Honeywell T775 Series 2000, Application Manual

Page: 14 / 56

T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
63-7147—05 14
Hot Water Reset – T775R
Application Description
The T775R is controlling the boiler water temperature with two
stages (relays), based on outside temperature using a reset
curve and an offset for Relay 2.
Sensor Designation
This device application requires two sensors.
• Sensor A is sensing hot water discharge temperature of
the boiler.
• Sensor B is sensing outside air temperature
Operation
In this example, when the outside temperature reaches
70° F (21° C), the desired water temperature of the boiler is
160° F (71° C). Likewise, when the outside temperature drops
to 20° F (-7° C), the hot water temperature needs to be
210° F (99° C). See Fig. 10.
Fig. 10. Hot water reset curve.
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:
— MOD 1
→
Reset
=
YES-BOILER
— # Relays = 2
— Relay 1
→
Reset = YES-BOILER
— Relay 2
→
Reset = YES-BOILER
Relay 1: Control to the discharge water temperature
Program for: (Refer to the reset curve in Fig. 10.)
— Boiler Max = 210° F (99° C)
— Outside Min = 20° F (-7° C)
— Boiler Min = 160° F (71° C)
— Outside Max = 70° F (21° C)
— Differential = 20° F (-7° C)
— Sensor A
— Heat
Relay 2: Control to the discharge water temperature
Program for:
— Setpoint Offset = -10° F (-12° C) (See Fig. 11)
— Sensor A
— Heat
Fig. 11. Program mode — Relay 2 offset.
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
outside temperature falls below 20° F (-7° C) . As the
outside temperature increases above 20° F (-7° C) ,
the boiler will be reset downward per the reset ratio
until it reaches the minimum setpoint, 160° F.
Assuming an outside temperature of 20° F (-7° C), Fig. 12
describes the actions of relays 1 and 2 to control the boiler
temperature. As the boiler temperature falls below 210° F
(99° C), Relay 1 activates (Relay 1 Differential is 20° F (-7° C),
so relay closes at 190° F (88° C). If Relay 1 cannot raise the
boiler temperature and the boiler temperature continues to fall
to 180° F °, Relay 2 activates (Relay 2 differential is 20° F
(-7° C) 180° to 200° F (82° to 93° C). When the boiler is able
to reach 200° F (93° C), then Relay 2 deactivates and Relay 1
remains active until the temperature reaches 210° F (99° C).
.
Fig. 12. Relay actions.
NOTE: The hot water reset application continues on the next
page.
M24871
SENSOR B
SP MAX A1
(BOILER MAX)
SP MIN A2
(BOILER MIN)
200
140
10
RELAY 2
SETPOINT
OFFSET -10°F
190
20
30 40 50 60
70
°F 80
150
160
170
210
220
180
°F
RESET B1
(OUTSD MIN)
RESET B2
(OUTSD MAX)
S
E
N
S
O
R
A
RELAY 1
SETPOINT
PROGRAM
RELAY 2
MENU
PROGRAM
RELAY 2
OFFSET
-10
F
o
ENTER
SETPOINT
OFFSET
FOR
RELAY 2
M24321
MENU
PROGRAM
RELAY 2
OFFSET
DIFFRNTL
HEAT/COOL
SETBACK
EXIT
M24877
BOILER DISCHARGE
TEMPERATURE
210°
180°
190°
200°
RELAY 1 ON
RELAY 1 OFF
RELAY 2 ON
RELAY 2 OFF
RELAY 1
DIFFERENTIAL
RANGE
RELAY 2
DIFFERENTIAL
RANGE