ASHRAE Journal - June 2014 - 16

Technical FeaTure

Temperature (°F)

During the "final exam," each control loop
Figure 1 VAV box control sequence is too tight.
of every system is checked to make sure its
80
control sequence actually does what it is
76
supposed to do. This is much more than
simply testing to see if a valve opens in
72
response to a call for heat, or a reheat valve
68
closes in response to a call for cooling. In
fact, from a technical commissioning perspective, those tests would have already
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been performed during the operational perTime (Minutes)
formance testing (OPT) phase.
Figure 2 VAV box control sequence is too loose.
Instead, the FPT phase calls for hands-on
verification that every controlled device
80
performs all of the functions it is required to
76
perform, including:
* The performance of all required steps it
72
is supposed to perform.
68
* A controller/actuator response speed
that is appropriate for the application.
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* A stable response.
Time (Minutes)
* All events are controllable to the setpoint.
* All adjustments take place automatically
Figure 3 Properly adjusted VAV box control sequence.
through the building automation system.
Figure 1 illustrates a control sequence that
80
is too tight. The figure shows a tempera76
ture that fluctuates hourly between 76°F
and 68°F (24°C to 20°C). The problem is
72
that when the temperature drifts from its
68
setpoint, the controller causes the temperature to cycle unacceptably. If a thermostat
controls this VAV box, the air flow damper
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Time (Minutes)
and heating control valve responses are too
rapid, which causes the cycling-and I can
practically guarantee that the occupants will complain
quickly enough-nor does the airflow increase at a rate
that it's too hot or too cold.
sufficient to maintain the setpoint.
But when called to resolve the problem, the building
In this case, when occupants complain, the maintemaintenance staff may not be able to properly diagnose
nance staff may simply see the temperature as too high
it because the temperature is fairly close to the setpoint
or too low. So to resolve overcooling, they reduce air
much of the time. So the maintenance staff leaves, havflow; to resolve overheating, they throttle back valves.
ing done nothing-except to accuse the occupants of
But neither of those solutions is the proper fix.
being oversensitive.
Figure 3 reflects proper system calibration, adjustment,
Figure 2 illustrates a different challenge: A controller
and tuning: When the setpoint is adjusted, the VAV airflow
that does not respond quickly enough. The problem is
damper and the reheat control valve respond correctly, the
when the room temperature is below its setpoint, the
temperature reaches the control setpoint within a reasonreheat valve or air flow damper does not change quickly able time interval, and afterward the temperature is stable.
enough to meet the setpoint; and when the temperature
In this case, most occupants would feel comfortable, as
is above its setpoint, the heating valve does not close
OPRs often call for a temperature of 72°F (22°C), ±2°F (1°C).
16

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