I've got a Worcester 24CDi which is working fine but for one issue. After using it from cold for HW for more than a minute or so, after I stop drawing HW it starts trying to fire up again. I can hear the fan spinning up and relay clicks and then it stops again. This start up and then stop process is happening every second or so and it does this in bursts of about a minute for the next 10 mins or so. The system is probably about 5 years old. Is this likely to be a faulty temp sensor? I intend to get someone out to it but was wondering if anyone has come across this before. Any pointers would be most appreciated.