Horstmann Electronic 7 User Operating Instructions Manual page 3

Programme Security Lock
If the 4 black squares are showing in the display then this indicates
that the automatic programme of switching times has been locked on
installation and cannot be altered by the user. This will ensure that the
programme stays matched to any preferential tariffs which should
mean that your water is heated as economically as possible. The
manual boost button will still be operative allowing one hour of extra
hot water each time the boost is operated.
TIMED
BOOST PROGRAMME
OFF PEAK PROGRAMME
Switched Output
In the "OFF" position the electrical supply to the immersion heater(s)
is disconnected even though the clock, display and indicator lights
continue to operate. In the "TIMED" position the over-night immersion
heater will be switched on and off at the programmed switch-time
settings, at other times the boost facility is available.
RUN
AM
CLOCK
OFF
SWITCHED
OUTPUT
TIMED
SUMMER
WINTER
TIMED BOOST
3