The only other thing it could be is if the hot water zone valve was stuck open and constantly flowing 75c water from the boiler through the coil. Anyone experienced this before/the best way of testing this. I imagine one way is to have the heating off and hot water and turn heating on and see...
The composite valve is ok reducing pressure to 3bar. The expansion vessel has the correct pre-charge of 3bar. There is no back pressure on the cold side from a mixer tap as mains pressure is also 3 bar. Can these valves some times just go wrong, will be 18 years old
I think it depends on the distance from the unit to the nearest cold water tap. A 5litre unit requires about 2 meters a 15 litre around for litres. Other wise it's a single check valve and 2 litre expansion vessel
Are the little 10 litre under sink water heaters ok piped up just places on the base of a unit/ the floor. Or is it best to mount to a wall under the unit? Thanks