Why your hot water relief valve is dripping
That little pipe near the bottom of your hot water system is the temperature and pressure relief valve, and it is supposed to discharge sometimes. Knowing when it is doing its job and when it is failing saves you both a plumber's visit and a surprising amount on your water bill.
What the valve is for
Every storage hot water system has a TPR valve — temperature and pressure relief. It is a safety device. If the water in the tank gets too hot, or the pressure inside gets too high, the valve opens and discharges through a pipe that runs to a safe drainage point.
You are storing a couple of hundred litres of water above 60°C under mains pressure. Without a working relief valve, a thermostat failure turns that into a pressure vessel. That is the entire reason the valve exists, and it is why the rule about not blocking it is absolute.
Never cap, plug or restrict this valve
Not with a cap, not with a plug, not by pinching the discharge line, not by putting a tap on it. If a drip is annoying you, the fix is to repair the cause, not to stop the water escaping. Anyone who suggests capping it should not be working on your plumbing.
Normal discharge versus a fault
Normal: intermittent discharge during a heating cycle. Water expands as it heats, the pressure rises, the valve relieves it. You might see a cup or so released in bursts while the system is heating and nothing in between. That is the valve working exactly as designed.
Not normal: a continuous drip, or a steady trickle that runs whether the system is heating or not. Something is wrong, and it is running twenty-four hours a day. A slow constant drip wastes a genuinely surprising volume of water over a quarter, and you are paying to heat a good share of it.
The three usual causes
1. The valve itself has failed
The most common one. Sediment and mineral deposits build up on the seat over the years and stop it sealing properly. Once a valve will not seat, it drips continuously. Replacement is a modest job, and it is a job for a licensed plumber — the valve is screwed into a pressurised tank of very hot water.
2. Mains pressure is too high
If incoming pressure exceeds what the system is rated for, the valve lifts to relieve it and keeps lifting. This matters well beyond the hot water unit: the same pressure is working on every joint, fitting, mixer and appliance in the house, and it shortens the life of all of them. The answer is a pressure limiting valve on the incoming main, not a new relief valve every couple of years.
If you have had more than one unexplained failure — a burst flexible hose, a leaking mixer, a relief valve that keeps going — pressure is worth checking. It is a quick test and it explains a lot of otherwise random plumbing problems.
3. The expansion control valve has failed
Some systems have an expansion control valve fitted specifically to absorb the pressure rise from heating, so the TPR valve does not have to. When it fails, the relief valve starts doing that work instead, and discharging far more than it should.
The thing to ease, and the thing to leave alone
Manufacturers generally recommend easing the TPR valve at set intervals — lifting the lever briefly so a flush of water clears sediment off the seat — and the recommended interval is in your system’s manual. Standing well clear when you do it is wise; what comes out is near-boiling.
One honest warning: on an older system that has never been eased, doing it for the first time sometimes stops the valve sealing afterwards. That is not really you breaking it — it tells you the valve was already at the end of its life and the sediment was the only thing holding it shut. But it is worth knowing before you try it on a Sunday evening.
If your taps run scalding, do not touch the thermostat
Stored water is kept above 60°C on purpose. It is a Legionella control, not a comfort setting, and turning it down creates a health risk in the tank.
The device that makes the water safe at the tap is a tempering valve, which blends the stored water down before it reaches bathroom fixtures. If your bathroom taps are dangerously hot, the tempering valve has failed or was never fitted. That is the thing to fix.
Related symptoms worth connecting
A dripping relief valve often turns up alongside other signs that a system is ageing. If you are also seeing lukewarm water, discoloured water, or water pooling underneath, the valve may be the least of it — water underneath is worth diagnosing separately, because one of the causes is terminal for the tank.
If the system is beyond its expected life anyway, it may be worth checking what that is before spending on valve work.
Getting it sorted
A dripping relief valve is a small job that is easy to put off, and the reason not to is that it is costing you money every day and it is sometimes a symptom of something larger. We diagnose the cause rather than just swapping the valve — if it is pressure, replacing the valve alone means you will be doing it again.
Hot water repairs across Wollongong, Kiama, Shellharbour, Gerringong and Berry.
Questions we get asked
Is it normal for a hot water relief valve to drip?
Some discharge during a heating cycle is completely normal — water expands as it heats and the valve relieves that pressure. Perhaps a cup or so, in bursts, while the system is heating. What is not normal is a continuous drip or a steady trickle that never stops. That points to a failed valve or a pressure problem, and it is quietly costing you money.
Can I just cap the pipe to stop it dripping?
No. Never cap, plug or restrict a TPR valve or its discharge line. It is the safety device that stops a tank of near-boiling water becoming a pressure vessel. Blocking it converts an annoying drip into a genuinely dangerous situation. If someone suggests capping it, get a different plumber.
What does TPR actually stand for?
Temperature and pressure relief. In Australia it is generally called a TPR valve; you may see it written as a T&P valve in American material. It opens if the water in the tank gets too hot or the pressure gets too high, discharging through a pipe that runs to a safe drainage point.
How often should the valve be operated?
Most manufacturers recommend easing the valve — lifting the lever briefly to flush it — at regular intervals, and the interval is set out in your system's manual. It clears sediment that would otherwise stop it seating properly. Worth noting that on an older system that has never been eased, doing it for the first time sometimes stops the valve sealing afterwards, which is a sign the valve was already at the end of its life.
Why does my hot water come out scalding?
Stored water is deliberately kept above 60°C to control Legionella, so the water leaving the tank is hot by design. The device that makes it safe at the tap is a tempering valve, which blends it down before it reaches bathroom fixtures. If your bathroom taps run dangerously hot, the tempering valve has likely failed or is missing. The fix is the tempering valve, never turning the thermostat down.
Does a dripping valve mean the whole system is failing?
Not necessarily. It is often the valve itself, which is a modest repair. But if the drip is caused by excessive mains pressure, the same pressure is working on every joint, fitting and appliance in the house, so it is worth diagnosing properly rather than just swapping the valve and waiting for the next thing to give.
Need it looked at?
We do same-day hot water repairs across Wollongong, Kiama, Shellharbour and down through Gerringong and Berry. Licensed plumber and gas fitter, 24/7 for emergencies.