42
DELTA T RESET (RETURN WATER RESET)
The control system is also capable of performing fluid tem-
perature reset based on evaporator fluid temperature difference
(Delta T), sometimes called return water reset. Because the
change in temperature through the evaporator is a measure of
the building load, the temperature difference reset is, in effect,
an average building load reset method.
Delta T Reset allows for the chilled water temperature set point
to be reset upward as a function of the fluid temperature differ-
ence (building load).
NOTE: Delta T (Return Water) Temperature Reset should not
be used with variable evaporator flow rate systems.
To use Delta T Reset, four variables must be configured: Cool-
ing Reset Select, Delta T No Reset Value (evaporator tempera-
ture difference at which no chilled water temperature reset
should occur), Delta T Full Reset Value (evaporator tempera-
ture difference at which the maximum chilled water tempera-
ture reset should occur), and Cooling Reset Deg Value (the
maximum amount of temperature reset desired).
To configure this option with the Carrier Controller display:
In the example in Fig. 40 using Return Water Temperature Re-
set, the chilled water temperature will be reset by 5°F (2.8°C)
when the Fluid Temperature Difference is 2°F (1.1°C) and 0°F
(0°C) reset when the Temperature Difference is 10°F (5.6°C).
Fig. 40 — Example: Return Water Reset
DISPLAY
NAME
PATH VALUE
Cooling Reset
Select
Main Menu
Configuration Menu
Reset Configuration
Default = 2
0=None,
1=OAT
2=Delta T,
3=4 to 20mA control
4=Space Temp
Delta T No
Reset Temp
Default = 0°F (0°C)
Range 0°F to 25°F
(0°C to 13.8°C)
Delta T Full
Reset Temp
Default = 0°F (0°C)
Range 0°F to 25°F
(0°C to 13.8°C)
Cooling Reset
Deg Value
Default = 0°F (0°C)
Range –30 to 30°F
(–16.7 to 16.6°C)
0
1
2
3
4
5
6
0
ENTERING-LEAVING WATER TEMPERATURE (deg F)
DEGREES RESET (deg F)
Cooling
Reset
Deg. Value
(cr_deg)
Delta T No Reset Temp
(dt_hr_no)
Delta T Full Reset Temp
(dt_hr_fu)
2 4
6 8
10
a30-5886
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