Relief valve leaking: when it's normal and when it's a warning
The relief valve is the one part of a hot water system designed to let water out. That makes "is it leaking?" the wrong question. The right one is when it discharges, and for how long.
Almost every storage hot water system in the country has a pipe running down its side or out through a wall, ending in the open air over a drain or a garden bed. When water comes out of it, most people read it as a fault.
Usually it is the opposite. That pipe is the system's safety outlet, and water coming out of it means the valve behind it is doing exactly what it was fitted to do. The job is not to stop it — it is to work out whether what you are seeing is the normal version or the warning version.
There is more than one valve, and they mean different things
People say "the relief valve" as though there is one. On a typical mains-pressure storage installation there are usually two or three devices that can put water on the ground, and they fail for different reasons.
| Valve | Where it sits | What it is for |
|---|---|---|
| Temperature and pressure relief valve | Screwed into the top or upper side of the cylinder | The last line of defence. Opens if the water gets too hot or too high in pressure |
| Expansion control valve | On the cold water line feeding the system | Takes the routine expansion of water as it heats, so the relief valve above does not have to |
| Pressure limiting valve | Usually near the meter or the system inlet | Holds the whole house down to a working pressure the fittings can live with |
Knowing which pipe is wet matters, because a discharge from the expansion control valve is a routine event, while a discharge from the temperature and pressure relief valve on the cylinder is the system telling you something is wrong upstream of it.
The normal version
Water expands when you heat it. A cylinder full of cold water that is brought up to temperature contains more water at the end of the cycle than it did at the start, and that extra volume has to go somewhere. In a mains-pressure installation the expansion control valve releases it.
Normal therefore looks like this:
- It happens while, or just after, the system is heating. On an off-peak or controlled-load tariff that is usually overnight, which is why people find the puddle in the morning.
- It stops. The discharge is intermittent, not constant.
- It is a modest amount — enough to wet the ground under the pipe, not enough to run all day.
- The water is clear.
If that is what you have, nothing is broken. It is worth knowing the pattern, though, because it is your baseline: the day it changes, you will notice.
The version that needs a plumber
Any of these means the discharge has stopped being routine:
- It runs continuously, including at times when the system cannot be heating.
- It has clearly increased — a trickle where there used to be an occasional drip, or a puddle that never dries out.
- The water is very hot, or there is steam. That points at the temperature side of the valve lifting, which is the serious one.
- It started suddenly, with no change to the system.
- It is rusty or discoloured, which is a cylinder condition question rather than a valve question — see which leak is urgent.
What causes a valve to discharge when it shouldn't
The expansion control valve has failed or was never fitted
If the valve whose job is to absorb expansion is not doing it, the expansion still happens — and the pressure relief valve on the cylinder takes it instead. That is why a system can start discharging from the top of the tank when nothing about the tank has changed. The fault is on the inlet line, not on the cylinder.
Mains pressure is too high
A pressure limiting valve that has failed, or a supply that has risen, can push the standing pressure in the house above what the relief valve is set to hold. The valve then lifts and stays lifted. The corroborating signs are elsewhere in the house: taps that bang when you shut them, noisy pipes, and tap washers that fail more often than they should.
The valve seat is fouled
Every time the valve lifts it flushes a little water past its seat, and that water carries scale and corrosion debris out of the cylinder. Eventually a fragment lodges on the seat and the valve cannot close fully. This is the classic cause of a valve that used to be intermittent and is now a permanent dribble.
The valve has simply aged
Relief valves are consumables, not permanent fittings. They sit in hot water, they operate repeatedly, and they are replaced as part of routine servicing rather than waiting for them to fail — which is one of the things a service is actually for. See how long a system lasts for where valve life sits against cylinder life.
The water is genuinely overheating
The least common cause and the only one that is urgent in its own right. If a thermostat or control has failed and the stored water is climbing past where it should be, the temperature element of the relief valve opens to dump it. Very hot discharge water, steam, or hot water at the taps that is hotter than it used to be all point this way. This one is a call now, not a call tomorrow.
A check valve has made the house a closed system
Some meters and backflow arrangements include a non-return valve. Where one is present, expanding hot water cannot push back towards the main, so the only way out is through a relief valve. Houses that gained a check valve during a meter change sometimes start discharging shortly afterwards, which looks like a coincidence and is not.
The checks that are yours to make
- Watch the timing over a day. Note when the pipe is wet and when it is dry. Discharge tied to a heating cycle is routine; discharge at three in the afternoon on an off-peak system is not.
- Work out which pipe it is — the one from the top of the cylinder, or the one from the valve on the cold inlet line. Photograph both. This single fact narrows the diagnosis substantially.
- Note the temperature, carefully and without putting a hand in the stream. Steam or scalding discharge changes the urgency.
- Look for the other symptoms of high pressure: banging pipes, taps that are hard to shut off, washers failing repeatedly.
- Note the age of the system, and when it was last serviced.
- Keep the area clear and draining. A discharge pipe needs to run to a safe place — not into a bucket, not into a garden bed that has been built up around it.
How urgent is it?
A relief valve that discharges during heating and then stops is not urgent at all. A valve that dribbles continuously is wasting water every hour and should be looked at in days rather than months, because a fouled valve rarely recovers on its own.
Discharge that is very hot, steaming, or accompanied by water at the taps that is hotter than it used to be is the exception: that is a temperature fault, and it is the one case here where the right response is to call a licensed plumber straight away rather than monitoring it.
How far can you take this yourself?
Observing, photographing and timing the discharge is yours, and it is genuinely useful — a plumber who is told which pipe is wet and when saves a diagnostic visit. Everything that involves touching a valve on a pressurised hot water installation is licensed plumbing work in every Australian state and territory, and the plumbing regulator for your state or territory is who licenses and inspects it. Where that line falls is set out in full on the licensed work pillar.
Frequently asked questions
Is it normal for water to come out of the hot water relief valve?
Some discharge is normal. Water expands as it heats, and that extra volume is released through a valve rather than building pressure inside the cylinder. Normal discharge happens while or just after the system heats, stops on its own, is a modest amount and runs clear. A continuous discharge, or one that is very hot or steaming, is not normal.
Can I cap or block a leaking relief valve pipe?
No, under no circumstances. That pipe is the pressure and temperature escape route for a sealed cylinder of very hot water. Capping, plugging, fitting a tap to it or restricting it removes the protection the system depends on and turns a minor fault into a dangerous one. If the discharge bothers you, the answer is to have the cause fixed.
Why is my relief valve dripping constantly?
The usual causes are an expansion control valve that has failed so the relief valve is absorbing expansion it was never meant to handle, mains pressure that has risen above what the valve holds, or debris from inside the cylinder lodged on the valve seat so it cannot close fully. Relief valves are also consumables and are replaced during servicing rather than run to failure.
Is a discharging relief valve an emergency?
Usually not. Intermittent discharge during heating needs nothing, and a constant dribble should be looked at within days because it wastes water and rarely recovers. The exception is discharge that is scalding or steaming, or hot water at the taps that is hotter than it used to be: that suggests the water is overheating and warrants a licensed plumber straight away.
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
- NSW Fair Trading. Plumbers, drainers and gasfitters — licensing