Vaillant Turbomax HW problem

[jamesy28]

Hi, I have a vaillant Turbomax VUW 242/1E boiler, the CH is working fine but our hot water is only luke warm. If the tap is turned right down do little more than a trickle it gets hot but as soon as you open it up the temp drops right down. The water temp dial on the boiler is turned right up.
I have noticed that when running the hot tap, the boiler fires up for about 20 secs, then switches off for about 10 secs and so on.
Any suggestions as to what the problem might be?
Thanks in advance, James

 

[Rob]

Do the radiators heat up slightliy when the tap is running?

Other than that it sounds like a blocked plate HE.

 

[jamesy28]

I don't think they do, can't tell right at the moment as we've had the heating on.
The boiler works fine and stays lit with the heating on, but as soon as HW is drawn i get same prob with or without heating on.
Is the plate HE the DHW Heat Exchanger?
Thanks, James