30 November 1998
TDR-XX-01. The normally open contacts of TDR-XX-01 on line 14 close, which energizes relay R-XX-06,
causing the system pump to start. Relay R-XX-01 also has a set of normally open contacts on line 10
which are closed during the occupied mode. This causes the chiller to be enabled, if flow through the
chilled water loop is proven by flow switch FS-XX-02, and pilot light PL-XX-03 (COOLING) is lit.
(4) When the occupied period ends, relay R-XX-01 is de-energized. The contact on line 10
opens to de-energize R-XX-05 and PL-XX-03. This causes the chiller to shut down. The normally open
contact of relay R-XX-01 on line 7 also opens, de-energizing time-delay relay TDR-XX-01. The normally
open contact of TDR-XX-01 on line 14 remains closed until the expiration of the time delay, allowing the
pump to continue running while the chiller completes its shut-down process. After the time-delay period
expires, this contact opens, de-energizing relay R-XX-06 and thereby shutting off the pump.
(5) When switch HS-XX-02 is indexed from the cooling mode to the heating mode, relays
R-XX-04, R-XX-05, and TDR-XX-01 are de-energized. As a result, the chiller is shut down and PL-XX-03
is turned off. The pump continues to run until the expiration of the time delay period of TDR-XX-01.
(6) With the system indexed to the heating mode, if the outside air temperature is below the PV
alarm setpoint of temperature controller TC-XX-01, the controller's PV contact on Line 3 will be closed.
This energizes relay R-XX-02 to start the system pump. Once flow is proven through the heating loop by
flow switch FS-XX-01, relay R-XX-03 is energized and pilot light PL-XX-02 (HEATING) is lit. R-XX-03
starts the boiler (Line 200) and also closes a contact between TC-XX-02 and IP-XX-01, placing valve
VLV-XX-01 under the control of temperature controller TC-XX-02.
(7) Temperature transmitter TT-XX-02 sends a hot water supply temperature signal to
temperature controller TC-XX-02. Controller TC-XX-02 maintains its setpoint by varying its output signal
to current-to-pneumatic transducer IP-XX-01. The pneumatic output from IP-XX-01 modulates three-way
valve VLV-XX-01 to mix boiler water and return water to maintain the temperature setpoint of controller
TC-XX-02. Outside air temperature transmitter TT-XX-01 sends an outside air temperature signal to
temperature controller TC-XX-01. As the outside air temperature falls, TC-XX-01 raises the setpoint of
d. Sequence of operation for DDC applications.
(1) Switch HS-XX-01 provides for manual indexing of the dual-temperature hydronic system
between the heating and cooling modes.
(2) When the heating mode is selected, the chiller shall be stopped. The distribution pump
shall continue to operate until the expiration of a time delay as recommended by the chiller
manufacturer. At the expiration of the time delay, the system changeover valves shall close to flow
through the chiller and shall open to flow through the boiler, 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
the distribution pump and the boiler 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 as shown. The DDC system shall accept a signal from a temperature sensing element and
transmitter located in the hydronic supply line and the DDC system output shall modulate the hydronic
heating system control valve to maintain the reset schedule setpoint in the hydronic heating supply line.
(3) When the cooling mode is selected, the boiler shall be stopped. The DDC system shall
accept a signal from a temperature sensing element and transmitter located in the system supply as