The Horstmann ServicePlus S21/S21R is a 24 hour programmer allowing
independent timing of hot water and central heating with up to
3 ON/OFF settings per 24 hours and a hot water boost button
and central heating advance override.
There is also a programming switch allowing permanent selection of:
OFF/ON/AUTO/ALL DAY.
Additionally, the ServicePlus programmer gives an advance warning
of the annual boiler safety check due date.
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