Combi System - Hot Water pressure drops

[nosdod]

Just recently my Combi has started mis-behaving. When I turn on the hot bath tap it starts filling fine then after a few minutes the flow drops to a trickle. If I turn off the tap, then on again the flow starts fine but then drops, there is also a hissing sound from the boiler.

I suspect a faulty diverter valve, but could this be caused by dirt in the Heating circuit or trapped air in the boiler ?

British Gas engineer suggested a trick with the mixer tap to clear it, something about turning hot and cold on together and putting your finger over the tap outlet .... what does this do ? (Apart from ensuring ALL of the kitchen gets a good soaking)

 

[ChrisR]

The BG man was having a brain failure. His Trick wouldn't do anything on a combi system. Where HW is lower presure than the cold - which yours isn't, you can get the CW to go back up the HW to try to clear an air lock. It's your FLOW that's restricted not pressure.

This sounds like a tap problem. Do all your hot taps do it?

And PLEASE tell us what the boiler is!!

 

[nosdod]

It's a Worcester 240 boiler. The problem is intermittent and has happened three times in the last couple of weeks. I've noticed it when trying to fill the bath, turn on hot tap, some 5 mins later check back only to find a trickle (well its dropped to less than half the rate it started coming out at) coming out of the tap. If I turn the tap off, then on again slowly, at a certain point the flow drops and there is the hissing sound from the boiler. At this point if I leave the bath tap off and try the basin hot tap, the same thing happens, flow drops boiler hisses.

The last time it happened I tried turning the basin tap on and off a few times to see if it would dislodge some muck in the system but I couldn't get it to clear. Leaving the system alone for 15-20 mins seemed to settle it down!

 

[COG]

Its not a boiler fault its your tap, either the mechanism is expanding or a small amount of water is trapped in the tap body and expanding as it gets hot ,fairly common .

 

[nosdod]

Thanks for the info, I'll strip the bath and basin taps down over the weekend and see if I can see what's happened - I guess it'll be fairly obvious if something isn't how it should be ....