problems with HW with vaillant boiler

[han]

hi,
recently we have been having problems with the hot water in our flat. when the HW in the bath is run, it is hot only for the first 30 seconds or so, then runs to cold.
There is no problem with the CH. The only thing i have noticed is the flames in the boiler look more yellow than the blue they normally are...

it is a vaillant combi heater vcw t3w.

I cant think of any reason why this is doing this, as boiler has been working fine until now, so would be very grateful if anyone has any ideas!
thanks in advance

 

[ChrisR]

Rubber Diaphragm split - not a mega job.

 

[han]

would it be possible to do myself?
thanks for replying

 

[kevplumb]

"There is no problem with the CH. The only thing i have noticed is the flames in the boiler look more yellow than the blue they normally are.."

corgi job like sharpish