1
Service
Service Manual
25
1
Service
(continued)
ModBus (Active / Inactive)
When BMS is set to ACTIVE (see BMS Active / Inactive)
and the boiler is being controlled through ModBus, set
parameter
H4
to ACTIVE. Otherwise, set the ModBus
parameter to INACTIVE. Note that the boiler can still be
monitored by ModBus with this parameter set to
INACTIVE. The default value is INACTIVE.
Modbus T/O (Timeout)
The amount of time the unit controls will wait to receive a
communication string from the BMS controller before
reverting back to its own internal parameters. This
parameter is adjustable by the installer by accessing
parameter
H5.
The default value is 10 seconds.
Cascade Address
The boiler designated as the Leader needs to be
programmed with address 0. All the Member boilers
require addresses from 1 to 7, and the addresses must be
different for each Member. The addresses can be in any
order, regardless of the order in which the units are wired
together. This parameter is adjustable by the installer by
accessing parameter
H6
. The outdoor air (if used) and
system supply sensor must be connected to the Leader
boiler. The default address is 1.
If installing the boilers in an existing system, the new boilers
should be programmed as the Leader and/or the higher
number addresses.
Cascade Type (L/L / EFF)
There are two (2) options for the way a Cascade divides the
load between its heaters. The first is Lead/Lag, designated
at L/L in the menu. This method is used when it is desired
to have the least amount of total flow through the boilers.
This method works much like the current Cascade, except
it will modulate the last two (2) boilers, instead of just the
last boiler. This change provides for smoother transitions
when a boiler turns on or off. When the last boiler reaches
100% and the calculated load is still increasing, it will start
the next boiler at 20% and reduce the previous boiler to
80%, thus eliminating the sudden jump in total output of
the Cascade. Therefore, the dead-zone we had in the
original Cascade is no longer necessary. When the
calculated load is decreasing and the last boiler gets down
to 20% fire, it will hold it there and start lowering the firing
rate on the next-to-last boiler. When the next-to-last boiler
reaches 35%, it will turn the last boiler off and raise the rate
of the next-to-last boiler to 55%, thus eliminating the
sudden drop in total output of the Cascade.
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 = 35%), 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 35%) / 2 =
52.5% in our example).
Maximum Cascade Outlet Set Point
This parameter determines the set point used by the individual
boilers in a Cascade. 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 parameter
H8.
The default maximum cascade set
point is 185°F (85°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
parameter
H9
. The default value is 10°F (6°C).
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 parameter
H10
. The default value is 20°F
(11°C).
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 parameter
H11
. 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 parameter
H12
.
The minimum setting is 0 minutes and the maximum setting is
10 minutes. The default is 60 seconds.
Boiler Size
Not used.
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