7 Intelligent Boiler Sequencing - IBS
Mk8 MM Manual Page | 142
7.3 Hot Water Sequencing
7.3.1 Implementing Hot Water Sequencing
For hot water sequencing, a temperature detector must be fitted to all the MMs (only Master MMs in Multi-
Burner operation if option 14 is set to 1 – Mk8 MM only) and option 1 must be set to 0 or 5.
As sequencing is based on firing rate, the MMs must have fuel flow metering entered, see option 57. The
burner rating is set as point 1 high fire in fuel flow commissioning. See section 3.5 for more information on
fuel flow commissioning.
The MMs can be configured for sequencing either in Commissioning Mode, or Online Changes; this allows
the commissioning engineer to implement/adjust sequencing later after the burners have been
commissioned.
Each MM in the sequencing loop must be set with an individual ID number through option 33; no two MMs
can have the same ID number in sequencing, Multi-Burner operation, and when connected to a DTI/Modbus
interface. The highest MM ID number should be set for that sequencing loop in parameter 57, so the system
only looks for communications with these MMs. The maximum number of MMs that can be in a sequence
loop is 10. If there is a DTI in the sequence loop, to control the sequence loop via the DTI, parameter 101
must be set to 1.
To enable sequencing, option 16 must be set to 1, or 3 for sequencing with DTI. If option 16 is set to 3, then
the DTI/Modbus interface is capable of some remote control. The individual and global required setpoint,
lead boiler select, sequence order, enable/disable and firing rate can be set remotely. If an MM’s firing rate is
set by the DTI/Modbus interface, then that MM will not follow the sequencing loop.
For hot water sequencing, option 53 must be set to 0 to disable the standby warming which is used in steam
sequencing. Options 40 and 41 must be set to 0 on all the MMs in the sequencing loop. If warming is
required for lag hot water boilers, then hot water sequencing can function like steam sequencing by setting
option 62.
In a sequence loop, there is one lead burner, and the rests are lag MMs. The lead burner identifies its own
firing rate by looking at its fuel flow metering data, proportional to the system’s load requirements. Having
established the percentage firing rate and maximum heating capacity, the MM will calculate the amount of
heat being contributed to the system by this burner.
The sequence scan time, see option 35, sets after how long the firing rates of all the MMs in the loop are
assessed. The scan time has a critical effect on the responsiveness of the sequencing system. Too long a
scan can result in the boilers not coming online quick enough to meet the load demand; too short a scan
time (shorter than the burner start-up time) can cause another boiler to be brought online before the previous
lag boiler has started firing. The scan time should normally be set at minimum, the start-up time for the
burner.
The lead burner looks at its firing rate and sends a command to the lag MMs to either contribute to load
because it cannot reach the setpoint, or to stop contributing to the load because the system has met the load
demand. Only one lead burner can be selected at one time, if more than 1 is selected as lead burner, then
the MMs will ignore the sequencing loop commands and return to independent firing. Parameter 2 sets how
often the ‘bus driver’ MM requests and transmit information to the other MM The ‘bus driver’ is always the
MM with the lowest ID number.
The MMs will start, continue or stop contributing to the load based on the change up and down thresholds,
see parameter 86 and 87. The next lag MM will be brought online if the lead burner cannot cope with the
load demand, and its firing rate is above the change up threshold. Alternatively, the MM will go into standby,
warming or offline if the last two lag MMs have a total combined firing rate less than the change down
threshold, because the system can cope with the load demand.
For example, if the change up threshold in parameter 87 is set at 90%, then if the last firing MM in the
sequence is above 90% firing rate, then upon the elapse of the next scan time, the next lag MM will be
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