111/232
Building Technologies
Boiler sequence controller RMK770
CE1P3132en
HVAC Products
7 Boiler control
23.04.2009
The release logic for 2-stage burner operation aims at ensuring an optimum switch-on
time for stage 2 which, in addition to a time criterion, also considers the amount of the
heat deficit, calculated with a temperature-time integral.
As soon as the burner’s basic stage is switched on, the minimum locking time for
burner stage 2 starts to run. This ensures that the burner always operates in the basic
stage for a minimum period of time.
The temperature-time integral is a continuous summation of the temperature differential
over time. In this case, the decisive criterion is the difference by which the boiler tem-
perature falls below the burner’s switch-on setpoint.
50
48
52
°C
44
46
t
a
t
release
TBo
TBoSetp
31
32D08
TBuOffPt
TBUOnPt
a
Release integral
Tbo
Actual value boiler temperature
TboSetp Boiler temperature setpoint
t
Time
TbuOffPt Burner’s switch-off temperature
t
release
Time until release
TbuOnPt Burner’s switch-on temperature
As long as the boiler temperature lies below the switch-on point – after the basic stage
has switched on – the controller builds up the release integral.
When the boiler temperature lies above the switch-on point, the release integral is
reduced again. Owing to the performance of the temperature-time integral, it is not only
the period of time that is considered, but also the extent of undershoot. This means that
when the undershoot is significant, the release according to the integral criterion is
reached earlier than with a small undershoot.
When the release integral (area “a” in the diagram) has reached the set value of the
release integral of stage 2 (point in time t
release
) and the minimum locking time has
elapsed, burner stage 2 is released. During the time burner stage 2 is released, the
controller switches burner stage 2 according to the switching differential.
The logic for locking burner stage 2 is based on the amount of excess heat, which is
also calculated with the help of a temperature-time integral.
As long as the boiler temperature lies above the switch-off point – after burner stage 2
has switched off – the controller builds up the reset integral. When the boiler tempera-
ture lies below the switch-off point, the reset integral is reduced again. The duration and
difference between switch-off point and boiler temperature are summed up.
b
50
48
52
°C
44
46
t
t
reset
TBo
TBoSetp
313
2D09
TBuOffPt
TBuOnPt
b
Reset integral
Tbo
Actual value boiler temperature
TboSetp Boiler temperature setpoint
t
Time
TbuOffPt Burner switch-off temperature
t reset Time to reset
TbuOnPt Burner switch-on temperature
Burner stage 2
Time criterion
Temperature-time
integral
Logic for locking
stage 2