Hot water relief valve dripping: normal or a problem?

What the relief valve does

The pressure and temperature relief valve is the brass valve near the top of a storage hot water system, with a lever and a drain pipe. If the pressure or temperature inside the tank ever gets too high, it opens and lets water out through the drain. It's a safety device, so it needs to work.

When a drip is normal

Water expands as it heats. On many systems, that extra water leaves through the relief valve drain, so a little water during each heating period is expected. Rheem's owner's manual says the same. If your system has an expansion control valve on the cold water side, that valve may discharge the expansion water instead.

When it's a problem

• It drips constantly, day and night, not just while heating.
• Water runs out in a steady stream.
• The water coming out is very hot or steaming.
• It started dripping after a new install, a plumbing change or a jump in water pressure.

The common causes

A worn relief valve

Over time, mineral deposits and wear stop the valve sealing properly. Rheem recommends replacing the relief valve at intervals of no more than five years, and sooner where the water leaves a lot of deposits.

Water pressure that's too high

Under AS/NZS 3500.1, static water pressure inside a building shouldn't be more than 500 kPa. If the mains pressure is higher, the relief valve can keep opening. A pressure limiting valve on the incoming water line fixes it, and protects your taps and fittings too.

A failed expansion control valve

Where one is fitted, a worn expansion control valve can drip constantly, or stop working so the relief valve takes the strain instead.

A thermostat fault

If the water coming out is very hot or steaming, the thermostat may have failed and the tank may be overheating. Switch off the power or gas to the system and call a plumber straight away.

What you should and shouldn't do

• Never block, cap or plug the relief valve or its drain. The valve is there to stop the tank from over-pressurising.
• Most owner's manuals recommend gently operating the easing lever every six months. Stand clear of the drain, because the water is hot, and lower the lever gently. If it won't stop dripping afterwards, call us.
• If the discharge is hot or steaming, switch off the power or gas and call.

Is it a repair or a replacement?

A dripping relief valve is almost always a repair: a new relief valve, a pressure limiting valve or a new expansion control valve. The tank itself is usually fine. See leaking hot water systems and hot water repairs, or read about water at the bottom of a hot water system.