This thread is now old but I encountered the same issue (Stelflow cylinder and a dual thermostat attached to the air pocket). The CH boiler would rarely switch off and was continually heating the cylinder to a water temperature well above the 60C I had set on the thermostat. Even setting the thermostat to 45C reduced this issue but did not remove it. A visiting plumber's solution was to turn up the CH boiler thermostat so that it ran much hotter but this was insane as it resulted in the water being far too hot and also led to very hot CH radiators. Of course, it did heat the cylinder so hot that the cylinder thermostat usually, but not always, switched off. In fact, my central heating had been running with this issue for about 5 years, since it was installed. I discovered the issue when replacing a motorised valve that had failed. Initially, I bought a new dual thermostat of the same design but it too had the same issue. However, I have now solved this which is why I also know that it had been this way since installation. Turns out that the air pocket in the Stelflow cylinders protrudes from the surface by about 2cm and the pocket itself reaches into the tank by about 20+cm. Hence, as supplied, the probes on the thermostat are near the outer part of the socket (they only protrude about 10cm from the rear of the thermostat).
When fitting the thermostat and the cover and electrical power are off, extend the coiled up thermocouple wires as far as they will go, taking care not to damage or detach them from the probe or control unit then pull the sensors as far out as possible. Note that the wires cannot be simply pulled out from the rear of the thermostat without first easing them out from inside the thermostat. This results in the ends of the sensors being about 20cm from the unit. Insert the sensors in the pocket, ensuring that the wires do not buckle, so that the sensors are as far into the pocket as possible. Also, wrap about a 4cm broad piece of insulation foam around the wires just after they enter the pocket (I used a small piece of kitchen surface sponge material for this purpose). This insulates the inside of the pocket to reduce convection from the pocket. Now attach the thermostat to the pocket using the two screws. Has to be said that this is also a poor arrangement for attachment. Take care not to over tighten the screws, as this will damage the plastic of the thermostat. Even once fitted, the thermostat can be pulled off using not much force. The manufacturers should do better than this. In any case, this arrangement solves the problems. The thermostat clicks on and off at a few degrees on either side of the actual tank temperature. I set it to 60C and the thermostat will switch on at about 56C and off at about 64C, so hot enough to kill legionella but not so hot as to burn users at the taps. The CH boiler now comes on and off as it should and, in fact, now spends much of its time off. As well as all now working correctly, I expect a substantial saving in gas usage and wear on the boiler. The supplied thermostat instructions are truly useless and I am guessing that this issue is affecting many thousands of users. I would go as far as saying that, as supplied and fitted as per the instructions, the system cannot work properly. I could find no workable solution on the internet forums and every plumber I asked (including G3 registered, etc.) provided me with illogical answers that could not possibly resolve this issue. Now that it has been solved it is, sort of, obvious, so one wonders why this approach is not already common practice. The issue should arise less with other types of pocket and thermostat but I am guessing that it is common among those using Stelflow cylinders and the accompanying dual thermostat in the air pocket. I hope this helps other forum members and those finding this via a search engine, as I spent many hours experimenting before reaching this solution.
