Water Heater fault

[Craig1984]

Hi,
I'm a gas safe registered engineer & have been asked to look at a britony 2t water heater as the water is only getting luke warm at all draw off's. Water heaters are abit before my time but I have my ticket for them & wanted the experience. Ive gone though fault finding & changed the diaphram as it was cheapest option for customer but its made no difference.I checked water section whlist it was stripped down & it was moving freely but the flange part (which the diaphram sits in) of the gas section seemed really stiff.
so, A is that normal & B could that be the fault?
Thanks

 

[mountainman]

A really basic water heater and a really old one too.
Time for a new appliance perhaps.

 

[Boilerbud]

Do you get correct gas pressure or gas rate when hot water tap full open with temperature set at max?