Viessmann Vitodens. Hot weather, no hot water

[decomplexity]

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).

 

[Ziggie]

Is the weather compensation adjustable? I guess that normally this would be installed in the room heating circuit rather than the hot water circuit.

If the weather compensator is adjustable, does changing the setting change the situation?

 

[decomplexity]

When weather compensation is operating, control over flow temperature using the temperature control knob on the front panel is overridden and a default ‘boiler temperature vs outside temperature’ curve is applied. Turning the knob does, however, provide some limited adjustment to the curve if the boiler temperature is consistently too low or too high, but I don’t know of any Viessmann documentation that specifies by how much the curve is shifted when the knob is in a particular position.
This is for the Vitodens 100 series; weather compensation on the 200 series is more elaborate and the installer can select one of many curves that is most suitable to the house construction and insulation.

Weather compensation is not installed ‘on the heating circuit’ – it is not like a room thermostat which reacts to the how hot the house gets – but is intended to allow earlier reaction to outside temperature changes by changing the flow temperature. How DHW (tap) temperature is controlled depends on the systems wiring plan and whether a cylinder demand box is installed (vide my 27th July post