PRESSURE TEMPERATURE RELIEF AND COLD WATER EXPANSION VALVES
A cold water expansion valve or pressure relief valve is a safety valve on all storage hot water systems. They are designed to relieve excessive pressure caused by the heating of water inside the cylinder. They come in two main types, a pressure temperature relief valve (hot water relief valve) which is installed directly into the hot water system, and a cold water expansion valve (cold water relief valve) which is installed on the water inlet valve assembly or before the hot water system.
Cold water expansion valves are set at a lower pressure than pressure temperature relief valves, which may explain why a cold water expansion valve is ‘dripping’ more than the pressure temperature relief valve.
SHOULD THEY LEAK?
Both cold water expansion valves and pressure temperature relief valves are designed to leak when the system is being heated, however when the tank is full of hot water and not being used, the valve will shut off and should not leak. If a constant leak is present, that indicates that the valve is malfunctioning and this could be due to something such as calcium being caught inside the pressure relief valve, or the pressure relief valve may be faulty and require replacement. It is not a safety concern if the valve is constantly leaking however it will dramatically increase your water and gas consumption.
DO I NEED A PRESSURE TEMPERATURE RELIEF VALVE OR COLD WATER EXPANSION VALVE?
Without a pressure temperature relief valve or cold water expansion valve the hot water cylinder will essentially become a bomb attached to your house.
The constant heating of the water will increase the pressure of the cylinder to such levels that it may explode sending boiling water and the hot water system in all directions. This is easily demonstrated in the MythBusters experiment shown in this link https://www.youtube.com/watch?v=9bU-I2ZiML0.
Soak pits are an approved point of discharge for drain lines (pressure temperature relief valve or cold water expansion valve) and shall be located so that the release of steam or hot water is controlled and does not cause a nuisance, is readily noticeable and will not cause damage to buildings or injury to persons.
Hot Water Systems that were installed pre 2018 may not have soak pits installed, however, soak pits are now a requirement on all new hot water system installations in Perth.