Gradually reducing flow on Hot Water

[Lemond2008]

Hi Folks. I have a bit of a wierd one on my Combi that I can't work out what is happening. When I run the hot water tap at anything less than say 1/4 or 1/3 pressure, the water comes out fine, but then slowly diminishes into a trickle and hence the boiler stops firing. Opening the tap a bit more then increases the flow again, but then the same thing happens.
Like I say, if the tap is more than about 1/4 on, then the flow stays constant and hot for eternity (or at least as long as I can bear to allow the cost of the burning gas!).
I am sure that I know the answer is bound to lie in the Diverter Valve, but I cannot understand how this would effect the flow. Does the mains feed not just flow through the heat exchnager and the DV uninterupted, or is there something in it which could be effectively 'turning off the tap' for me?
BTW the boiler is a Vaillant 242 if that helps......

Cheers

 

[spacethegas]

don't quite understand.

what about opening the tap fully as normal ?

is it all taps ?

it probably doesn't matter but is it vcw242, vuw242 ?

 

[namsag]

It will be your taps and not your boiler at fault seen this several times . Simple just turn the tap on enough so its not affected

 

[Lemond2008]

Thanks for the replies. I'm pretty convinced it's not the taps as all three hot taps do the same, and the remaining 3 cold taps flow fine and consistantly.
Turning the hot taps on any more than about 1/4 -1/3 full pressure and they work fine and consistantly also. It is only on the reduced opening that the flow diminishes over about a 30second time period.
Boiler is a vuw242e.

I wouldn't be bothered that much, but it is a right pain with the kitchen tap - I could expain fully if you want, but it's probably best to just trust me!

 

[spacethegas]

It will be your taps and not your boiler at fault seen this several times . Simple just turn the tap on enough so its not affected

this is your answer.

 

[namsag]

Yep , seen it where the heat of the hotwater expands the washer(or whatever) closing the opening down thus the flow which is why the cold are not affected.
Bristan taps i have on my own bath do it.
What would you suggest it is

Edit cant see anything in a turbo max causing it

 

[Lemond2008]

Thanks namsag - with the explaination as well as the statement it makes it easier to understand. I never had it in my old house, but that was not on a Combi - I guess it is more of a problem with Combi systems as the first 'slug' of water is alway cold.
Also - yep, these are Bristans too. I guess I'll just have to live with blasting water or scolding hands............