750-265 24
Fig. 9. CH outdoor reset with TOD.
DHW Loop Demand & Rate (DHW DR)
The Domestic Hot Water (DHW) Demand and Rate source
compares a sensor to a setpoint.
A Burner demand will exist if the sensor temperature falls
below the setpoint minus a hysteresis value. Once the burner
demand signal is on, it remains on until the sensor
temperature is above the setpoint plus a hysteresis value, or
until the other selected demand source input (i.e. Remote Stat
or DHW Switch), if any, turns off.
Pump demand may be driven by the a remote stat, or by the
sensor alone.
A Proportional-Integral-Differential controller operates to
generate the source's requested modulation rate.
Table 9. Central Heating Parameters. (Continued)
Parameter Comment
CH P-gain
CH I-gain
CH D-gain
0-100
These parameters are the gains applied to the proportional, integral, and differential terms of
the PID equation for the CH loop.
CH outdoor reset enable Enable, Disable
If outdoor reset feature is enabled and the sensor is functioning properly, then the current
outdoor temperature is used to determine the setpoint by interpolation.
This lookup function uses two X,Y points to determine a line on the graph as shown in Fig. 9.
The Y coordinate of the top-right point depends on the time-of-day input (J10-2):
• If it is off then the CH setpoint is used.
• If it is on the CH TOD setpoint provides the Y coordinate and the lower left point is also re-
calculated to shift the graph in a parallel way, as shown in Fig. 9.
For outdoor temperatures lower than the minimum, the water temperature provided by the
appropriate setpoint is used.
For outdoor temperatures higher than the maximum, the minimum water temperature is used.
For outdoor temperatures between the minimum and maximum, a linear interpolation is used
to find the setpoint.
SETPOINT TEMPERATURE
OUTDOOR TEMPERATURE
180
160
140
120
100
80
60
40
0 20 40 60 80
(MIN OUTDOOR,
CH SETPOINT)
18°F
(MAX OUTDOOR, MIN WATER)
(MIN OUTDOOR,
CH TOD SETPOINT)
CH OUTDOOR RESET WITH TOD AND BOOST
TOD SETPOINT
SETPOINT
MINIMUM OUTDOOR TEMP
MAXIMUM OUTDOOR TEMP
MINIMUM WATER TEMP
CH ODR PARAMETERS
PARALLEL SHIFT
TOD = OFF
TOD = ON
BOOST MAX
SETPOINT
(COULD BE
HIGHER THAN
CH SETPOINT)
18°F
BOOST TIME
M24976
BOOST MAX SETPOINT
Table 9. Central Heating Parameters. (Continued)
Parameter Comment
CH ODR minimum water
temperature
CH ODR maximum outdoor
temperature
Degrees or None
These two parameters determine the lower left point on the graph.
CH ODR boost time
CH ODR boost max setpoint
mm:ss
Degrees or None
If CH outdoor reset is not active or if the CH ODR boost time parameter is zero, then the
boost function is inactive.
Otherwise, the boost time provides a time interval. Each time this time interval elapses and
demand is not satisfied, then the setpoint is increased by 18°F, up to the maximum provided
by the CH ODR boost max setpoint. However, if the latter is not a valid temperature for
boosting the setpoint, then the boost function is inactive and an alert is issued.
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