Electronically Controlled Pwm Heating Circuit Pump; General; Introduction; Notes - Siemens LMU54 Series Basic Documentation

General

Introduction

Notes

Task of ∆T control
with PWM pump
Siemens Building Technologies
HVAC Products
3.8 Electronically controlled PWM heating circuit
pump
In heating systems, the return temperature is the result of the
• amount of heat currently consumed,
• adjusted pump speed (multispeed pump), and
• current piping network characteristics
In the case of condensing boilers, well defined low return temperatures are required in
order to improve the efficiency of condensation.
However, the temperature differential between flow and return (∆T) should not be too
great since the heat exchangers should keep the differential within given limits.
Also, the maximum pump speed is hardly ever required (saving electrical energy and
reduction of noise in connection with thermostatic radiator valves).
∆T control acts on the PWM heating circuit pump and on the flow temperature setpoint :
1st stage: Maximum limitation of the flow temperature
∆T limitation
2nd stage:
∆T supervision
3rd stage:
A description of the control functions of stages 1, 2 and 3 is given in the following.
• A heating circuit pump with PWM is only supported in those hydraulic systems that
have a PWM pump included (refer to chapter 10).
If the selected hydraulic system that does not permit the use of a PWM pump, the
connected and parameterized pump will be controlled in a correspondingly stepwise
fashion via the PWM line
• Control of the PWM pump is based on weather-compensated flow temperature
control according to the heating curve.
If no weather compensation is active (no outside sensor connected), the default value
used is an outside temperature Ta of 0 °C on which the calculations will be based
If a modulating pump is used, adjustment of the pump speed alone does not suffice.
A different pump speed changes the volumetric flow through the heating plant and thus
the amount of heat delivered by the heating system.
This means that when the volumetric flow changes, the flow temperature must be
adjusted to the new situation.
In other words, it must be ensured that, at a given operating point, the amount of heat
delivered by the heating system must be the same although the volumetric flow
changes.
Since in the case of weather compensation, the heating curve defines the flow
temperature for a maximum volumetric flow, ∆T control should be based on that setting.
Basic Documentation LMU54... / LMU64...
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CC1P7494en
07.11.2002