Hot Water System Leaking? Where It's Coming From and What to Do

If you've found your hot water system leaking, the first job is working out where the water is coming from. A small drip from the relief valve drain line is often normal. A leaking fitting is a repair. Water weeping from the base of the tank itself means the tank has corroded through, and that's a replacement.

Here's how to tell which one you've got, how to shut the system down safely, and what happens next.

Hot water system leaking? Find the source first

Dry everything off with a towel first, then watch for a few minutes. It's surprisingly hard to trace a leak on a wet unit, because water runs down the jacket and pools at the lowest point regardless of where it started.

Look at three places, in this order:

• The copper drain line running down from the relief valve near the top of the tank
• The pipe connections and valves at the top or side of the unit, including the tempering valve if there is one
• The base of the tank, where the outer jacket meets the ground or the stand

Those three points line up with the three causes below, from least to most serious.

1. The relief valve is dripping

Every storage hot water system has a pressure and temperature relief valve, usually a brass valve near the top with a small lever and a copper line running down to the ground or a drain.

Is it normal for a hot water system to leak a little?

From this valve, yes. When water heats it expands, and that extra volume has to go somewhere. On many systems a small amount discharges from the relief valve line during each heating cycle. A few drips an hour, or a small wet patch at the end of the drain line after the unit has been heating, is the valve doing its job.

Why is my hot water relief valve leaking constantly?

A steady trickle or a constant stream is different. The usual causes are:

• The valve itself has worn out. Relief valves are a service item, and manufacturers generally recommend replacing them around every five years.
• Scale or grit has lodged on the valve seat, so it no longer closes fully. Homes on tank or bore water, common on acreage around Dural and Kenthurst, tend to see more of this than homes on mains supply.
• The mains pressure coming into the house is too high, so the system spends its life close to the valve's limit. A pressure limiting valve on the incoming supply fixes the cause rather than the symptom.
• The thermostat has failed and the water is overheating. This is the one to take seriously, and a reason not to ignore a valve that suddenly starts discharging a lot of very hot water.

Most manufacturers suggest easing the relief valve lever gently every six months to keep the seat clear. If you do this and the valve won't stop running afterwards, it's due for replacement anyway.

Whatever you do, never block, cap or tie off a leaking relief valve. It is the safety device that stops the tank from over-pressurising.

2. A fitting, valve or tempering valve is leaking

If the water is coming from a pipe connection rather than the relief valve drain line, you're usually looking at a threaded joint that has loosened or corroded, a failed isolation valve, or a tempering valve that's past its best.

The tempering valve is the one that mixes hot and cold so no more than 50°C reaches your bathroom taps, while the tank stays at 60°C to stop legionella. Like relief valves, tempering valves wear and need periodic servicing. If you're not sure whether you have one, our post on what a tempering valve does explains where it sits and what goes wrong with it.

A leaking fitting on an otherwise healthy tank is a straightforward repair. The main thing is not to leave it, because a slow drip onto the base of the unit or the floor underneath does damage over time, especially on units installed inside a laundry or cupboard.

3. The tank is weeping from the bottom

Is a hot water system leaking from the bottom repairable?

If the water is coming from the tank itself, no. A hot water system leaking from the bottom almost always means the inner cylinder has corroded through. That happens once the sacrificial anode (the rod inside the tank that corrodes so the tank doesn't) has been used up and nobody has replaced it.

You can't patch a corroded tank. It's a replacement, and the leak will only get worse, sometimes gradually and sometimes all at once.

Before you conclude the worst, check that the water isn't running down from a fitting or the relief valve above and collecting at the base. That's why drying the unit off first matters.

We see a lot of these in the Hills District in homes built in the 1980s and 90s, where the original gas storage unit has been replaced once and the second one is now reaching the same age. In newer estates like Kellyville and The Ponds, the builder-grade units installed when the houses went up are reaching their first replacement too. If you're weighing up how much life your unit has left, how long hot water systems last covers the typical ages by system type.

Should I turn off my hot water system if it is leaking?

For a small, occasional drip from the relief valve drain line, no. That's normal operation.

For anything else, yes. Turn it off, and do it in the right order so you don't damage the unit or burn yourself.

Electric storage

1. Switch off the hot water circuit at the switchboard. It's usually labelled "HWS" or "hot water". Do this first, because an element running in a tank that's lost water will burn out.
2. Close the cold water isolation valve on the inlet pipe into the tank. Turn it clockwise.

Gas storage

1. Turn the gas control knob on the unit to "off" (or "pilot" if the instructions label it that way).
2. Close the cold water isolation valve on the inlet.
3. If you smell gas at any point, leave it alone, get everyone outside and call a licensed gasfitter. If the smell is strong, call 000.

Heat pump

1. Switch it off at the isolator switch next to the unit or at the switchboard.
2. Close the cold water isolation valve.

With the cold supply closed, the tank can't refill, so the leak will slow and stop. Don't try to drain the tank yourself by opening the relief valve. The water inside is at 60°C or more and it comes out fast.

If water is spreading into the house or you can't find or close the valve, our guide to what to do about a burst pipe or leak walks through turning off the water at the meter.

How long can a leaking hot water system last?

It depends entirely on the source.

Where it's leaking	What it usually means	Typical outcome
Relief valve, small drips during heating	Normal expansion	Leave it, ease the lever every six months
Relief valve, constant trickle	Worn valve, scale or high mains pressure	Replace the valve, check the pressure
Pipe fitting or tempering valve	Loose or failed joint or valve	Repair
Base of the tank	Corroded inner cylinder	Replace the system

A dripping relief valve can go on for months without doing harm, though it wastes water and it's telling you something. A tank leaking from the base is on borrowed time. Some hold on for weeks at a slow weep, others split with little warning. If it's an indoor unit, don't wait to find out which.

What you can safely do yourself

• Dry the unit off and trace where the water is coming from
• Check whether the relief valve drip happens only while the unit is heating
• Ease the relief valve lever gently, standing clear of the drain line, as per the manufacturer's instructions
• Turn the unit off at the switchboard or gas control, and close the cold water isolation valve
• Move anything stored around the unit that could be damaged by water

What needs a licensed plumber

Replacing a relief valve, tempering valve or isolation valve, adjusting or replacing a thermostat, checking and fixing mains pressure, replacing the anode, and replacing the tank itself. Anything touching the gas supply or burner needs a licensed gasfitter, and in NSW gas work is signed off with a compliance certificate. Hot water installations also need to meet AS/NZS 3500, the plumbing code, including the relief valve drain line and tempering valve.

Getting it sorted

If your hot water system is leaking and you're not sure whether it's a valve or the tank, send us a photo or give us a call. We'll tell you straight whether it's a repair or a replacement, and we give you a fixed price before any work starts. There's no call-out fee, and our labour is covered by a lifetime warranty.

We look after homes across the Hills District, Hornsby Shire and the Upper North Shore. Have a look at our hot water system repairs and replacement page for what's involved.