CEMP-E
TI 810-11
30 November 1998
Table 3-3. Controller configuration parameters
Outside air temperature controller
Discharge air temperature controller
(TC REV)
(TC DIR)
Scaled low range of the PV input = -35 C (-30 F)
Scaled low range of the PV input = 5 C (40 F)
Scaled high range of the PV input = 55 C (130 F)
Scaled high range of the PV input = 60 C (140 F)
Setpoint = -1 C (30 F)
Scaled low range of the CPA input = 32 C (90 F)
Proportional band = 37.5 %
Scaled high range of the CPA input = 49 C (120 F)
Manual reset = 50 %
Minimum setpoint (optional) = 32 C (90 F)
Minimum output = 4 mA
Maximum setpoint (optional) = 49 C (120 F)
Maximum output = 20 mA
m. The heating coil temperature control loop, with its fixed controller setpoints, is similar to the
preheat coil temperature control loop in that it also controls the coil valve at all times. Figure 3-5 shows
the heating coil controlled in a fixed temperature application. Temperature sensing element and
transmitter TT sends a signal to heating-coil temperature controller TC. The operations of the control
devices affected by the output signal of TC are identical, as previously described in paragraph 3-3.b. For
DDC applications, the DDC panel takes the place of heating coil temperature controller TC.
Figure 3-5. Heating coil temperature control loop with heating coil controlled at a constant temperature.
n. The heating coil temperature control loop functions continuously, without regard to the operating
condition of the HVAC system. This has the advantage of maintaining a minimum temperature in the
ductwork when the HVAC-system supply fan is off.
5. MIXED AIR TEMPERATURE AND ECONOMIZER CONTROL LOOPS.
a. The use of up to 100 percent outdoor air to provide free cooling when the outdoor air conditions
are favorable is called an economizer cycle. Whenever the enthalpy of the outdoor air is less than the
enthalpy of the return air, conditions are favorable for an economizer cycle to provide cooling. While
ideally the decision to allow the introduction of outside air beyond the minimum ventilation requirements
for cooling would be based on a comparison of the enthalpies of the outside and return air, enthalpy
sensors do not have the reliability of temperature sensors and they require more maintenance. Because
3-5
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