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The weather compensator (WC) is intended to override the boiler outflow temperature set-point (as shown on the Vitodens panel). This is fine except in hot weather when the demand temperature (from the WC’s performance graph) is too low for hot water to reach a useful temperature. The cylinder demand box fixes this by (in effect) overriding the WC’s thermostat when DHW cylinder thermostat demands heat. As soon as the cylinder water is hot enough, the demand box tells the boiler “OK – your target outflow temperature is now set by the WC again. So in unusually hot weather – and without a cylinder demand box – the boiler may never fire to heat DHW at all.
OP does not mention which wiring plan his/her system uses (e.g. Sundial S,Y or W), but Viessmann say that WC is not possible with S-Plan. It is, vide an earlier post on this forum, but needs additional wiring and is forces the system to run ‘DHW priority’ (not necessarily bad thing).
OP does not mention which wiring plan his/her system uses (e.g. Sundial S,Y or W), but Viessmann say that WC is not possible with S-Plan. It is, vide an earlier post on this forum, but needs additional wiring and is forces the system to run ‘DHW priority’ (not necessarily bad thing).