Drains from hot water cylinder relief valves
Guidance about drains from temperature/pressure relief valves in storage water heating systems, including acceptable discharge locations.
This guidance has been has been updated to clarify requirements relating to the installation of drains from temperature/pressure relief valves in storage water heating systems.
Temperature/pressure relief valves (TPR) are a safety valve designed to automatically release water if the temperature or pressure in a water heating system exceeds safe levels.
This guidance should be read in conjunction with the acceptable solution provisions in G12/AS1 Section 6.7 Relief valve drains and AS/NZS 3500.4 Heated water services Section 5.11 Temperature/pressure relief valve and expansion control valve drain lines (cited in G12/AS3).
Every TPR valve must be fitted with a drain*. TPR valve drains must:
- Be made of copper pipe
- Have a diameter no smaller than the TPR valve outlet for the length of the drain
- Have no restrictions or valves
- Fall continuously from the TPR valve to the point of discharge
- Be interconnected only with a drain line from a cold water expansion valve connected to the same water heater, and
- Have a maximum length and number of changes of direction in accordance with G12/AS1 or G12/AS3.
TPR valve drains must discharge in a visible position where the release of steam or hot water does not present a hazard or damage to other building elements, or to people. Acceptable TPR valve drain discharge locations include:
- To a tundish, discharging via an air gap at least twice the diameter of the drain.
- Outside a building, with the end of the drain located between:
- 75mm - 300mm above the grate of an external gully trap
- 75mm - 300mm above a gravel pit of not less than 100 mm in diameter.
- 200mm - 300mm above an unpaved surface, and directed away from building foundations.
- Where the water heater is located externally, the end of the drain must discharge away from the operator during the operation of the valve.
- Where located where freezing is likely, the drain must discharge over a tundish via an air gap before the drain enters a zone where freezing is likely. The drain from the tundish must be one size larger than the TPR valve outlet.
- The drain must not discharge directly into a safe tray.
*Note: TPR valves serving free outlet (push through) storage water heaters are not required to have a drain to comply with the minimum requirements of G12/AS1. The provision of a drain from these TPR valves is however recommended.
MBIE has received several enquiries about copper TPR valve drains discharging above a tundish or an external gully trap grate connected to a PVC-U sanitary plumbing or drainage system. This is acceptable, provided the water heater served by the TPR valve is connected to a controlled heat source. A controlled heat source has controls or devices that ensure the water temperature in the storage tank is no greater than 90°C. Water temperatures are typically controlled by a thermostat, which incorporates an over-temperature thermal cut-out device.
For TPR valve drains from water heaters that are connected to an uncontrolled heat source, such as a wet-back or solar water heater, where the drain discharges over a tundish or an external gully trap, the sanitary plumbing or drainage system must be constructed of a material that can withstand frequent high temperature discharges.
In some cases, PVC-U pipework has melted because of TPR valve drain discharges from water heaters connected to uncontrolled heat sources, causing a failure of the plumbing system.
Regular maintenance and easing of TPR valves
The easing lever on a TPR valve should be manually operated regularly (at least every six months) to ensure the valve opens and closes properly, to remove any mineral build up and check that it is not blocked. Water should flow freely from the drain line when the lever is lifted. If the valve does not discharge water, it should be checked by a licensed plumber.
It is important that the TPR valve easing lever is raised and lowered gently.
Note: Water discharged from the TPR valve drain line will be hot. When operating the easing lever on a TPR valve, exercise care to avoid any splashing of water by standing clear of the end of the drain.