Viessmann Vitodens. Hot weather, no hot water

[ShaunCorbs]

Why would weather come hold hot water off

If it’s not hot water priority and just simple controls it won’t know if heating or hot water is being demanded so will just run in wc mode

 

[townfanjon]

Why would weather come hold hot water off

It shouldn’t

 

[townfanjon]

I wonder if this has the v cylinder demand box installed , thats why I asked how long it had been installed for .

 

[frogisland]

Update

Boiler fired up for hot water during the night when it was cooler. Just now (09.30) not firing up for hot water. Outdoor temperature sensor is on a north facing wall, current temperature just below 26 degrees. Disconnected the cables from the sensor, resistance measured about 4.25KOhms, should circa 9KOhms according to the manual data @ 25 degrees.

Left sensor disconnected, switched boiler back on and it fired up for hot water.

However, just talked to Veissmann, they said very likely not the sensor itself. For the outside temperature sensor to work properly there should be a cylinder demand control box wired in (there isn't). Suggested leave the sensor disconnected. When i get an engineer in i'll have the wiring looked at. It was installed as part of a large extension about 5 years, i remember the contractors electrician struggling integrating with the existing electrics + outside sensor! (he didn't understand what the pump over-run did also + also other issues with the lighting circuit).

View attachment 39680

 

[townfanjon]

should be a cylinder demand control box wired in (there isn't).

I did wonder .
However , theres still something else wrong , because its worked up until now

 

[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