30 November 1998
Figure 4-9g. DDC control system schematic for central plant high-temperature hot water hydronic heating
Figure 4-9h. DDC control system ladder diagram for central plant high-temperature hot water hydronic
Figure 4-9i. DDC control system equipment for central plant high-temperature hot water hydronic heating
Figure 4-9j. DDC control system I/O table and data terminal strip layout for central plant high-
temperature hot water hydronic heating system.
b. General Sequence of Operation.
(1) The primary pumping system and the high-temperture hot water converter operate
continuously whenever the outside air temperature is low enough for the building to require heating.
When this occurs, an outside air temperature controller starts the primary pump, enables the high-
temperature hot water converter's control system, and enables the secondary pumps to operate. The
outside air temperature controller raises the primary hot water supply temperature as the outside air
(2) The primary hot water supply temperature controller maintains its setpoint by modulating the
high-temperature hot water converter valve to maintain the scheduled primary hot water supply
(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 water
from the primary hot water supply with secondary hot water return in order to maintain the zone 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 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 to turn on 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. The contacts of relay R-XX-06 allow the primary water temperature control system to
(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 relay R-XX-05 to