Damper Control and Tenant Override Logs
Damper Control
The damper position is calculated by the mode and demand from the
space sensor. Included in this calculation is an Integral function. This
prevents the damper/airflow from stagnating at a position somewhere
above the setpoint because the supply air temperature or duct pressure
isn’t quite enough to satisfy the space at the currently calculated pro
-
portional position. The Integral causes the calculation to keep adding
a small amount of the proportional error back into the damper/airflow
position each time a new position is calculated. The amount the Integral
adds back in is user adjustable. This value is presented as a number
between 0.0 and 10.0. That means that if the integral is less than 1.0,
you are adding a percentage from 0 to 100% of the error back into the
calculation. If you increase the Integral above 1.0, you are adding more
than 100% back in.
With just proportional control, a 1 ºF error would cause a 50% increase
in damper/airflow if the Integral is not included. (1 °F is half of the 2
ºF Proportional Window).
If you had set the Integral to 1.0, the calculation would add 2% to the
current damper/airflow calculation each time. The calculation occurs
once every 10 seconds, so it would take a little over 4 minutes to reach
the programmed 100% maximum.
Example:
1. 50% Remaining / 2% Integral = 25 Moves to get to a 100%
Maximum
2. 25 Moves times 10 seconds = 250 seconds or a little over 4
minutes to reach the 100% maximum damper/airflow
position.
Of course, different space temperature errors and different Integral values
cause this calculation to operate slower or faster. It is up to the user to
determine the optimum setting that provides the tightest temperature
control without causing the damper to continue to hunt or modulate,
causing premature wear of the actuator gears and motor.
On pressure dependent terminal units, the damper position is maintained
to within ± 3% of the calculated position. No attempt is made to position
the damper exactly on the calculated position. This reduces wear and tear
on the actuator gears and motors and the amount of airflow involved is
not affected by that small amount of damper error.
On pressure independent terminal units, the airflow is maintained to
within roughly 3% of the terminal unit size constant but no tighter than
16 CFM on the smallest terminal units. The actual control window is
based on the formula:
Window = Terminal unit size X Square Root (1 / 750)
Where Terminal unit size refers to the total rated CFM of the
terminal unit.
This sliding window allows the control to be much tighter on the smaller
terminal units than can be achieved on the larger terminal units as far as
CFM readings. On a large terminal unit, 25 CFM may not be noticeable,
but on a small terminal unit, 25 CFM may be more than the minimum
airflow setting for the space.
If the pressure sensor is disconnected or fails on a pressure independent
terminal unit, the controller automatically reverts to pressure dependent
operation and generates an alarm to alert the user that a failure has oc-
curred.
On either type of terminal unit, a space sensor failure will force the
damper to position itself to the 50% of the maximum damper position
setpoint, and it will not change until the sensor is repaired or replaced.
Tenant Override Logs
If you require tenant billing for push-button override usage, a
MiniLink
Polling Device must be installed on each local loop. The MiniLink Poll-
ing Device has the ability to track the amount of override time generated
by each space sensor equipped with push-button override.
Storing and retrieving these logs requires a dedicated computer run
-
ning the Prism front-end software program. No other method exists for
retrieving these logs. That means that all of your units will be connected
together on the communications loop and the loop will be terminated at
a CommLink device connected to an on-site computer.
Warning:
This computer must be on 24 hours a day 7 days a
week running the Prism software in order for tenant
logging to be tracked.
The tenant logs are kept on the dedicated job site computer’s hard drive.
The only limitation to the number of logs stored is the capacity of the
hard drive on the computer to which it is being logged.
Note:
For proper time and date stamping of the tenant log, you
must configure the air handler to broadcast the time so
that the VAV/Zone Controllers can read it and use it in
their tenant and trend logs.
25
Non-Modular ZCAP Technical Guide
Sequence of Operations
