I would appreciate it if anyone out there could give me some advice on a boiler problem i've got. I have a Worcester Greenstar 28i Junior Combi boiler. The heating is working perfectly but I am have no hot water. When I turn a hot tap on the boiler fires up and hot water is initially produced but within 10 to 20 seconds the water goes cold again. The flow from the tap then starts to slow to just a trickle and I have to open it up further to get a flow but then the water only runs cold and no matter how long it runs it never heats up again.