Control of Heat Generator Sequencing

The heating control strategy module can switch 1 or 2 heat generator stages at the same time depending on the difference between the calculated system supply setpoint and the actual system supply temperature. The control region of following stages is defined by parameter SST-10, while the switching of the next heat generators is based on a temperature differential as defined by parameters SST-11 and SST-12 and shown in the next figure. When the system supply temperature is within the control region the temperature control is only done by switching the second or modulating stage.

Fig. 7.   Heat generator demand

The following conditions must be true to switch on and off heat generator(s) in sequential or parallel mode (see also previous figure).


Sequential mode:

To switch on 1 heat generator stage:


To switch on 2 heat generators stages:


To switch off 1 heat generator stage (optimised off):

or


To switch off 2 heat generators:

Parallel mode

To switch on 1 heat generator stage:


To switch on 2 heat generator stage:


To switch off 1 heat generator stage:


To switch off 2 heat generators:


Heat generators can be switched in sequential mode with or without optimized off function or in parallel mode. The sequential or parallel mode is selectable by parameter OPM-02, according to next table.

Table 6.   Heat generators group operation mode

OPM-02

Heat generators group operation mode

0; 1

sequential mode

2

sequential mode, last two active boiler switch off in parallel mode

3

parallel mode


ATTENTION:

Whenever the user changes the operation mode parameter OPM-02, the strategy will automatically reset and start the heat generator sequence again.


In sequential mode both stages of a heat generator must be on before a next heat generator can be started.

In case of modulating boilers only the last boiler modulating stage will be controlled and all the others run at full power.

In parallel mode all 1st stages of the heat generators must be switched on before a 2nd stage of a heat generator can be started.


The following chapters describe the heat generator sequences for the three operation modes, for the cases that only one heat generator stage is switched on at the same time.

Sequential mode:

Sequential mode means that the heat generators in a group start in sequence and stop in sequence. In sequential mode both stages of a heat generator must be “On” before a next heat generator can be started, except when two heat generators have to switched on at the same time.

See an example of 3 staged heat generators in sequential in the next figure:

Fig. 8.   Staged boilers sequential mode

BL1; BL2 and BL3 refer to boilers 1 to 3

ST1 and ST2 refer to stage 1 and stage 2

ST1-04, ST2-04 are the blocking times to switch on heat generators

ST1-05 and ST2-05 – are the blocking times to switch off heat generators


Sequential mode, last two active boilers switch off in parallel mode:

This operation mode will keep the last two boiler in stage 1 when switching off. This can increase the heat generator efficiency for some boiler types avoiding too frequent total switch off. Sequential mode means that the heat generators in a group start in sequence and stop in sequence. In sequential mode both stages of a heat generator must be “On” before a next heat generator can be started, except the case when two heat generators have to be switched on at the same time.

When switching off, always the last two boilers have to be in stage 1 before a boiler will be switched off. See an example of 3 staged heat generators in the next figure:

Fig. 9.   Sequential mode, last two active boilers switch off in parallel mode

BL1; BL2 and BL3 refer to boilers 1 to 3

ST1 and ST2 refer to stage 1 and stage 2

ST1-04, ST2-04 are the blocking times to switch on heat generators

ST1-05 and ST2-05 – are the blocking times to switch off heat generators

Parallel mode:

In parallel mode the heat generators start and are switched off in parallel mode.


Fig. 10.   Parallel mode

BL1; BL2 and BL3 refer to boilers 1 to 3

ST1 and ST2 refer to stage 1 and stage 2

ST1-04, ST2-04 are the blocking times to switch on heat generators

ST1-05 and ST2-05 – are the blocking times to switch off heat generators


When two heat generators or generator stages have to switch on at the same time, we always consider that we cannot start both stages at the same time. This means that depending on the initial state, we can switch on:

Table 7.   Switch on two stages or heat generators at the same time

Sequential mode

Parallel mode

Initial state

final state

Initial state

final state

ST1 = 0001

ST2 = 0001

ST1 = 0111

ST2 = 0001

ST1 = 0000

ST2 = 0000

ST1 = 0011

ST2 = 0000

ST1 = 0011

ST2 = 0001

ST1 = 0111

ST2 = 0011

ST1 = 0111

ST2 = 0000

ST1 = 1111

ST2 = 0001

ST1 = 0111

ST2 = 0001

ST1 = 0111

ST2 = 0111

ST1 = 1111

ST2 = 0001

ST1 = 1111

ST2 = 0111

Table 8.   Parameter List

Description

Value Range

Default Value

Unit

SST-09: time to reach setpoint

1...600

6

min

SST-10: control region following stages

1...20

5

K

SST-11: temperature span switching on boilers

2...30

20

K

SST-12: temperature span switching off boilers

2...30

10

K

OPM-02: group 1 operation mode:

0; 1 = sequential

2 = sequential mode, last two boiler switch off in parallel mode

3 = parallel

0...3

1

-


The operation mode will be transmitted between the heat generators and heating strategy via communication signal which will connect automatically.