If it is a heat exchange hot water system then check the valve that feeds the tank, it might be faulty. The popper valve used here has a 1/2 inlet towards the tank and a 3/8 as well. Turn off the water to the hws undo the valve from the tank, get as 1/2 galvanised plug and screw it into the 1/2 female thread. Turn the water back on to the valve, if water comes out of the 3/8 hole the valve is cactus, if not its the tank that is buggered