What is a pressure relief valve on a hot water system and why is it dripping?

Water expands when heated, and in a sealed storage system that expansion has to go somewhere. The temperature and pressure relief valve is the safety device that releases it, and its drain line usually discharges onto the ground or into a tundish where you can see it.21

Normal versus not

• Some discharge during a heating cycle is normal and expected. A small amount of water under the drain line after the system has heated is not a fault.
• Continuous dripping, all day, is not normal.
• Heavy or hot discharge is a warning and warrants attention promptly.
• No discharge ever, on a system several years old, may mean the valve has seized rather than that everything is fine.

The causes of continuous discharge

• A failed valve seat, usually from mineral deposits, which is the most common cause and an inexpensive replacement.
• Incoming mains pressure above what the system is rated for, which means a pressure limiting valve is needed or has failed.
• A failed expansion control valve on systems fitted with one.
• A thermostat fault causing the tank to overheat, which is the serious case.

Manufacturers generally recommend easing the valve lever periodically so the seat does not seize, and replacing the valve on a service interval rather than on failure. On an older system where nobody has ever touched it, easing the lever can also be what causes it to start leaking, which is a good reason to have it done when a plumber is present rather than on a Sunday.