Ok you lot it’s my turn to try and assist this fellow so here we go.
Firstly the valve you are referring to is called a TPR valve (temperature and pressure relief) it is designed so that is the water rose to a temperature greater than 99 degrees C, or the water pressure exceeded the valves rated specifications then it will open and discharge or release the pressure via the valve to waste.
However, with over 35 years experience with gas fired water heaters it is commonplace for this to happen on ocassion due to the extreemly rapid heat up capability of the gas burner used in the water heater.
To overcome this problem install a pressure limiting valve on the inlet to the cylinder and a cold water releif valve on the same line rated at 70% of the maximum setting of the hot water relief valve. This will allow the expansion to discharge cold water when the main burner is firing and keep your gas operating costs to a minimum.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians