Viessmann VITODENS 242-F Installation And Service Instructions Manual page 154

Function description
Internal extensions (accessories)
The internal extension is integrated into
the control unit casing. The following
alternative functions can be connected
to relay output sK. The function is
assigned via coding address "53" in
group "General":
■ Central fault message (code "53:0")
■ DHW circulation pump (code "53:1")
(only for weather-compensated oper-
ation)
Internal extension H2
154
(cont.)
■ Heating circuit pump for heating circuit
without mixer (code "53:2")
■ Circulation pump for cylinder heating
(code "53:3")
An external gas isolation valve can be
connected to connection gD.