When water is heated it expands, this is called thermal expansion. When heated from cold the volume of water can increase by as much as 3%. For a 180L hot water cylinder (HWC) this could be as much as 5.4 litres. HWCs are designed to relieve expanded hot water through an open vent pipe on the roof or by way of a relief valve and pipe to an outside wall or drain. There have been a number of cases where these vent/relief pipes have been blocked off by owners trying to stop the dripping. In some cases, where a secondary safety vacuum break was not fitted, the water cylinder has exploded with devastating consequences. Walls and roofs have been blown off and houses moved on their foundations. These days’ cold water relief valves are installed to the inlet side of the HWC to save hot water.

To test whether the dripping relief pipe is normal expansion or a faulty valve:

• turn off the power supply to the tank for 5 minutes. 
• if the dripping stops then this is normal thermal expansion. 
• if it continues then the valves are probably faulty.