how can i fix my shower?

Hmmm?

Over flow from the cold water tank?

If so its back feeding, NRVs are needed to be renew

Again manufacturer to the rescue

Plumbstar Tom
August 2009

sounds like you've blown the pressure relief device
(PRD) contact makers for replacement a good diy person can easily replace it. shower must then re-commisioned by turning the temperture controle to it's hottest position,remove spray head and then switch on
check that shower head is not blocked as this can blow the (PRD) again and you'll be back to square 1 again

ta1painter
August 2009