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Building Technologies Division User Manual LMS14… CC1U7471en
6 The settings in detail 28.01.2015
6.39.6 Limitation to the maximum differential
If the modulating heating circuit pump is controlled to maximum speed, speed control is
no longer capable of increasing the volumetric flow. In general, the heating circuit pump
should be controlled at reduced speeds. For this reason, when the maximum speed is
reached, adherence to the nominal boiler temperature differential is abandoned and
only the parameterized maximum value Temp differential max (2316) is used for limiting
the boiler’s temperature differential.
But limitation of the boiler’s temperature differential to the maximum is not raised
abruptly, but rather slowly in a linear manner from the last valid limitation level to the
maximum.
Raising of the boiler’s temperature differential to the maximum follows a ramp. The
slope of the ramp depends on the time constant used for speed control (Filter time
const speed ctrl (889/1189/1489)) and the difference between the nominal and the
maximum value of the boiler temperature increase. The ramp is defined such that the
transition from the nominal to the maximum differential is made with factor 5 of the
relevant time constant.
If, for instance, a time constant of 3 minutes is parameterized for speed control, a
nominal value of 20 °C and a maximum value of 35 °C for the boiler’s temperature
increase, the boiler temperature differential is raised by exactly 1 K per minute until the
maximum value is reached.
Line no. Operating line
889 Filter time const speed ctrl
1189 Filter time const speed ctrl
1489 Filter time const speed ctrl
2316 Temp differential max
2317 Temp differential nominal
Line no. Operating line
8310 Boiler temp
8311 Boiler setpoint
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