Trane Tracer ZN517 Installation And Operation Manual page 82

Chapter 9 PID control
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piece of equipment contributes a set amount to the final output. When
setting the error deadband for staged outputs, the main goal is to reduce
equipment cycling.
Follow these guidelines when adjusting the error deadband:
• Ask how tight control should be. A smaller error deadband results in
tighter control, but control should not be so tight that the stages cycle
on and off too frequently.
For example, for a VAV air-handler turning on cooling stages, control
can be somewhat loose. The individual VAV boxes control their valve
to the space depending on the supply air temperature. If the supply
air temperature is relatively warm, the VAV box allows more air flow.
If the supply air temperature is somewhat cool, the VAV box con-
stricts the air flow.
The contribution of each stage can change depending on external cir-
•
cumstances, so make adjustments under worst case conditions.
Adjust the error deadband for cooling tower fan stages on very warm
days, and adjust the error deadband for boiler stages on very cold
days.
With the preceding guidelines in mind, use the following procedure to
determine error deadband.
To adjust the error deadband for staged outputs:
1. Run the system manually.
If possible, do so under worst-case conditions for the site. Although it
is not always possible for a technician to do this, it is possible for a
well-trained customer.
2. Find the smallest change in temperature, ∆T, that the first stage can
contribute (the quantity could also be building static pressure for fans
or flow for pumps).
Pay attention to possible changes in external circumstances, such as
the amount of water flow. If the system uses a lead-lag approach to
the equipment, it will be necessary to find the minimum ∆T for all
stages.
3. Multiply ∆T by 0.45 (the error deadband should be slightly less than
half of ∆T).
Keep in mind the resolution of the sensor. You may need to round the
error deadband to a more reasonable value.
4. Run the system with the new error deadband.
Cycling should be reduced as much as possible.
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