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Tap During Preheat
Although preheat cannot become True while Tap demand is
true, it is possible that Tap demand may occur after Preheat
has started. If the conditions for Tap demand are met, that is, if
tap demand does become true during preheat, this is noted by
setting the Tap during Preheat flag (which was cleared initially
when preheat began).
If this flag is set:
• If the Inlet temperature is less than the temperature it had
when preheat started, and the 1 minute timer is not already
running, then, a 1 minute timer is started.
• If any of these occur:
• The 1 minute timer is running, and it elapses, OR
• The DHW temperature equals or exceeds the Inlet
temperature, OR
• Both:
DHW sensor temperature >= TOFF, AND
The preheat minimum on time has expired
• Then:
• Tap demand is forced on, that is, a “Set” event is
generated for it, which starts its minimum on timer.
• Preheat becomes false (inactive).
Preheat Modulation control
Preheat provides its own setpoint and hysteresis values.
These are used by the burner on/off hysteresis logic in place of
the normal DHW values, as shown in Table 15.
However, the preheat function does not modulate and does not
use the PID function. Whenever preheat is active, the
minimum modulation rate is used. (As usual, a modulation rate
of 0% may be used as an output because this value always will
be clipped to the minimum modulation rate by the rate limit
section.)
When preheat ends the DHW PID integrator will be restarted
since it may have accumulated a value during the preheat time
which is not relevant because it was not in control. (This is
done in the same way as for the end of an override: preheat is
essentially a rate override.)
DHW Storage
DHW Storage provides a source of demand for the DHW
system that will keep the DHW pump on and maintain the
water temperature for a programmable period of time after the
normal DHW demand has been satisfied. The DHW storage
feature has its own setpoint and hysteresis values, so they can
differ from the values used during normal DHW demand.
Plate preheat on hysteresis Degrees or None
The preheat on threshold is calculated as:
T
ON
= Plate preheat setpoint - Plate preheat on hysteresis
If the preheat block is False, then it is Set (becomes True) when:
1. Tap demand is false, AND
2. The preheat delay-after-tap time has elapsed, AND
3. DHW sensor temperature <= T
ON
, AND
4. The above have remained true for the time specified by:
Plate preheat on recognition time
That is, whenever conditions 1, 2, or 3 are not true, a preheat recognition timer is reset.
Whenever they are all true then the timer is allowed to run. If the time elapses then the
preheat block becomes true (preheat is active, and this causes the plate demand to be true).
Whenever preheat first becomes active, the Inlet temperature is sampled and saved, a Tap
during Preheat flag is cleared, and a 1 minute timer is marked as inactive. (All of these are
used by the Tap during Preheat logic.)
Whenever preheat demand becomes true, a minimum on timer is started to measure the time
specified by the Plate preheat minimum on time parameter. Preheat demand will remain true
until this time elapses (except that it may convert to Tap demand under the conditions
described for “Tap during Preheat”). If preheat loses control of the burner due to priority, the
minimum on timer will be restarted so that it provides the full minimum on time, when priority
returns to preheat.
Plate preheat minimum on time mm:ss or None
This parameter provides the minimum on time for preheating.
Plate preheat off hysteresis Degrees or None
The preheat off threshold is calculated as:
T
OFF
= Plate preheat setpoint + Plate preheat off hysteresis
If the preheat block is True, then it becomes False when:
• Tap during Preheat is recognized (see below) OR
• Both
• DHW sensor temperature >= T
OFF
, AND
• The preheat minimum on time has elapsed.
Table 13. Preheat Demand Parameters. (Continued)
Parameter Comment
Table 14. Preheat Modulation Values.
Preheat Inactive Preheat is active
Setpoint DHW setpoint Plate preheat setpoint
Hysteresis On DHW off hysteresis Plate preheat on
hysteresis
Hysteresis Off DHW off hysteresis Plate preheat on
hysteresis
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