Siemens RVL471 - Frost Protection for the Boiler

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Siemens Building Technologies Basic Documentation RVL471 CE1P2524E
HVAC Products 12 Function block "Boiler" 23.10.2002
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 x < 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
1
0
INT
max.
FG
B2
1
0
Y
B2
t
FGI
RSI
2522D09
0
1
0
t
t
t
t
Y
B1
max.
T
Kw
+ 0,5
*
SD
T
Kw
- 0,5
*
SD
T
Kx
RSI
FG
B2
Release of burner stage 2
FGI Release integral
INT Integral
RSI Reset integral
SD Switching differential
tTime
TKw Boiler temperature setpoint
TKx Actual value of boiler temperature
Y
B1
Control signal for burner stage 1
Y
B2
Control signal for burner stage 2
12.4.3 Frost protection for the boiler
Frost protection for the boiler operates with fixed values:
• Switch-on point: 5 °C boiler temperature
• Switch-off point: Minimum limit of the boiler temperature plus switching differential
If the boiler temperature falls below 5 °C, the burner will always be switched on until the
boiler temperature has crossed the minimum limit by the amount of the switching differ-
ential.