30 November 1998
system, and enables the secondary pumps to operate. The outside air temperature controller raises the
primary hot water supply temperature as the outside air temperature falls.
(2) The primary hot water supply temperature controller maintains its setpoint by modulating the
converter steam valve to maintain the scheduled primary hot water supply temperature.
(3) During the occupied mode, the secondary hot water pumps run whenever the primary hot
water pump runs. Each zone temperature controller modulates its zone control valve to blend primary
hot water supply with secondary hot water return in order to maintain the zone's space temperature
setpoint, which is adjustable by the zone occupant.
(4) During the unoccupied mode, the secondary hot water pumps are cycled by the zone night
thermostat to maintain the low limit setpoint and to prevent freezing. During this mode, the space
temperature controller setpoint is determined by an adjustment within the system's HVAC control panel.
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 and R-XX-05, and pilot light
PL-XX-02. The contacts of relay R-XX-04 energize the primary hot water pump, and the contacts of
relay R-XX-05 enable the secondary pumps. The auxiliary contacts of the primary pump starter energize
relay R-XX-06, whose contacts then allow the primary water temperature control system to operate.
(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 relay R-XX-04 to stop the primary pump, and to de-energize
relay R-XX-05 to stop each secondary pump. The auxiliary contacts of the primary pump starter open,
de-energizing relay R-XX-06, whose contacts then remove valve VLV-XX-01 from control, causing it to
(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 converter steam valve
VLV-XX-01 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 loops for the other secondary zones function 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