Function Description Air Pressure Switch; Fan Pressure Monitoring Function (Ldw1); Combustion Chamber Pressure Monitoring Function (Ldw2); Boiler Water Temperature Sensors And Flue Gas Temperature Sensors - Viessmann VITOCROSSAL CRU 800 kW Service Instructions Manual

Air pressure switch

Fan pressure monitoring function (LDW1)

The switching threshold of air pressure switch 1 (plug
aCA ) is monitored in all fan ramp-up phases and
checked during modulating burner operation. This
ensures a minimum pre-purge.
The air pressure switch triggers a fault shutdown of the
burner control unit under the following circumstances:
If the static pressure check was unsuccessful after
■
approx. 5 minutes.
■ If the air pressure falls outside the permissible range
during the pre-purge phase (tolerance time approx. 5
minutes).
■ If, in control mode, the air pressure switch fails or the
air pressure falls outside the permissible range.

Combustion chamber pressure monitoring function (LDW2)

To monitor the combustion chamber pressure, the
switching threshold of air pressure switch 2 (plug
A) is monitored in all operating phases (except dur-
aCA
ing safety and stabilising time).
The air pressure switch triggers a fault shutdown of the
burner control unit under the following circumstances:
■ If the combustion chamber pressure falls outside the
permissible range in the pre-purge phase, in control
mode or in the post-purge phase after 2 attempts.
■ Only for multi-boiler systems with flue gas cascade:
Checking and monitoring of the flue gas damper
function

Boiler water temperature sensors and flue gas temperature sensors

The temperature sensors are of a dual sensor design.
Connection to burner control unit
A/ B Boiler water temperature sensors
§ §
A/ B Flue gas temperature sensors
aG aG
The fault shutdown is indicated by fault codes "F F5"
and "F F7" on the burner control unit display (see
page 38). The fault shutdown can be cleared by press-
ing reset button R.
LDW1 setting: 4 mbar (0.4 kPa)
The fault shutdown is indicated by fault code "F FB" on
the burner control unit display (see page 32). The fault
shutdown can be cleared by pressing reset button R.
LDW2 setting: 10 mbar (1 kPa)
NTC 10 k
Ω
20
10
8
6
4
2
1
0.8
0.6
0.4
10 30 50
Temperature in °C
Fig. 33
Function description
70 90 110
57