Vaillant uniSTOR HP circuit stat hysteresis issue

[drcarrera]

Hi

We've just had a Vaillant ASHP system installed together with a uniSTOR HP cylinder. However, there's a problem with the primary circuit thermostat in that the hysteresis seems to be in the order of 20 ^oC . As the ASHPs output at 50 ^oC this means that if we set the stat to 50, it doesn't trigger until it drops to near 30 ^oC which is too cold for showers etc. If we increase the stat to, say 60, so it triggers at 40, it's then permanently on as obviously the water won't ever reach that temperature.  The stat for the immersion which sits next to it seems fine, with a hysteresis of around 5 ^oC

Does anyone have any experience of this type of cylinder/stat and know if the hysteresis is adjustable in any way, or is it just faulty? 

It's the right hand stat in the diagram below.

 

 

TIA.

Chris