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Building Technologies
Modular Heating Controller RMH760B
CE1P3133en
HVAC Products
6 Boiler temperature control
05.02.2007
The release logic for 2-stage operation aims at ensuring an optimum switch-on time for
stage 2 which, in addition to a time criterion, also considers the amount of 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 will always operate with the
basic stage for a certain 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
TBoSetpt
313
2D
08
TBuOffPt
TBUOnPt
a Release
integral
TBoSetpt Boiler temperature setpoint
TBuOffPt Burner’s switch-off temperature
TBuOnPt Burner’s switch-on temperature
TBo
Actual value of the boiler temperature
t
Time
t
release
Time to release
As long as the boiler temperature lies below the switch-on point – after the basic stage
has been switched on – the controller will build up the release integral. If the boiler
temperature lies above the switch-on point, the controller will reduce the release inte-
gral. Through the generation 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 after the integral criterion will be reached earlier
than with a small undershoot.
When the release integral (area “a” in the diagram) reaches the set value of the release
integral of stage 2 (point in time t
release
) and the minimum locking time has elapsed,
stage 2 will be released. During the period of time burner stage 2 is released, the
controller will activate and deactivate stage 2 according to the set 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 the second
stage has been switched off – the controller will build up the reset integral. If the boiler
temperature lies below the switch-off point, the controller will reduce the reset integral.
The duration and the difference between switch-off point and boiler temperature will be
summed up.
Burner stage 2
Time criterion
Temperature-time integral
Logic for locking stage 2
