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Discharge-air temperature control
CNT-APG002-EN 31
®
After the initial installation and testing, the technician noticed that the
discharge-air temperature was oscillating in a 10°F (5.6°C) band around
setpoint. Slowing the sampling frequency to 30 seconds stopped the oscil-
lations (see Chapter 5, “Troubleshooting”). The technician also increased
the proportional and integral gains to make the discharge-air tempera-
ture reach setpoint faster.
Figure 21 shows the discharge-air temperature and valve position over a
two-hour period. During the unoccupied period, the hot-water valve is
completely open. Eventually the discharge-air temperature rises to
almost 100°F (37.8°C). At the twelve-minute point, the HVAC system
changes from the unoccupied to the occupied state, and the hot-water
valve is adjusted to meet the discharge-air setpoint. The valve closes com-
pletely for nearly 20 minutes until the discharge-air temperature drops
below setpoint. Achieving a stable discharge-air temperature takes
approximately 30 minutes. Note that once setpoint is reached, the valve
position remains stable between 10% and 15%. A stable valve position
over time indicates that the loop has been tuned for optimal performance.
Figure 21: Hot-water valve position and discharge-air temperature
Time (minutes)
DA Temperature and valve position
Discharge-air temperature
setpoint (°F)
Discharge-air
temperature (°F)
Valve position (%)
Heat valve position during
change from unoccupied
to occupied state