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, this could be as much as 5.4 litres. Hot water cylinders 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 or 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 hot water cylinder to save hot water.

To test whether your dripping relief pipe is experiencing normal thermal expansion, or if it has a faulty valve:

  • Turn off the power supply to the tank for five minutes.
  • If the dripping stops, this is normal thermal expansion.
  • If the dripping continues, the valves are probably faulty.

No. Hot water must be stored at over 60 degrees Celsius to prevent the growth of legionella bacteria in the hot water tank. A tempering valve is fitted to the hot water outlet to lower the temperature to 55 degrees Celsius before it is consumed.

The Building Code requires that the temperature of hot water delivered to sanitary fixtures used for personal hygiene purposes like showers, baths and basins should not exceed 45 degrees Celsius for rest homes, schools and early childhood centres, and 55 degrees Celsius for all other buildings. A tempering valve reduces the temperature of the hot water that is stored in your hot water cylinder before it is consumed.

No. If you have a low-pressure hot water cylinder that is restricted by a pressure-reducing valve, a limiting valve will not affect the hot water, only the cold. In fact, in some cases limiting valves improves the mixing of hot water in your shower.

The pipes may be inadequately clipped, or the water pressure could be too high and therefore need a limiting valve installed.


Call (09) 263 7621 or complete the form below.

    Site Safe logo
    2020_New Gold Mission Supporter

    As registered Master Plumbers our work is covered by the Master Plumbers Guarantee for 12 months, giving you peace of mind.