Limitation Of Boiler Temperature Increase - Siemens LMS14 User Manual

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6.11.13 Limitation of boiler temperature increase

Use of a speed-controlled pump in heating operation is aimed at meeting the request
for heat at minimum pump speeds whenever possible. Reduced pump speeds lead to
lower volumetric flow. To ensure that reduced volumetric flow is able to meet the
demand for heat, the flow temperature must be increased.
Also, reduced volumetric flow leads to lower return temperatures. Higher flow
temperatures and lower return temperatures result in higher boiler temperature
increases. To protect the boiler from thermal stress due to excessive boiler temperature
increases, the increase is limited. This limitation is accomplished by limiting the boiler
temperature setpoint as a function of the current return temperature.
Limitation of the boiler temperature setpoint means that the flow temperature setpoint
calculated for the heating circuit will not be reached. Speed control for the pump
responds by increasing the speed. As long as the pump is not controlled to the
maximum speed, the boiler temperature increase is limited to the parameterized
nominal increase (Temp differential nominal (2317)).
If the pump is already controlled to the maximum speed, the volumetric flow cannot be
further increased. In that case, the boiler temperature increase is no longer limited to
the nominal but to the maximum increase (Temp differential max (2316)). The transition
from limitation to nominal boiler temperature increase aimed at limiting the maximum
boiler temperature increase is made via the ramp in a linear manner.
The slope of the ramp is determined by the time constant for speed control (Filter time
const speed ctrl (889, 1189, 1489)). For the transition, the time constant is multiplied by
5. If the pump speed returns to a level below the maximum speed, the transition is
made in the other direction with the same time constant.
Limitation of boiler temperature increase
2316
2317
Pump PWM
PWM-Max
PWM-Min
Figure 39: Limitation of boiler temperature increase
User Manual LMS14...
6 The settings in detail
5*
Filter time constant
speed control
7471d70e/0911
Time
Time
CC1U7471en
07.05.2014