Downdraft Compensation - Siemens Desigo RXB Technical Manual

Function
Note
Controller output
LTHW radiators with
motorized valve actuators
LTHW radiators with
thermal valve actuators
Note
Siemens
Building Technologies
8.1.5

Downdraft compensation

This function is only active in Comfort mode.
In situations where (owing to large internal heat gains) there is no heating demand from
the room despite a low outdoor temperature (supplied via bus), large window surfaces
can impair indoor comfort (through radiated cold, downward flow of cold air,
condensation).
A radiator located under the window can be used to slow the downward flow of cold air
and compensate for cold radiation.
To achieve this, the radiator is switched on whenever the outdoor temperature drops
below a predefined value (the outside temperature 0% valve position).
The maximum heating output (set under Max. valve position ) is reached at the coldest
outdoor temperature (which can be set under Max. outdoor temperature valve position).
100%
Max. valve position
25%
0%
Outdoor temp.
max. valve position
The controller adds the values representing the valve position for downdraft
compensation and the valve position for the heating sequence.
– If the room temperature rises as a result of the downdraft compensation feature, the
heating sequence reduces the opening of the associated valve, so correcting the
room temperature.
– When the sequence reaches zero, the room temperature is increased by the residual
heat from the downdraft compensation feature.
The heat output calculated by the controller is achieved as follows:
The valve is opened to the heat output value [%].
• The minimum heat output is 25% (LED08: 10%) :
400 seconds "OPEN" (1s On, 1 s Off) 1200 s "CLOSE" (1s On, 19 s Off)
• Heat output 50%:
400 seconds "OPEN" (1s On, 1 s Off) 400 s "CLOSE" (1s On, 19 s Off)
• Heat output 80%:
1600 seconds "OPEN" (1s On, 1 s Off) 400 s "CLOSE" (1s On, 19 s Off)
The long cycle time ensures that the valves are fully opened and closed.
In a network containing several room controllers, the opening of the Siemens thermic
actuators is staggered to prevent the heating load from fluctuating.
If a thermal radiator valve actuator and a thermal heating/cooling valve actuator work
in parallel, the controller controls them alternately.
RXB (KNX) Applications library
Control sequences
Heating output
Thermic
Motoric
Outdoor temp.
0% valve position
CLC and RAD description of functions for CC-02 CM110384en_03
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OA
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