CleaverBrooks CFC-500 Operation, Service And Parts Manual page 159

Parameter
DHW modulation sensor
Plate Heat Exchanger
Plate heat exchanger demand for DHW comes from one of two
sources:
• Tap Demand - detected primarily as a temperature
decrease rate when hot water is "tapped", e.g. when a tap is
turned on.
• Preheat demand - used to keep a plate exchanger
preheated so it is warm enough to be ready to provide hot
water, and also so that Tap demand can be detected (it has
to be warm if a temperature drop rate is to be detected).
One of the selections for the DHW demand switch parameter is
"Plate Heat Exchanger". This selection acts as an enable for
the Tap Demand and Preheat Demand subsystems. If this
choice is selected then the logic described in Fig. 15 and Table
13 is used to generate DHW demand; however if this is not
selected, then the logic is inactive and does not apply.
Table 11. Domestic Hot Water Parameters. (Continued)
Inlet (S1), Outlet (S3S4), DHW (S6S7), Auto DHW (S6) or Inlet(S1), Auto DHW (S6) or Outlet
This parameter selects the source of modulation control for the DHW system. If the selected
input is not a temperature (e.g. S1 is steam pressure for a steam control) then an alert occurs
and the DHW control subsystem is suspended.
• If Inlet is selected then the sensor on J8 terminal 4 provides DHW temperature.
• If Outlet is selected then this sensor controls DHW modulation.
• If DHW (S6S7) is selected then this sensor controls DHW modulation.
• If one of the two Auto: DHW(S6) or Inlet(S1) or DHW(S6) or Outlet options is selected,
then the modulation sensor is determined by the automatic detection function described
for the DHW demand switch parameter.
• If Auto DHW (S6) or Inlet is selected then the Inlet sensor is used if DHW (S6) is a
heat demand switch input.
• If Auto: DHW (S6) or Outlet is selected then the Outlet sensor is used if DHW (S6) is
a heat demand switch input.
Comment
Tap Demand
For a plate-type heat exchanger, a set of parameters is used to
detect demand when the DHW system is tapped (as in turning
on a tap). This tapping is detected either as a drop in DHW
temperature that exceeds a certain rate, or as a temperature
threshold that is exceeded (on the low side) for a period of
time. When either of these events occurs, tap demand
becomes True. Once tap demand is True, it remains on for a
minimum time. At the end of this time tap demand will end
when one of two criteria occurs, based upon comparing the
Inlet temperature to the DHW and Outlet temperatures.
Because tap demand has two criteria for starting, and two
other criteria for stopping, it is modeled as a Set/Clr block
driven by two OR gates, which in turn are connected to the four
criteria sources. The tap demand is also modeled to have a
"force" input, which forces it to recognize a "Set" event: this is
used when Preheat has had control and is now relinquishing
this control to Tap demand.
The plate heat exchanger subsystem will operate as shown in
Fig. 15
35
CB FALCON
750-265