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Siemens
Heating and D.h.w. Controller RVL481
CE1P2541en
Building Technologies
12 Function block "Boiler"
20.05.2008
12.4.2 Control with a 2-stage burner
For 2-position control with a 2-stage burner, the variables that can be set are the
switching differential and the minimum burner running time – which now apply to both
stages – plus the following variables:
•
The release integral (FGI) for the second stage. This is the variable generated from the
temperature (T) and time (t). If the maximum limit is exceeded, the second burner
stage is released and can switch on, provided the minimum waiting time for the second
stage has elapsed. Prerequisite is that the minimum locking time for the second stage
has elapsed.
FGI =
∆
T dt
t
0
∫
where:
∆
T
= (w
−
0,5 × SD
−
x) > 0
•
The reset limit (RSI). This is the variable generated from the temperature (T) and time
(t). If the maximum limit is exceeded, the burner will be locked and switches off
RSI =
∆
T dt
t
0
∫
where:
∆
T
= (x
−
w + 0,5 × SD) > 0
•
The minimum locking time for the second stage, which is the period of time on comple-
tion of which the second stage can switch on at the earliest
The controller compares the actual value of the flow temperature with the setpoint. If it
falls below the setpoint by half the switching differential (x < w
−
0.5 × SD), the first
burner stage will be switched on. At the same time, the minimum waiting time for the
second burner stage commences and the release integral is being generated. The
controller ascertains for how long and by how much the flow temperature remains be-
low w
−
0.5 × SD. It continually generates the integral based on the time and the pro-
gression of temperature.
If, on completion of the minimum locking time, the flow temperature is below
w
−
0.5 × SD, and if the release limit reaches the set maximum limit, the second burner
stage will be released and switched on. The flow temperature starts rising.
When the flow temperature has exceeded the setpoint by half the switching differential
(x = w + 0.5 × SD), the second burner stage is switched off again, but will remain re-
leased. The first stage continues to operate. If the flow temperature drops, the second
stage will be switched on again at × < w
−
0.5 × SD. The setpoint is now maintained by
the second burner stage.
If, however, the flow temperature continues to rise (x > w + 0.5 × SD), the controller
starts generating the reset integral. It determines for how long and to what extent the
flow temperature stays above the setpoint by half the switching differential. It continu-
ally generates the integral based on time and the progression of temperature. When the
reset limit reaches the set maximum limit, the second burner stage will be locked and
the first stage switched off.
The minimum locking time and calculation of the release integral at × < w
−
0.5 × SD
are started when the switch-on command for the first burner stage is given.
Due to the time-temperature integral, it is not only the duration of the deviation that is
considered, but also its extent, when deciding whether the second stage shall be
switched on or off.
SD Switching
differential
w
Boiler temperature setpoint
x
Actual value of boiler temperature
Setting parameters
Control process
