Quick answer
The pressure relief valve on a hot water system releases water if temperature or pressure inside the tank gets too high. An occasional small drip can be normal, but constant dripping, a steady stream or bursts of hot water point to a worn valve, high mains pressure or a faulty thermostat. A licensed plumber should check it.
The pressure relief valve on your hot water system is a safety device, and when it starts dripping, spurting or running constantly it’s telling you something. Sometimes a little water is normal. Often it’s a sign that the valve is worn, mains pressure is too high, or the system is overheating.
This guide explains what the relief valve does, how it differs from a hot water tempering valve, what’s normal and what isn’t, and when you need a licensed plumber. It doesn’t include DIY repair steps. Valves on a pressurised tank of very hot water are not a home repair job.
What does a pressure relief valve on a hot water system do?
It stops the tank from building up dangerous temperature or pressure. Most mains-pressure storage systems have a temperature and pressure relief valve (often called a TPR or PTR valve) near the top of the tank, with a drain line running down to the ground or a tundish.
If the water inside gets too hot or the pressure climbs too high, the valve opens and releases water. Without it, a faulty thermostat or a pressure spike could damage the tank or make it unsafe.
Many systems also have an expansion control valve on the cold water inlet. It releases small amounts of water as the tank heats, taking some of the load off the relief valve. Your plumber can tell you whether yours has one.
Is it normal for the relief valve to drip?
A small amount of water discharged while the tank is heating can be normal on some systems, because water expands as it warms. What isn’t normal is ongoing loss of water.
| What you see | What it may mean | Urgency |
|---|---|---|
| A few drops while heating, then dry | Normal expansion on some systems | Monitor |
| Constant drip, day and night | Worn valve seat or debris in the valve | Book a plumber |
| Steady stream of water | Failed valve or high mains pressure | Book a plumber soon |
| Bursts of very hot water or steam | System overheating, possible thermostat fault | Urgent, call a plumber |
| Water leaking from the tank itself, not the valve | Tank failure | Urgent, turn off water to the unit if safe |
| Drain line hot to touch for long periods | Valve releasing hot water continuously | Book a plumber |
Why is my hot water relief valve leaking?
The common causes are:
- A worn valve. Relief valves have seals and springs that wear over time. Scale from Adelaide’s harder water can stop the valve seating properly.
- High mains pressure. If water pressure coming into the house is high, the tank pressure rises too. SA Water notes that homes built after November 2005 are required to have a pressure limiting valve where mains pressure exceeds 500 kPa. Older homes may not have one, and existing valves can fail.
- A faulty expansion control valve. If this valve on the cold side stops working, the relief valve does more of the work.
- Thermostat or element faults. If the system overheats, the relief valve releases hot water to protect the tank.
- Debris after mains work. Sediment stirred up by water main repairs can lodge in valves.
Our guide to low water pressure explains pressure limiting valves in more detail.
What is a hot water tempering valve?
A tempering valve mixes hot and cold water so the water reaching your bathroom taps is at a safe temperature. It’s a different device from the relief valve and usually sits on the pipework near the hot water unit.
The SA Government says the maximum temperature for heated water at plumbing fixtures used mainly for personal hygiene (showers, baths, basins and bidets) must not be higher than 50 °C. At the same time, heated water needs to be stored above 60 °C in the hot water system to prevent bacteria growth such as legionella. The tempering valve bridges that gap.
The SA Government also points out that burn risk rises sharply with temperature: at 60 °C, it can take just five seconds for an adult and one second for a child before a third-degree burn occurs. At 50 °C, it takes around five minutes.
How do I know if my tempering valve is faulty?
Signs of a tempering valve problem include:
- water at the bathroom taps that’s too hot to hold your hand under
- hot water that fluctuates between hot and lukewarm
- low hot water flow at bathroom outlets while the kitchen is fine
- no hot water at bathroom taps while the tank is working
The SA Government notes that systems installed before 1998 are unlikely to have temperature control devices fitted, and that all water temperature control devices need regular maintenance and performance testing. If you’re not sure whether you have one, a licensed plumber can check.
What can I safely check myself?
Homeowner checks are limited to looking and noting.
- Note when the valve drips: only while heating, or all the time.
- Check whether water is coming from the valve drain line or from the tank itself.
- Look for rust, bulging or water pooling at the base of the tank.
- Check the manufacturer’s instructions for any valve maintenance they recommend for your model.
- Note the age of the system, which is usually on the label.
Don’t adjust the thermostat, remove valves or block the drain line. Blocking a relief drain to stop the drip defeats the safety device. If water is coming from the tank itself, turn off the cold water isolation valve to the unit if you can do it safely, and for electric units switch off the hot water circuit at the switchboard.
What does a plumber do when the relief valve leaks?
A licensed plumber will usually start by working out why the valve is releasing water, not just swapping it. Replacing a valve without fixing high pressure or an overheating thermostat means the new valve will soon start leaking too.
A typical visit may include:
- checking the incoming mains pressure and the pressure limiting valve
- testing or replacing the expansion control valve on the cold side
- checking the thermostat and heating controls on electric units, or the gas controls on gas units
- replacing the relief valve with one that matches the system’s specifications
- checking the drain line runs freely to a safe discharge point
- testing the tempering valve and delivered temperature at bathroom outlets
The plumber should explain what they found and quote before replacing parts. If the tank shows rust, weeping welds or water around the base, they’ll talk you through replacement rather than repair.
Repair or replace the hot water system?
Often the valve can simply be replaced. But if the system is old, the tank is rusting, or several parts are failing at once, a plumber may suggest replacement instead.
If you’re replacing, South Australia has requirements for the type of water heater most homes can install. Our guides to heat pump vs gas hot water and hot water system replacement cost explain the options and the cost drivers.
When should you call a licensed plumber?
Call a licensed plumber if:
- the relief valve drips constantly or runs in a steady stream
- the valve releases bursts of very hot water or steam
- the tank itself is leaking
- bathroom water is too hot or keeps fluctuating
- you’re not sure your system has a tempering valve
Valve replacement and adjustment is licensed plumbing work, and after completing it the plumber must give you a certificate of compliance within 7 days. For gas systems, any gas work must be done by a licensed gas fitter. Our hot water systems page and the Adelaide plumber hub explain more. If you have no hot water at all, see our guide on no hot water.
Getting it sorted
If your pressure relief valve or tempering valve is playing up, send one enquiry through our contact form or call 08 6800 6079. We pass it to one local plumber, who calls back within 24–48 hours to diagnose the problem and quote before any work starts.
Sources
Stock photo, for illustration only. This guide is general information for Adelaide homeowners, not professional advice. In South Australia, electrical, plumbing and gas work must be carried out by an appropriately licensed tradesperson, and rules change, so check current requirements with the relevant authority.
