Multi-Stage Boiler Control (Reset) - T775P - Honeywell T775 Series Application Manual And Cross Reference

Multi-Stage Boiler Control (Reset) – T775P
Application Description
The T775P is providing multistage boiler control based on the
boiler's discharge water temperature. The T775P uses the
fourth output relay to energize the primary pump.
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 is used to control
3 boiler 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
In this 3-stage example, as the heating load increases,
additional stages of heat will cycle ON as the boiler water
temperature decreases. The T775P will stage three boilers to
provide sufficient heating. See Fig. 20 for staging behavior.
The primary circulating pump energizes whenever any stage
is energized.
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 = 3 (T775 assigns pump to Relay 4)
→
— Options Reset = YES-BOILER
→
— Options On Delay and Off Delay:
Seconds = 0 to 3,600 (default is 0)
→
— Options WWSD = YES or NO
Temperature = 30 to 100° F (-1 to 38° C)
— STG4/Pump: (Relay 4 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)
Return to the Setup menu, and select Alarms:
— High Alarm = YES
— High Limit = 220° F (104° C)
Stages 1-3: Control to the discharge water temperature
Program for: (Refer to the reset curve in Fig. 21.)
— 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 = 18° F (-8° C)
— Sensor A
— Heat
— Setback = -10° F (-12° C) (Optional)
1
See Notes in "Programming Example" on page 18 for an
explanation of Prepurge and Postpurge times
T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
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.
STAGE 3 ON
STAGE 2 ON
STAGE 1 ON
1 206°F
STAGE ENERGIZES
1 DISCHARGE WATER TEMPERATURE
Fig. 20. Boiler Control Staging Behavior (when effective
SP MAX A1
(BOILER MAX)
S
E
N
S
O
R
A
SP MIN A2
(BOILER MIN)
1
Fig. 21. Reset Curve with Optional Setback Offset.
Wiring
All output relays should have a common power wiring source,
which may or may not be the same as the T775 power wiring.
See Fig. 22 on page 20.
NOTE: The Multi-Stage Boiler Control (Reset) application
continues on the next page.
19
STAGES
HYST.
STAGE 3
STAGE 2
STAGE 1
-33% 0% 33%
200°F 194°F
THROTTLING RANGE (18°F)
STAGE DE-ENERGIZES
setpoint = 200° F).
°F
220
210
200
190
180
170
SETBACK
OFFSET
160
-10°F
(OPTIONAL)
150
140
20
10 30 40 50
RESET B1
(OUTSD MIN)
SENSOR B
67%
188°F
M28043
70
60 80 °F
RESET B2
(OUTSD MAX)
M24869
63-7147—3