30 November 1998
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 and de-
energize relays R-XX-01, R-XX-02, and R-XX-03; pilot light PL-XX-01 turns off. The transfer of contacts
of relay R-XX-01 transfers the temperature setpoint adjustment of TC-XX-03 from TSP-XX-01 to
TSP-XX-02; likewise, relay R-XX-02 transfers the 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
element and transmitter located as shown. The DDC system shall start and stop the pumps at the
outside air temperature 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 heating
supply line and the DDC system output shall modulate the converter steam control valve to maintain the
reset schedule setpoint in the hydronic heating supply line.
(2) When the system time schedule places the system in the occupied mode, a space
temperature sensing element and transmitter located as shown shall signal the DDC system, which shall
maintain the space temperature setpoint by modulating the secondary hydronic system zone valve.
(3) When the system is in the unoccupied mode, the space temperature setpoint shall be as
12. SINGLE BUILDING HYDRONIC HEATING WITH HOT WATER BOILER CONTROL SYSTEM.
a. Description of the hydronic heating system. This hydronic heating system consists of a hot water
boiler that provides hot water to a primary pumping system. The primary pumping system supplies hot
water to secondary pumping systems that provide hot water to separate zones of space temperature
control. This control system varies the hot water flow through the boiler. Therefore, it can be used only
with boilers whose operation is not affected by flow variation. A variation of this system which utilizes
constant flow through the boiler is depicted in Chapter 5. Figures 4-8a through 4-8f show the system
design for this type of heating system using a SLDC control panel. Figures 4-8g through 4-8j show the
system design for this type of heating system using DDC controls.
Figure 4-8a. Control system schematic for single building hydronic heating system with hot water boiler.
Figure 4-8b. Control system ladder diagram for single building hydronic heating system with hot water
Figure 4-8c. Control system equipment for single building hydronic heating system with hot water boiler.