7 Intelligent Boiler Sequencing - IBS
Mk8 MM Manual Page | 143
brought online. If at the next scan time, the firing rates of the last two online lag MM are 30% and 40%
respectively, and the change down threshold in parameter 86 is set at 80%, then the last lag MM will go into
standby, warming or off depending on how the sequencing mode is set.
If a lag burner fails to start when requested, the scan time will be decreased by the offset set in parameter 1,
until it is automatically ignored from the sequencing communication loop. If a lag burner fails to modulate
after being requested to contribute to the load requirement, then that MM will be ignored from the
sequencing communication loop after a time delay set in parameter 5.
After a power recycle, the number of MMs which are initially set on when the MMs start up again, is set in
parameter 3.
7.3.2 Two Port Valve Operation
In hot water sequencing, when there is no demand for all of the lag boilers to be on, the system will close the
two port valves on the lag boilers that do not need to be on. This will stop the hot water circulating in the
system from passing through the boiler and the heat being wasted. It is possible to use Terminal 78 which is
a switched neutral to control the two port valve. This works by switching to neutral once the burner has
stopped firing but if the temperature of the boiler ever gets above the required set point then Terminal 78 will
switch back even if the boiler is not firing.
• When the two port valve is closed on an OFF lag boiler, if at any time, the residual heat in that boiler
is above the required setpoint, the valve is opened immediately and the heat is let through the
system. The two port valve then stays open.
• When the lead boiler recognises that it needs a lag boiler to come online after the sequence scan
time, the lag boiler will then run its relay tests. This will open the two port valve immediately and it
will then stay open.
For the two port valve to close, the lag boiler must be in standby mode, and the actual setpoint must be on or
below the required setpoint; it must be in this condition for at least one minute.
Terminal 78 on the MM is a switched neutral connection for controlling a two port valve that would normally
be installed in the boilers return pipe connection to the common return header. This facility ensures that
boilers that are switched ‘offline’ do not contribute return temperature water to the flow header thereby
diluting the flow temperature to the building.
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