Siemens RWF40.002B97 - Operating modes; Low-fire operation; High-fire operation

18/57 CC1B7865en 05.12.2002 HVAC Products
5. Operating modes
Low-fire operation means that only small amounts of heat are drawn from the boiler. A 2-
position controller maintains the setpoint, switching the burner on and off like a thermostat.
This mode of control is known as the thermostat function. An adjustable switching
differential ensures that the burner’s witching frequency can be selected, aimed at
reducing wear.
W
HYS1
HYS3
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Modulating and 2-stage operation:
Actual value between «HYS1» and
«HYS3»
High-fire operation means that large amounts of heat are drawn from the boiler so that
the burner is continuously running. If the heating load during thermostat operation rises
to a level where the actual value begins to fall below the switch-on threshold «HYS1»,
the controller will not immediately switch to a higher burner output, but makes a dynamic
test of the control deviation first and switches to the higher output only when an
adjustable threshold «Q» is exceeded (A).
ð Section 5.6 «Response threshold Q»
- In high-fire operation – depending on the application – the burner can be fired in
modulating or 2-stage operation, then burning larger amounts of fuel than in low-fire
operation. The binary input «D1» can be used to switch between modulating and 2-
stage operation
- When contact is open: Modulating burner operation
- When contact is closed: 2-stage burner operation
In diagram area (1), the thermostat function is active. The modulating mode of burner
operation is shown in area (2). In high-fire operation, a modulating controller acts on an
actuator via relay 2 (open) and relay 3 (close).
A
B
q
A
q
(2)
(1)
W
HYS1
HYS3
db
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(3)
In area (3), the actual value exceeds the upper switch-off threshold «HYS3» and the
controller switches the burner off (B). The controller only starts low-fire operation when
the level falls below the switch-on threshold «HYS1» again. If «Q» is exceeded, the
controller switches to high-fire operation (A).
ð Section 5.6 «Response threshold Q»
5.1 Low-fire operation
Thermostat function
5.2 High-fire operation
Operating mode
changeover
5.2.1 Modulating burner, floating output