Unvented cylinder won't stop heating up when CH is turned on

[John.g]

Looks that way all right but a bit strange that with HW only selected that the cylinder temperature doesn't continue rise, if the zone valve is passing it can only mean that the HW programmed on time is spot on and shuts the boiler down or the CH comes on shortly afterwards.
Might be worth extending the HW on time and letting the CH off until/if the cylinder continues heating.

Probably no good though because boiler will shut down anyway once SP temp is reached on HW only.

 

[bluto32]

Thanks again for your help. Having got hold of a thermometer, I was wrong about the water being hotter than before when the pressure reading was higher.

I have since discovered that the pressure gauge is attached to a tiny pressure reducing valve between the cylinder and a shower I don't often use. The gauge is just after this valve and before the shower. There was some trapped air in this part of the pipe which was causing havoc. I think the trapped air was being heated due to the pipe in question being close to a very hot central heating pipe under the floor somewhere. The expanding air may have increased the pressure in that section of the pipe where the gauge was. The pressure reducing valve was stuck closed, not allowing any backflow when other hot water taps were turned on. Letting hot water out of that shower seems to have displaced the trapped air and sorted things for now.