The boiler is a Main HE condensing. The problem only occurs when Hot Water Only is selected. If run from stone cold, it will run for a while and then off/on every few minutes but when hot water is selected with the tank already warm, the problem occurs. In summer it could take a day to heat up the water.
The system was power flushed and converted to have an expansion chamber but problem is still there

jimmo, May 2014

thanks very much - will run those checks

jimmo, May 2014

With the hot water tank cool/cold select hot water only and then monitor the flow and return pipes at the boiler,the flow pipe should gradually warm up and return should be cool,if the flow pipe gets quite hot quickly the water flow is too slow,
you say it was power flushed so the pump could be a factor.If the flow and return seem to heat up more or less together it could be the bypass valve that needs adjusting

toptrish, May 2014