I have a fully pumped central heating and domestic hot water system operated by separate motorised valves. The central heating works ok but the domestic hot water pipes are not heating up. The bolier is ok; the control programmer is ok, the cylinder thermostat is ok; the motorised valve is opening and closing ok but the return flow pipe just does not get warm / hot. I've drained and refilled the whole system 3 times thinking that I must have an airlock but I still have the same problem; central heating radiators fill, bleed and heat up ok. I can only think that I have an airlock that I can't shift!!! Any ideas?