CEMP-E
TI 810-11
30 November 1998
(6) Outside air controller TC-XX-01 raises the setpoint of hot water supply temperature controller
TC-XX-02 as the outside air temperature falls and lowers the setpoint as the outside air temperature
rises.
(7) Temperature controller TC-XX-02, with its temperature transmitter in the hot water supply
piping, through current-to-pneumatic transducer IP-XX-01, modulates high-temperature hot water
converter valve VLV-XX-01 to maintain the supply water temperature setpoint of controller TC-XX-02.
d. Sequence of operation for DDC applications.
(1) Switch HS-XX-01 provides for manual switching of the dual-temperature hydronic system
between the heating and cooling modes.
(2) When the heating mode is selected, the system changeover valves shall close to the central
plant chilled water flow and shall open to flow through the converter, and the distribution pump shall be
under control of the DDC system. The DDC system shall accept a signal from a sunshielded outside air
temperature sensing element and transmitter located as shown. The DDC system shall start and stop
pump at the outside air temperatures shown. The DDC system shall reset the hydronic heating supply
temperature setpoint in a linear schedule based on the outside air temperature. The DDC system shall
accept a signal from a temperature sensing element and transmitter located in the hydronic heating
supply line and the DDC system output shall modulate the converter's steam control valve to maintain
the reset schedule setpoint in the hydronic heating supply line.
(3) When the cooling mode is selected, the converter steam valve shall be closed. The DDC
system shall accept a signal from a temperature sensing element and transmitter located in the system
return. The DDC system shall continue to operate pump to circulate water through the system. When
the system return water temperature drops below the setpoint shown, the DDC system shall allow the
changeover valves to close to flow through the converter and to open to the central plant chilled water
flow, and place the control of pump under control of the system time schedule. During the occupied
mode, pump shall operate continuously. In the unoccupied mode, pump shall stop.
15. CENTRAL PLANT HIGH-TEMPERATURE HOT WATER DUAL-TEMPERATURE HYDRONIC
CONTROL SYSTEM.
a. Description of the hydronic system. This hydronic heating and cooling system consists of hot
water from a high-temperature hot water converter, chilled water from a central plant, and related
pumping systems. Figures 4-11a through 4-11f show the system design for this type of heating system
using a SLDC control panel. Figures 4-11g through 4-11j show the system design for this type of heating
system using DDC controls.
Figure 4-11a. Control system schematic for central plant high-temperature hot water dual-temperature
hydronic system.
Figure 4-11b. Control system ladder diagram for central plant high-temperature hot water dual-
temperature hydronic system.
Figure 4-11c. Control system equipment for central plant high-temperature hot water dual-temperature
hydronic system.
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