Knight WH 399, Service Manual

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Service Manual
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Service
(continued)
The other Cascade divider method is Efficiency Optimization,
designated as EFF in the menu. This method is used, as the
name implies, when it is desired to have the most efficient
system. When the first boiler reaches a certain rate (default =
90%), it lowers its rate to 45% and turns on the next boiler at
45%. The two (2) boilers then modulate at the same rate.
As the calculated load increases further and both boilers ramp
up to 90%, it lowers the rate of the first two (2) boilers to 60%
and brings the next boiler on at 60%.
The three (3) boilers
then modulate together. As the calculated load decreases, the
boilers will reach a lower threshold (default = 30%), at which
time the last boiler (the third in our example) will turn off and
the Cascade will increase the rates of the remaining boilers to
provide the equivalent total output as before ((3 x 30%) / 2 =
45% in our example).
Efficiency optimization is automatically selected when boilers
of different sizes are programmed into the Leader control (see
Boiler Size on this page).
Maximum Cascade Set Point
This parameter determines the set point used by the individual
boilers in a Cascade when a system sensor is connected to the
Leader boiler. When a boiler is commanded to fire by the
Leader boiler, it will attempt to achieve this temperature at its
outlet. The Leader boiler will limit the modulation of the last
boiler to fire in order to hold the temperature at the system
supply sensor to the user set point. If any of the boiler outlet
temperatures reach the maximum cascade set point, the boiler
will then modulate down on its own in order to keep its outlet
temperature within the maximum cascade set point. Therefore,
this parameter can be used to limit the outlet temperatures
of all the boilers in a Cascade. Note that this parameter does
not apply when the boiler is heating an indirect DHW tank.
This parameter is adjustable by the installer by accessing the
Maximum Cascade Set point parameter . The temperature range
of this parameter is 32°F (0°C) to 180° (82°C). The default
maximum cascade set point is 180°F (82°C).
Cascade Offset
This parameter determines how much the temperature must
go above set point before the lead boiler will turn off. This
parameter can be adjusted by the installer by accessing the
Cascade Offset parameter . The temperature range of this
parameter is 0°F (0°C) to 20° (11°C). The default value is 10°F
(6°C). In Cascade, this parameter will override the boiler offset
parameter.
Cascade Off-On Differential
This parameter determines how much the temperature must
go below the turn off temperature (Set point + Offset) before
the lead boiler turns on. This parameter can be adjusted by the
installer by accessing the
Cascade Off-On Differential parameter .
The temperature range of this parameter is 0°F (0°C) to 60°
(33°C). The default value is 20°F (11°C). In Cascade, this
parameter will override the boiler differential parameter.
Minimum On/Off Time
In order to prevent units in a Cascade from short cycling,
this parameter defines the minimum ON and OFF time for
each unit. The installer can adjust this time by accessing the
Minimum On/Off Time parameter . The minimum setting is 0
seconds and the maximum setting is 10 minutes. The default
is 30 seconds.
Minimum Next On Time
In order to reduce the risk of temperature overshoot with
a Cascade, this parameter defines the minimum time delay
from starting one unit until the next unit may be started. The
installer can adjust this time delay by accessing the
Minimum
Next On Time parameter . The minimum setting is 0 minutes
and the maximum setting is 10 minutes. The default is 60
seconds.
Boiler Size
When boilers of different sizes are connected together in a
Cascade, the Leader boiler has to know the size of each boiler
in that Cascade. The
Boiler Size parameters allow the installer
to program the size based on the Cascade address. This screen
shows the Cascade address and the size of the boiler with that
address (in BTU/hr):
1. When the Boiler Size screen is first accessed, Cascade
Address 0 (Leader) is shown.
2. Press the NAVIGATION dial to access the first digit of the
boiler size. Rotate the NAVIGATION dial to adjust the
first digit. To access the next digit, press the NAVIGATION
dial again. Continue this process until the correct boiler
size (to the nearest 1000 BTU/hr) is shown. Press the
RIGHT SELECT [SAVE] key.
3. Rotate the NAVIGATION dial to select the address of the
next boiler in the Cascade. Repeat Step 2.
4. Once the size of the last boiler in the Cascade has been
entered and saved, press the LEFT SELECT [EXIT] key to
return to the Control Modes menu.
5. If no other parameters are to be adjusted, press the RIGHT
SELECT [HOME] key to save the new settings and return
to the Status screens.
The SMART SYSTEM control automatically uses the Efficiency
Optimization Cascade type when controlling boilers of different
sizes.
Circulation pumps
System Pump Delay
The system pump delay parameter sets the length of time the
system pump (if connected) will run after a SH demand has
been satisfied. This parameter is adjustable by the installer by
accessing the
System Pump Delay parameter . The time range
for this parameter is 0.1 second to 40 minutes. The default
time is 30 seconds.
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