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4 Stage with Pump Output and Reset
– T775P using a T775S Expansion
Module
Application Description
The T775P is providing multistage boiler control based on the
boiler's discharge water temperature. Four stages and a
dedicated pump output are used in this example. The T775P
uses the eighth output relay to energize the primary pump.
NOTE: The pump output is always the last relay output. In
this application example, four relays are used for
staged boiler control. A T775S expansion module is
added to provide the additional relay for the pump.
However, the pump must be the last relay, so it is
configured as Relay 8 on the T775S module.
Any number of stages from 1 to 12 can be configured (T775P
and T775L models). The throttling range is divided equally
among the stages.
Sensor Designation
This device application requires three sensors.
• Sensor A is sensing discharge water and controls 4 stages.
• Sensor B is sensing outside temperature and is used for
reset.
• Sensor C is sensing the return water temperature.
NOTE: Control can be to either Sensor A or C.
Operation
As the heating load increases additional stages of heat will
cycle ON as the boiler water temperature decreases. In this
example, the T775P provides four stage control when the
effective setpoint is 200° F (93° C). (See Fig. 25.) The primary
circulating pump energizes whenever any stage is energized.
NOTE: The pump output must always be the last relay on the
controller or expansion module (Relay 4, 8, or 12).
NOTE: For applications with 3 stages or less with a pump out-
put, an additional expansion module is not needed.
Programming Example
Program in Setup:
Press and hold the MENU button for 5 seconds to enter
Setup mode. Select the Outputs menu, and then select:
— # Stages = 4 (T775 assigns pump to Relay 8)
→
— Options
Reset = YES-BOILER
— STG8/Pump: (Relay 8 controls the pump output)
Enable = YES
Exercise = YES or NO
Prepurge = -300 to 300 seconds (default is 0)
Postpurge = 0 to 300 seconds (default is 0)
Stages 1-4: Control to the discharge water temperature
Program for: (Refer to the reset curve in Fig. 26.)
— Boiler Max = 210° F (99° C)
— Outside Min = 20° F (-7° C)
— Boiler Min = 160° F (71° C)
— Outside Max = 70° F (21° C)
— Throttling Range= 20° F (-7° C)
— Sensor A
— Heat
T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
1
See Notes in "Programming Example" on page 18 for an
explanation of Prepurge and Postpurge times.
IMPORTANT
After the desired value is selected, be sure to press
the or or HOME button in order to save that
value in the controller's memory.
IMPORTANT
By programming the boiler setpoint at 210° F
(-99° C) the T775 has established 210° F (-99° C) as
the highest operating point that will be allowed when
the temperature falls below 20° F (-7° C). As
temperature increases above 20° F (-7° C), the boiler
will be reset downward per the reset ratio until it
reaches the minimum setpoint of 160° F (-99° C), at
70° F (21° C) and above outdoor temperature.
STAGE 4 ON
STAGE 3 ON
STAGE 2 ON
STAGE 1 ON
-25%
205°F
1
(when the effective setpoint = 200° F (93° C)
SP MAX A1
(BOILER MAX)
S
E
N
S
O
1
R
A
SP MIN A2
(BOILER MIN)
NOTE: The 4 stage with pump output and reset application
continues below.
21
STAGES
HYST.
STAGE 3
STAGE 2
STAGE 1
0%
25%
200°F
195°F
190°F
THROTTLING RANGE (20°F)
STAGE ENERGIZES
STAGE DE-ENERGIZES
DISCHARGE WATER TEMPERATURE
1
Fig. 25. Staging behavior
°F
220
210
200
190
180
170
160
150
140
20
10
30
40
50
RESET B1
(OUTSD MIN)
SENSOR B
Fig. 26. Reset curve.
STAGE 4
50%
75%
185°F
M28010
70
60
80
°F
RESET B2
(OUTSD MAX)
M24878
63-7147—05
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