30 November 1998
(2) When the system is indexed to heating, the boiler and pump operate continuously whenever
the outside air temperature is low enough for the building to require heating. When this occurs, the
system boiler is started and the system pump is energized. The boiler functions under its own control
system to maintain a constant boiler water temperature.
(3) The control system modulates a three-way valve to mix supply and return hot water to
maintain the scheduled hot water supply temperature setpoint. As the outside air temperature falls the
supply water temperature is raised.
(4) When the system is indexed from the heating mode to the cooling mode, the boiler is shut
down. The pump continues to run to dissipate the heat from the piping system and the boiler until the
return water temperature drops below the setpoint of the return water thermostat. When this occurs, the
distribution pump stops and the changeover valves are then permitted to close to the boiler and open to
(5) When the system is indexed to cooling and changeover has occurred, the chiller and pump
are energized during the occupied mode. At the conclusion of the occupied period, the chiller is de-
energized but the pump continues to circulate water for a period of time while the chiller completes its
shutdown cycle, and then the pump stops.
(6) When the system is indexed from cooling mode to heating mode, the chiller enters its
shutdown cycle and the pump continues to circulate water through the chiller for a period of time while
the chiller completes its shutdown cycle.
c. Detailed sequence of operation.
(1) Switch HS-XX-02 allows for manual indexing of the dual-temperature hydronic system
between the heating and cooling modes. Switch HS-XX-03 allows for manual operation of the dual-
temperature changeover valves.
(2) When switch HS-XX-02 is indexed from the heating mode to the cooling mode, relays
R-XX-02 and R-XX-03 are de-energized, heating pilot light PL-XX-02 is turned off, and time-delay relay
TDR-XX-01 is energized. One set of normally open contacts of relay R-XX-03 (Line 200) open to de-
energize the boiler and a second set of normally open contacts open to interrupt the input signal to
IP-XX-01. A set of normally open contacts of relay R-XX-02 on Line 13 open and normally open contacts
of time-delay relay TDR-XX-01 on Line 14 close, transferring control of pump relay R-XX-06 from the
heating mode to the cooling mode. If the return water temperature is above the setpoint of TSL-XX-01,
relay R-XX-04 (Line 9) remains de-energized. As a result, its normally closed contact on Line 8 remains
closed and the pump continues circulating water through the system to dissipate heat. When the
temperature of the return water falls below the setpoint of TSL-XX-01, relay R-XX-04 is energized. The
normally closed contacts of relay R-XX-04 on line 8 are now open, de-energizing time-delay relay TDR-
XX-01. Once the time-delay period expires, the normally open contacts of TDR-XX-01 on line 14 open,
de-energizing relay R-XX-06, causing the pump to stop. When relay R-XX-04 is energized as a result of
the return water temperature falling below the setpoint of TSL-XX-01, a set of normally open contacts on
line 12 are closed, which allows EP-XX-01 to become energized, when switch HS-XX-03 is in the
"cooling" position, causing the changeover valves to open to the chiller.
(3) When the system enters the occupied mode, as determined by timeclock CLK-XX-01 on Line
0, relay R-XX-01 on Line 1 is energized and pilot light PL-XX-01 (OCCUPIED) is lit. When relay
R-XX-01is energized, the normally open contact on Line 7 is closed, which energizes time-delay relay