30 November 1998
(2) When switch HS-XX-02 is indexed from heating to cooling, relays R-XX-02 and R-XX-03 are
de-energized which causes the converter valve to close and the system pump to momentarily shut down.
Simultaneously, relay R-XX-04 is energized, causing its normally open contact on Line 10 to close.
Assuming that the temperature of the return water is above the setpoint of TSL-XX-01, the system pump
is operated through the normally closed contact of relay R-XX-05 on Line 11. The pump continues to run
until the temperature of the return water cools down below the setpoint of TSL-XX-01. When this occurs,
pilot light PL-XX-03 (COOLING) is lit and relay R-XX-05 is energized which allows the pump to stop and
energizes solenoid actuated pneumatic valve EP-XX-01 on Line 12. When EP-XX-01 is energized, two-
position changeover valves VLV-XX-02 and VLV-XX-03 close to the hot water converter and open to the
central plant chiller. Hereafter, the system pump operates through the contacts of relay R-XX-01 only
when the system is in the occupied mode.
(3) When hand switch HS-XX-02 is indexed from cooling to heating, EP-XX-01 and relays
R-XX-04 and R-XX-05 are de-energized and cooling pilot light PL-XX-03 is turned off. When EP-XX-01
is denergized, changeover valves VLV-XX-02 and VLV-XX-03 open to the conveter and close to the
central plant chilled water system.
(4) On a fall in outdoor air temperature to the setpoint of the PV contacts of outside air
temperature reset controller TC-XX-01, relay R-XX-03 is energized and heating pilot light PL-XX-02 is lit.
The pump is started through the normally open contacts of relay R-XX-03 (Line 9). When the pump
starter is energized, its auxiliary contacts (M01 on Line 3) close. This energizes relay R-XX-02 whose
normally open contacts close to allow the output of temperature controller TC-XX-02 to be received by
current-to-pneumatic transducer IP-XX-01. On a rise in outdoor air temperature to 18 degrees C (65
degrees F) the PV contacts of temperature controller TC-XX-01 open and relay R-XX-03 is de-energized,
stopping the pump. The pump starter's auxiliary contacts open, which de-energizes relay R-XX-02 and
causes the converter valve to close.
(5) Outside air temperature reset 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.
(6) Temperature transmitter TT-XX-02 sends a hot water supply temperature signal to
temperature controller TC-XX-02. Temperature controller TC-XX-02 maintains its setpoint by varying its
output to current-to-pneumatic transducer IP-XX-01. The pneumatic signal from IP-XX-01 modulates high
temperature hot water converter valve VLV-XX-01 to maintain the hot water supply temperature setpoint
of temperature 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 as shown. The DDC