Cylinder temperature slowly rises when boiler + system off! Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[the_meek]

I'd appreciate any advice on this problem relating to a conventional boiler with 3 way valve, pump and hot water tank with indirect coil.

I noticed the hot water temperature was fairly suddenly too hot. This seemed to happen fairly quickly without any adjustments to the system.

I suspected one of the thermostats had broken but each works fine. The cylinder stat turns the pump/boiler on/off. The boiler stat turns the main burner/pump on/off ok.
I checked the 3 way valve and i can see the motor unit turning to the 3 positions as the system calls for the 3 options of water, heat or both.
The heating side pipe is cold when the heating is off.

As a final check this morning i turned the heating off and ran the hot water until the tank was below the cyclinder stat temperature so the boiler came on.
I tracked the cylinder temperature on a thermocouple connected next to the stat and recorded on a PC. The temperature reached a lowest of about 27C then rose rapidly to about 50C over about 30 mins. (I had turned the cylinder stat down to about 45C for the test.)
Then over the next 8 hours the cylinder temperature rose slowly and uniformly to 63C. There were no sharp changes in temperature so i am sure the boiler did not come back on, and the heating was off all the time.

Any ideas what is causing the slow drift in temperature, much above the required temperature?

Can the cylinder be fouled so the heat input from the indirect coil is more than enough to heat the contents when the natural circulation mixes the hot and cold water?

Can the water be circulating naturally and over heating the cylinder (heat from where?)? If so, can i de-scale it.

The creeping temperature happens over such a long time and consistently i'm not sure what is happening.

I replaced the cylinder stat with a new one- same result.
The boiler working discharge temp is 65-70C maximum.

[the_meek]

I did another test yesterday.

Turned off the boiler controller at the switch (should stop any calls for heat and stop pump) and isolated the valves either side of the pump (will stop natural circulation).
Left boiler on pilot light.

After 8 hours or so the cylinder water temperature did not rise (ok). However noticed the back plate of the boiler was too hot to touch.
It does appear the boiler is staying hot "on its own" or is firing without command from the controller. To support this theory i heard violent steam/water hammer type noises from the pipework suggesting the stagnant water in the boiler was too hot and was raising steam/cavitating locally.

Any ideas please?
How can the boiler be staying so hot with the timer and rest of system turned off at the mains?