Your hot water system may be releasing water from its pressure and temperature relief (PTR) valve because water expands during heating. A small, occasional discharge can be normal for some systems. Continuous dripping, steady flow or a sudden increase may indicate excessive pressure, overheating, debris or a faulty valve.
Never block, cap or modify the outlet. The PTR valve is a critical safety device that releases excess pressure or temperature from the storage cylinder.
Common causes
- Thermal expansion: Water expands as it heats, so some systems release a limited amount during a heating cycle.
- Excessive pressure: High incoming pressure or a fault with pressure-control components can cause repeated discharge.
- Excessive temperature: Overheating or a temperature-control fault can activate the valve and requires prompt professional assessment.
- Debris or deterioration: Deposits, debris, a worn seal or an ageing valve may prevent it from closing properly.
An “overflow valve” may mean the PTR valve, although some systems also have an expansion control valve on the cold-water side. From a safe distance, check which labelled valve or drain line is releasing water. Do not operate a valve to identify it.
When is discharge a concern?
Limited, intermittent discharge during heating may be normal, depending on the system and manufacturer’s instructions. Arrange an assessment if you notice:
- continuous dripping or steady flow
- heavy or suddenly increased discharge
- unusually hot water or steam
- banging, boiling or other unusual noises
- water overflowing the drainage area or damaging property.
There is no universal acceptable discharge volume. Without touching the valve or hot pipework, note when the discharge begins, whether it stops after heating and whether the amount has changed.
What should you do?
Keep clear of the outlet because hot water or steam can cause severe scalding. Keep children and pets away. Do not touch or dismantle the valve, operate its lever, plug the discharge pipe or obstruct drainage.
For heavy uncontrolled discharge, steam or suspected overheating, keep everyone away. Follow the manufacturer’s shutdown instructions only if you can do so without approaching hot water, steam or electrical hazards; otherwise contact an emergency plumber.
Diagnosis, pressure or temperature checks, and valve repair or replacement are not homeowner tasks. Under the national plumbing standard AS/NZS 3500.4—which governs hot water safety, relief line sizing, and scald prevention—these valves must discharge safely without risking scalding or property damage. If the leak is persistent, heavy, or associated with overheating, contact our Sydney plumbing team. Sydney households have trusted Fixed Today for over 15 years for prompt, transparent hot water advice. Describing the brand, approximate age, and whether your cylinder is gas, electric, or heat pump when calling helps our plumber arrive with the correct pressure ratings and replacement valves on the first visit.