23 750-265
Fig. 8. Setpoint and hysteresis.
Whenever the burner turns on, the turn-off threshold is
raised by 18°F,and then it is decreased in steps. The
time of each step is provided by the hysteresis step time
parameter. If the time (T) is not-zero, then the following
schedule is followed until the off threshold reaches its
orignial value:
Whenever the burner turns off, the turn-on threshold is
lowered by doubling the on hysteresis, and then increasing it
by 2 degrees F per step until it reaches its original value.
The time of each step is provided by the CH hysteresis step
time parameter. The number of steps required to reach the
original on hysteresis is the on hysteresis value divided by 2
degrees F.
* Assuming that on hysteresis is sufficiently large.
CH time-of-day setpoint Degrees or None
This setpoint is used when the time-of-day input (J10-2) is on.
If the ODR function is inactive then the setpoint is used as-is.
If the ODR function is active (input on J10-2) then this setpoint provides one coordinate for
the shifted (because TOD is on) outdoor reset curve, as described in “CH outdoor reset
enable” on page 24.
CH off hysteresis
CH on hysteresis
Degrees or None
The off hysteresis is added to the setpoint temperature to determine the temperature at which
the demand turns off.
Similarly, the on hysteresis is subtracted from the setpoint to determine the temperature at
which demand turns on.
These may be set to None to indicate that no setpoint has been defined.
The On and Off hysteresis are adjusted at the time the burner changes from off to on, and
from on to off, as shown in Fig. 8. This gives the PID algorithm some room to be more
aggressive in tracking the load, which can result in overshoot (undershoot).
CH hysteresis step time seconds
Time of each step. A step time of zero - diables this feature.
Table 9. Central Heating Parameters. (Continued)
Parameter Comment
SETPOINT + OFF HYSTERESIS
SETPOINT
SETPOINT - ON HYSTERESIS
BURNER
+18°F
+8°F
+6°F
+4°F
+2°F
1 MINUTE
SETPOINT - 2 * ON HYSTERESIS
1 MINUTE
OFF
ON
ANTI-SHORT-CYCLE DELAY
SETPOINT AND HYSTERESIS
2°F
ANTI SHORT CYCLE DELAY TIME
TOD SETPOINT
SETPOINT
ON HYST.
OFF HYST.
CH PARAMETERS
SYSTEM PARAMETER
TOD SETPOINT
SETPOINT
ON HYST.
OFF HYST.
DHW PARAMETERS
TOD SETPOINT
SETPOINT
ON HYST.
OFF HYST.
LL MASTER PARAMETERS
M24975
Time since turn-on Turn-off threshold
0 <=
time <1T
Setpoint + Off hysteresis + 18°F
1T <=time <2T
Setpoint + Off hysteresis + 8°F
2T <= time <3T
Setpoint + Off hysteresis + 6°F
3T <=time <4T
Setpoint + Off hysteresis + 4°F
4T <=time <5T
Setpoint + Off hysteresis + 2°F
5 <= time
Setpoint
Time since turn-on Turn-on threshold
0<=timer <1T Setpoint - 2 * On hysteresis
1T<=time <2T
Setpoint - 2 * On hysteresis +
1*2°F*
2T<=time <3T Setpooint - 2 * On hysteresis +
2* 2
°F
nT<=time <(n+1)T
Setpoint -2 * On hysteresis + n *
2°F
(on hysteresis/
2F*T<=time
Setpoint
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