automatic air vent on valliant vcw t3w

i can only get my HOTwater on if i have my heating on question should my expantion tank on back of boiler be full of water or full of air/?

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.

Isnt this the Sine 18 with waxstat diverter ?

The diaphragm water flow switch is pretty reliable although the stuffing box can seize up, particularly if the boiler is left unused for a few weeks.

Tony

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.