Triton T30i handwash

No air relief valve on this.

Is the nozzle on the arm clean and clear

Is the water heating up properly if not you could have afaulty element in the heating tank

The other possibility is you may have a faulty flow controlvalve

It needs a new stabilising valve assembly http://www.showerdoc.com/shower-spares/triton/TRITO-PARENT-1-Triton-T30i-handwash

Which one will depend on the output of the hand wash, 3kw of 7kw