Hot water system leaking? What to do first, and what the leak usually means

A hot water system leaking onto the floor never turns up at a good time. The useful thing to know is that plenty of leaks come from a valve or a fitting, and those can be repaired. Some come from the tank itself, and those can't. Below is what to switch off first, how to track down the source, and what each spot usually means.

Quick answer: If water is running, pooling, or coming from the tank, turn off the energy to the unit first (the hot water circuit at the switchboard for electric and heat pump units, the gas control or gas isolation valve for gas), then close the cold water valve going into the unit. Then look for where the water starts. If all you can see is a little water from the relief valve drain pipe during heating, read the relief valve section before you switch anything off, because some of that is often normal. A relief valve dripping all the time, or a drip from a union or valve on the pipework, can normally be repaired. Water coming from the body or base of the tank, with everything above it dry, means the tank has most likely rusted through and needs replacing.

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A small drip from the relief valve drain during heating is a different case: check the relief valve section below first. For a steady leak, a puddle that keeps growing, or water from the tank body, do these in order: energy off first, water off second. If you shut the water and leave the unit running, a tank that keeps leaking can lose water while it is still trying to heat. An electric element that ends up out of the water can burn out.

Electric storage tank

Switch off the hot water circuit at the switchboard. It's often labelled "HWS" or "Hot Water". Some units also have an isolating switch on the wall nearby. If water has reached a power point, the switchboard or the electrical cover on the unit, don't touch anything wet. Keep everyone clear and get an electrician to make it safe.

Gas storage tank

Turn the gas control knob on the unit to OFF, or close the gas isolation valve on the pipe going into it. On most of these valves the handle sits across the pipe when it's closed. If you can smell gas, don't flick any switches. Get everyone outside and ring the gas emergency number on your gas bill.

Gas continuous flow

Switch off the power point the unit plugs into (normally on the wall beside it), then close the gas isolation valve underneath. In a cold snap, don't leave an outdoor unit switched off for days if you can avoid it. Most have frost protection that only works with the power on.

Heat pump

Switch it off at the switchboard or at the isolating switch near the unit.

Solar with a gas or electric booster

Turn off the booster the same way as above, and switch off the power to the solar pump or controller if there is one. If the tank is up on the roof, stay off the roof.

Then close the cold water

Most units have an isolation valve on the cold pipe going in. Where it sits varies between models, so follow the pipes. The hot outlet pipe will feel warm, the cold one won't. Turn the valve clockwise until it stops. Don't force a valve that won't move. If it's seized or you can't find it, turn off the main stop tap at the water meter. That cuts water to the whole house, but it stops the leak being fed.

Closing the cold water stops new water going in, but a storage tank holds a lot of water and it can keep leaking slowly until it's drained. Put down towels or a tray, move anything you care about out of the way, and leave the draining to the plumber. The water inside is hot enough to scald, and the drain valve on an older tank can start leaking once it has been opened.

The temperature and pressure relief valve (often called the PTR or TPR valve) is the brass valve near the top of a storage tank with a small lever on it. It's a safety valve. If the pressure or temperature inside the tank gets too high, it opens and lets water out through its drain pipe. How it's dripping tells you a lot.

A few drips while the unit is heating

Water expands as it heats. On a mains pressure system with no expansion control valve, that extra water has to go somewhere, and the relief valve lets a little of it out. Manufacturers describe a small discharge during heating as normal. If it drips for a while each time the unit heats and then stops, that's all it is. If it's more than a bucket of water a day, or it bothers you, a plumber can fit an expansion control valve on the cold line so the tank's relief valve doesn't have to do it.

Dripping all the time, even when the unit isn't heating

That points to a worn valve or a bit of grit on the valve seat. Most owner's manuals describe a lever test and how often to do it, so follow yours: lift the lever gently, let some water run out of the drain pipe, then lower it gently. Doing that once can flush a bit of grit away. This only works with the cold water still on. If you've already isolated the unit, or there's any chance the tank itself is leaking, leave the lever alone and let the plumber test it. Stand clear of the end of the drain pipe, because the water is hot, and don't force a lever that won't move. If it still drips afterwards, the valve needs replacing. Our hot water maintenance guide covers that lever test in more detail.

Running hard, or letting out very hot water or steam

A strong, constant flow from the relief valve drain, especially very hot water or steam, can mean the tank is overheating because a thermostat has failed. Turn the unit off at the gas control or the switchboard and leave it off until a plumber has checked it. Never block the drain pipe or tie the lever down to stop the water. That valve is what protects the tank from failing under pressure.

High mains pressure

If the water pressure coming in from the street is high, the relief valve can open even when the unit is fine. The plumbing standard limits pressure at the outlets inside a home to 500 kPa, so where the supply runs higher a pressure limiting valve gets fitted on the cold line. If that valve has failed, or there never was one, the relief valve can drip constantly. A plumber can test the pressure and tell you which it is.

Replacing a relief valve is licensed plumbing work. The tank has to be isolated and partly drained, and the new valve has to match the rating for that unit.

The pipework around a hot water system has several joints and valves, and any of them can weep. These leaks tend to be slow, and most are repairs rather than a reason to replace the unit.

• Unions and fittings. The nuts that join the cold and hot pipes to the unit can drip when a seal ages or the pipes get knocked. A drip here often runs down the side of the tank and looks like a tank leak from below. Resealing or replacing the fitting fixes it.
• The cold water valve. The isolation valve on the cold inlet often has a non-return valve built in, and sometimes a pressure limiting valve next to it. A drip from the spindle when you turn it, or from the valve body, means it needs servicing or replacing.
• The expansion control valve, where fitted. Some installations have an expansion control valve (ECV) on the cold line. It lets out the extra water from heating so the relief valve on the tank doesn't have to. A drip from the ECV while the unit heats is it doing its job. Constant dripping means the valve is worn or the mains pressure is too high.
• The tempering valve. Many systems have a tempering valve on the hot outlet that limits how hot the water is at the bathroom taps. A leak from its body or connections is a valve repair, and the valve has to be set correctly afterwards, so it's a plumber's job.

It's tempting to put a spanner on a weeping nut. On old brass and copper it's easy to crack a fitting or twist the pipe, and then a slow drip becomes a fast one. All hot water work is licensed work in Victoria, and our guide to what plumbing you can do yourself sets out where the line sits. Switching off, isolating and looking is fine. Undoing anything is not.

Most storage tanks are steel lined with a glass-like enamel, with a sacrificial anode rod inside that corrodes so the tank doesn't. Once the anode is used up, the steel starts rusting from the inside, and eventually it rusts through.

Signs the leak is the tank:

• Water seeping from under the outer jacket or around the base, with the pipes and valves above all dry
• Rust streaks running down the outside, or bubbling paint near the bottom
• Rusty or brown water from the hot taps only
• A leak that gets a little worse each day

A leaking tank can't be patched. The inner cylinder sits behind insulation and an outer jacket, and once it has corroded through in one spot the rest of it isn't far behind. Tank leaks also tend to get worse, sometimes quickly, so treat one as urgent even if it is only weeping now.

Before you assume the worst, check the relief valve drain and the fittings on top one more time. A drip at the top can run down inside the outer jacket and come out at the base. That alone is a good reason to have it looked at before buying a new unit.

Continuous flow (instantaneous) units

There's no tank to rust through, so leaks come from the water connections underneath, an internal fitting or the heat exchanger. If water is dripping from inside the case of a gas unit, switch it off at the power point, close the gas valve and leave it off. The case should only be opened by a licensed gasfitter. A leaking internal fitting can be replaced. On an older unit with a failed heat exchanger, replacing the unit can make more sense. Outdoor units can also split a pipe or fitting in a hard frost if the power has been off.

Heat pumps

Check the condensate hose first. Clean water from that hose is normal. A leak from the tank or its fittings is the same as any storage tank: fittings are repairs, the tank body isn't. A refrigerant leak doesn't leave water. It shows up as a unit that runs but doesn't heat. Start with the no hot water checks, and keep in mind that refrigerant repairs need a technician with a refrigerant handling licence, often arranged through the manufacturer's warranty.

Solar hot water

A ground-mounted tank with panels on the roof leaks in the same places as any storage tank. Some systems also have a frost valve up at the panels that lets out a little water on cold nights to protect them, and that is by design. Water running off the panels during the day, or a roof-mounted tank that is leaking, needs a plumber. Don't climb up to check.