Viessmann MatriX Service Instructions Manual page 23

Air pressure limiter
Function
The air pressure limiter A will
signal in the following operating
conditions:
H prior to the fan start (idle state
check)
H in the pre−purge phase
H in the control mode, subject to at
least the starting output being
produced. Outputs which are lower
than the starting output are not
monitored.
A
Fault shutdown
The air pressure limiter triggers a
fault shutdown on the gas burner
control unit (see page 24) in the
following situations:
H if the idle state check is
unsuccessful after 5 attempts
(anti−cycling function) (i.e the air
pressure limiter contact does not
open within 30 s)
Initial start−up, inspection and maintenance
H if, in the pre−purge phase, the
minimum flow rate has not been
produced (fault shutdown via the
anti−cycling function)
H if, in the control mode, the air
pressure limiter fails or the air
pressure lies outside the
permissible range
The fault shutdown is shown with
fault display "L" on the gas burner
control unit (see pages 24 and 31)
and cannot be rectified by pressing
the reset button (see page 31).
The fault shutdown can only be
rectified if the gas burner control unit
is switched off by switching off the
power supply to the control unit.
Before the gas burner control unit is
set to fault ("L") through a signal
from the air pressure limiter, a
re−attempt will take place five times
every two hours. If these attempts
are not successful, the gas burner
control unit will be set to fault ("L").
This indicates a serious fault. A fault
shutdown by the air pressure limiter
("L") will not be displayed as a fault
at the control unit, as it is normally
(such as lack of gas "A") classified as
a maintenance stage.
The air pressure limiter is always set
to 1.0 mbar. This guarantees hygienic
combustion. This will also ensure a
minimum volume for pre−purge, as
the air pressure limiter still regulates
at minimum output.
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