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Examples
CNT-APG002-EN 47
®
Examples
This section presents troubleshooting scenarios from a hot-water valve
application. The three examples have the same symptom but different
solutions to the problem.
Example 1
A hot-water valve cycles closed every few minutes. Although the space
temperature remains fairly stable, the discharge-air temperature swings
across a range of 10°F (5.6°C).
The technician follows the troubleshooting procedure described in this
chapter. However, nothing seems to work. The program is the same as the
one used in “Discharge-air temperature control” on page 29, and is known
to work well. Reducing the sampling frequency reduces the cycling, but
does not eliminate it. Reducing the gains helps reduce the cycling further,
but now the discharge-air temperature takes too long to reach setpoint.
The valve continues to cycle closed, though not as often.
The technician goes back through the troubleshooting steps and notices
when checking the programming logic that the hot-water valve closes
when the chilled-water valve is open. The technician decides to check the
position of the chilled-water valve while the application is running with
its original settings. Figure 33 illustrates what the technician found. The
chilled-water valve opens when the discharge-air temperature goes above
setpoint, thus forcing the hot-water valve to close.
Figure 33: Hot and chilled-water valve positions
Time (minutes)
Discharge-air temperature and valve position
Chilled-water valve
position (%)
Discharge-air
temperature setpoint (°F)
Discharge-air
temperature (°F)
Hot-water valve
position (%)