reset setpoint
3. LWT Reset signal
Reset is 0 if the reset signal is less than or equal to 4 mA. Reset is equal to the Max Reset
Delta T setpoint if the reset signal equals or exceeds 20 mA. The amount of reset will vary
linearly between these extremes if the reset signal is between 4 mA and 20 mA. An
example of the operation of 4-20 reset in Cool mode is shown below.
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