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Drains from hot water cylinder relief valves

Guidance for plumbers, plumbing designers and building consent authorities

The Department has received several enquiries about drain lines from pressure relief and temperature and pressure relief valves discharging to a PVC-U plumbing and drainage system. In some cases, the PVC-U pipes and stacks have melted causing a failure of the plumbing system and extensive damage.

It appears that this is due in part to the increasing use of solar water heating, and other heat sources that cannot be controlled by an energy cut-out device, requiring relief valves to discharge to prevent hot water cylinder explosions.

Relief valves are designed to reset after discharging and can discharge again and again until the heat source is reduced or a fault, such as replacing a failed thermostat, is fixed.

What is the function of relief valves?

Pressure relief valves are designed to relieve excess pressure to make sure valve-vented storage water heaters do not explode.

Temperature and pressure relief valves are designed to keep the temperature below 99°C and to relieve excess pressure to make sure valve-vented storage water heaters do not explode.

What do the compliance documents say?

Acceptable Solution G12/AS1, and Standards AS/NZS 3500.4: 2003 (cited in G12/VM1) and NZS 4607: 1989, all require:

  • the drain line from the relief valve to be in copper
  • discharge to an appropriate place that does not cause damage to the building.

AS/NZS 3500.2 (cited in G13/AS3) says the range of temperatures likely to discharge to the plumbing and drainage system must be taken into account when selecting materials for use in the plumbing system. Therefore, if the drain line discharges over a tundish into the plumbing system, the plumbing system has to be constructed of a material that will take the high temperature discharge from relief valves.

Failure to comply with these simple requirements could cause the PVC-U plumbing system to melt when a relief valve discharges, resulting in a failure of the plumbing system.