Cylinders venting water

[Wistow]

I have recently noticed, normally about late afternoon, that our hot water cylinder is venting hot water and i dont know why, turning on a hot tap for 10 seconds stops it for about a day. The water flow is about a litre every 10 minutes.

The system is a solid fuel boiler, not lit at the moment, an immersion running for 2 hours a night set at about 63 degrees, the top of the cylinder has a vertical pipe with a tee, one going up to the loft and discharges into the tank which feeds the central heating system it is this pipe which has the running water.

Searching appears to give me 2 reasons for this water too hot, and coil leaking for central heating into the hot water system. Are the any other reasons? Before i change the cylinder, have i missed anything. The pipework and cylinder are about 20 years old, the cylinder is covered in cream coloured foam

 

[cross thread]

Cold water tank ball valve passing

 

[Johntheo5]

Normally, the pipe coming off the very cylinder top is the HW outlet & vent, the heating coil flow and return come in the side, if the former then only reason I can suggest is that the the immersion is on permanently and the thermostat has failed causing the water to vent but if so it would vent into the cold water (CWST) (big) storage tank.
If the coil is leaking, then if the CWST level is higher than the F&E tank level, the water level will rise in the F&E Tank but will not force it out the vent as the vent is higher than the CWST water level. Have another look at it.

 

[Wistow]

Thanks for your replies
The cold water tank ball valve is not leaking and shuts off cleanly when it is supposed to.
The immersion is on a timer and shows a light when on, i have checked the state on/off of the imersion at different times and it is only on at the correct times, the hot water is at the correct temprature at the tap the tank does not feel hotter than normal.
The vent from the cylinder vents into the small tank which feeds central heating side of the system. The outlet for this tank is at the same level of the large cold water tank in the loft.

It is actually the overflow from the small tank which is flowing but i can see the vent pipe from the cylinder flowing into this tank.

This has only been happening for about a week.

Is the only way of checking if the heating coil in the cylinder is perforated by draining the central heating systen and see if the problem stops

 

[Johntheo5]

Can you post a few photos of the tank (s) and cylinder, you may have a unvented cylinder.

 

[jeff the gasman]

It’s your heating pumping over when hot water is heating by the sounds of it!