If you can only get hot water with the heating on the most likely reason is the diaphragm in the water section is holded/worn.
The expansion vessel should contain air pressurised to about 0.7 Bar (with the system at zero pressure).
It is normal to have to drain a small amount of water out of the vessel on these older vaillant expansion vessels. These are high quality vessels and can easily give a service life of 20 years.
However more than a cupful may indicate a punctured vessel, in this case plumb in an additional vessel onto the system.
Yep wax stat diverter, Vaillant managed to get the design right where Potterton didin't.
I often find the screw holding on the water section is seized due to a leaking stuffing box. A small flame off the blowlamp sometimes shifts it otherwise down the breakers yard for a lower section.
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