The temperature and pressure relief valve: the part you must never block
It is a small brass valve with a lever and a pipe running away from it. It exists for the one day everything else fails, and a surprising number of owners have been tempted to cap it.
A temperature and pressure relief valve (often called a TPR valve, or just the relief valve) is a safety valve fitted to a storage hot water system. It opens by itself if the water in the tank gets too hot or the pressure climbs too high. Then it lets water out through its drain pipe until things are safe again.
You should never plug it, cap it or tighten it shut. That is the main thing to take from this page. The rest explains why.
What does the relief valve actually protect against?
A storage tank is a sealed steel vessel full of water that is being heated. Water expands as it warms, and if the thermostat or control ever fails, the water keeps heating and the pressure keeps rising. A cylinder under that kind of stress can split a seam, and a ruptured tank full of near-boiling water is a genuine danger to anyone nearby.
The relief valve is the last line of defence. It does nothing on a normal day. It opens only when temperature or pressure goes beyond what the tank was built for.
Think of it like a fuse. You do not judge a fuse by how often it works, and you certainly do not replace it with a nail.
Where is it, and how do you recognise it?
On most storage systems it sits on the side or top of the tank, close to the top, with a small lever or test handle. A drain pipe runs from it, usually down the side of the unit and out to a safe spot, often through the wall or to a tray. That pipe is part of the safety system, not a stray bit of plumbing.
| Part | What it is for |
|---|---|
| The valve body | Opens when temperature or pressure goes beyond its rating |
| The test lever | Lets a licensed plumber confirm the valve moves freely |
| The drain pipe | Carries discharged water safely away, so it must stay open and unobstructed |
| The stamped rating | Shows the temperature and pressure it was made to open at. It must match the tank |
Is a drip from the pipe normal?
Sometimes, yes. As cold water heats up in a storage system it expands, and some systems release a little of that expansion through the relief valve. A short drip while the tank is heating, which stops once the water is hot, is often just that.
A steady stream, a drip that never stops, or water that runs when the tank is already hot is a different story. That points to a valve that is worn, dirty with mineral scale, or reacting to a real pressure problem. We cover how to tell the two apart in relief valve leaking: normal, or a warning.
Why should you never block the valve or its pipe?
It is tempting. The drip is annoying, the puddle stains the paving, and a cap or a bit of tape seems like a cheap fix. Do not do it.
A blocked relief valve cannot relieve anything. The tank is then relying entirely on a thermostat that, by assumption, has already failed. A drain pipe that has been capped, kinked or run uphill can also stop a working valve from doing its job.
If the discharge bothers you, the answer is a licensed plumber looking at why it is happening. The answer is never to remove the valve's ability to open.
How often does a relief valve need attention?
The valve is a mechanical part with a spring and a seat, and mineral scale in hard water can stop it sealing or stop it opening. Many manufacturers and installers recommend having it checked as part of a regular service. The exact interval depends on your model and your water, so use the one your manufacturer nominates rather than a figure from a website.
Keep an eye on the warranty too. Servicing and approved parts are often conditions, and what actually voids a hot water warranty explains how that works.
What is genuinely yours to do
- Find the valve and its pipe. Know where they are, and make sure nothing is stacked against the pipe or growing over its outlet.
- Look at the pipe outlet now and then. Staining, mineral crust or a wet patch tells you the valve has been opening.
- Write down what you see. When does the water come out, how much, and is the tank hot at the time? A plumber can work with that.
- Leave the lever alone. Testing it is a job for the person servicing the system.
- Ask at the next service. Is the valve the right rating, is it showing scale, and is the drain pipe laid correctly?
If the valve is discharging heavily or the water coming out is scalding and constant, treat it as urgent and call a licensed plumber. Keep clear of the pipe outlet while you wait.
Frequently asked questions
What is a temperature and pressure relief valve?
It is a safety valve on a storage hot water system that opens automatically if the water gets too hot or the pressure rises too high. It releases water through a drain pipe until conditions return to safe levels. It does nothing on a normal day, and its whole job is to prevent a tank from being over-stressed if the thermostat or controls fail.
Is it normal for the relief valve to drip?
A brief drip while the tank is heating can be normal, because water expands as it warms and some systems release a little of that expansion. A constant drip, or water that keeps running once the tank is hot, is not normal and is worth having a licensed plumber look at.
Can I block or cap the relief valve pipe?
No. A blocked valve or a capped drain pipe means the tank has no safety release if the thermostat fails. If the discharge is a nuisance, the fix is finding out why it is happening, not removing the valve's ability to open.
Can I replace a relief valve myself?
No. Replacing one involves isolating and draining a pressurised tank of very hot water, and it is licensed plumbing work in every Australian state and territory. A valve of the wrong rating can be as dangerous as no valve at all.
How do I know if my relief valve needs replacing?
Signs include a continuous discharge, mineral crust around the outlet, or a lever that is stuck. Only a licensed plumber can confirm the cause and say whether the valve or something else, such as the system pressure, is at fault.
Do all hot water systems have a relief valve?
Most storage systems have one. Continuous flow units have no storage tank and work differently, so they are set up with different safety arrangements. Check your manual or ask a licensed plumber what your particular model uses.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)