Viessmann WB1A Series Installation And Service Instructions Manual page 62

Water flow switch activates
plug 33.
yes
Differential air pressure switch enables
start-up.(Close contacts between terminals
X8.7 and X8.8)
yes
(symbol A appears).
5581 775 1.3
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no
no
Display message
no
Fault F5/F9
after approx. 51 seconds
no
Fault F5/F9
after approx. 51 seconds
Fault F4
Fault F4
Fault F4
Check water flow switch. Replace
if necessary.
Check cabling and plug-in connectors of the
blower , as well as blower itself. Check
proper power supply to the blower.
Check flue gas system and differential air
pressure switch.
unit and it's plug-in connections.
104VRAC (DC)
between brown and blue must be present
during start-up, plug 35
check gas supply pressure.
Check condensate line and flue gas
system for leaks.