30 November 1998
c. Detailed sequence of operation.
(1) Outside air temperature transmitter TT-XX-01 signals outdoor air temperature to temperature
controller TC-XX-01. On a fall in outside air temperature to 16 degrees C (60 degrees F), the process
variable (PV) contacts of TC-XX-01 close to energize relays R-XX-04, R-XX-05, and R-XX-06 and pilot
light PL-XX-02. The contacts of relay R-XX-04 energize the primary hot water pump, the contacts of
relay R-XX-05 enable the secondary pumps, and the contacts of relay R-XX-06 enable the boiler control
(2) On a rise in outside air temperature to 17 degrees C (62 degrees F), the PV contacts of
controller TC-XX-01 open to de-energize relays R-XX-04 to stop the primary pump, to de-energize relay
R-XX-05 to stop both secondary pumps, and to de-energize relay R-XX-06 to disable the boiler control
circuit and to turn off pilot light PL-XX-02.
(3) Controller TC-XX-01 raises the setpoint of primary hot water supply temperature controller
TC-XX-02 as the outside air temperature falls, and lowers the setpoint as the outside air temperature
(4) Temperature transmitter TT-XX-02 signals the primary hot water supply temperature to
temperature controller TC-XX-02, which then maintains its setpoint by varying its signal to current-to-
pneumatic transducer IP-XX-01. The pneumatic signal from IP-XX-01 modulates primary hot water valve
VLV-XX-01 to mix boiler water and primary return water to maintain the primary supply water
temperature setpoint of controller TC-XX-02.
(5) Temperature transmitter TT-XX-03 signals the zone space temperature to controller
TC-XX-03, which then maintains its setpoint by varying its current output signal to transducer IP-XX-02.
The pneumatic signal from IP-XX-02 modulates secondary zone control valve VLV-XX-02. Zone control
valve VLV-XX-02 mixes primary supply water with secondary return water to maintain the zone space
temperature setpoint. The temperature control loop for the other secondary zone functions identically.
(6) Throughout the occupied mode, the contacts of time clock CLK-XX-01 are closed to energize
relays R-XX-01, R-XX-02, and R-XX-03, and to turn on pilot light PL-XX-01. The contacts of relay
R-XX-01 connect temperature setpoint device TSP-XX-01 to allow manual adjustment of the setpoint of
temperature controller TC-XX-03. Relay R-XX-02 provides the same function for TSP-XX-03 in
adjustment of controller TC-XX-04. Relay R-XX-03 closes contacts in the starter control circuits of the
secondary pumps. The secondary system pumps will start whenever the contacts of relay R-XX-03
(occupied) and R-XX-05 (heating) are closed in their respective pump starter circuits.
(7) During the unoccupied mode, the contacts of time clock CLK-XX-01 are open, de-energizing
relays R-XX-01, R-XX-02, and R-XX-03, and pilot light PL-XX-01 turns off. The transfer of the contacts of
relay R-XX-01 transfer temperature setpoint adjustment of TC-XX-03 from TSP-XX-01 to TSP-XX-02;
likewise, relay R-XX-02 transfers temperature setpoint adjustment of TC-XX-04 from TSP-XX-03 to
TSP-XX-04. The contacts of relay R-XX-03 open the secondary pump starter circuits to place the
secondary pumps under the respective night thermostats TSL-XX-01 and TSL-XX-02. When the zone
space temperature falls to 13 degrees C (55 degrees F), the zone's secondary pump is energized and
remains energized until the temperature rises to 14 degrees C (57 degrees F).
d. Sequence of operation for DDC applications.
(1) The DDC system shall accept a signal from a sunshielded outside air temperature sensing