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Discuss Vaillant diverter valve works but not when hot water cylinder is refilled and tap open... in the Plumbing Forum | Plumbing Advice area at


I am not going to make any repairs myself on my boilers but no plumbers have been able to solve my issue. I am seeking for help on this forum to find the cause of the issue I face so that I could communicate to the plumber. I tried on french forums since I am from France but I have not got any feedback on my issue and I am quite desesperate now.
I have a vaillant boiler plus a hot water cylinder. Diverter valve was changed 3 years ago because it did not move correctly.
For a month now I have had a new problem with this diverter valve but slightly different . The valve works perfectly for central heating / DHW alternation. I checked the valve during these 2 phases through the small holes in the valve, and the axis that pushes or pulls the "plug" inside the valve moves well during the transition between DHW and heating and vice versa. The pipe temperatures are ok, no hot water leak in the heating circuit that would indicate a partially blocked valve.
The problem happens only when I draw hot water (shower .) and the temperature of the hot water cylinder drops below a certain temperature, it triggers the production of DHW (as expected) and every time, the valve does not move into DHW position but stays on central heating position that leads to a major issue: I have no DHW produced and very hot water (80°C) goes into the heated floor. If I restart the production of DHW a few minutes later (after a central heating cycle) everything works well again.
I observed the valve during this transition and the axis does not move, there is only a vibration of the axis that is similar to the beginning of the normal movement but nothing more. I think it means that an electric signal is sent to the valve to ask it to move into DHW position but the valve does not make the full movement and the axis does not push the "plug" in the diverter valve. The status code on the boiler indicates that the valve is in DHW mode.
It seems to me to be an electrical issue, the signal sent to the valve may be not long enough to move the axis.
I can not find the reason why when drawing hot water with a request for DHW the valve does not work properly while it works perfectly when no hot water is drawn. Does anyone have an idea of the cause of this problem?
Thanks for your help,

Is the cylinder thermostat operating correctly?
Yes. it triggers correctly the production of hot water when the temperature inside the cylinder drops around 20°C. I can see the temperature inside the cylinder. It is really a matter of diverter valve that only in case a hot water tap is open does not move to its correct position. It does not happen when no tap is open. In this case the diverter valve moves into its correct position.


Advent Win
The diverted wont necessarily open to hot water when a tap opens. You would have to run sufficient hot water for the cylinder stat to sense a drop in temp at the point its attached to the cylinder.
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Need some pictures. Is this a conventional cylinder or a thermal store?
I will post some pictures tonight. Yes you are right, to test the diverter valve, I open a hot water tap and let the hot water run for some time until the temperature in the cylinder drops and triggers hot water production. This is how I detected this issue with the diverter valve.
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Here are some photos.


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