It is and it's optional
The VR66 has been wired or set up incorrectly, D.70 on the boiler is set to parallel heating, or the HW motorised valve is faulty.I've noticed that vsmart heats my cylinder when heating is on even though HW is set to off in the profiles.
I've only asked for hot water on between 5 and 6am every day, but it seems to heat it whenever heating is on (in that it opens the HW valve when heating is on).
Cylinder is a megaflo so just using the cylinder stat at the moment. Thinking of getting the vr10 though, just got to make sure it will fit along with the copper rods in the cylinder.
I thought d.70 was only used on combi boilers?
However, if using a non-Vaillant cylinder the overheat thermostat should be wired in series with the zone valve to maintain G3 regulations if using a VR10. Any work relating to the safety devices on an UV cylinder must be carried out by someone with the relevant UV qualifications.
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