Hot Water System Leaking? Where the Water Comes From Decides the Fix

Find the leak before you spend anything

A wet patch under a hot water unit tells you almost nothing on its own, because water runs to the lowest point and the puddle is frequently nowhere near the failure. The most useful thing you can do before making a phone call is dry the unit and the slab completely, lay paper towel across the base and around each fitting, and come back in 30 minutes. The first sheet to go wet is your answer.

Of the 8 rows above, 2 are not faults at all. Those 2 account for a meaningful share of the calls that come in about leaking hot water systems, and recognising them saves you a call-out entirely.

The relief valve: when a drip is normal and when it is not

Every storage hot water system in Australia carries a temperature and pressure relief valve near the top of the cylinder, with a copper pipe running down the side to a safe discharge point. It is a safety device with an unglamorous second job: water expands by roughly 2% when heated from mains temperature to 60 degrees, that volume has to go somewhere, and the valve lets it out.

This is why a drip during the heating cycle is normal and expected. The unit fires, the water expands, the valve weeps for a while, and it stops. What is not normal is a valve that runs continuously, that dribbles when the unit has been idle for hours, or that has gone from an occasional drip to a steady trickle over a few weeks.

• A valve that will not reseat. Scale and grit collect on the seat over the years and hold it fractionally open. Replacement is the fix, and it is one of the smaller jobs on a hot water system.
• Mains pressure that is too high. If the incoming pressure sits above the valve rating, the valve relieves constantly and a new valve will do exactly the same thing within weeks. The actual fix is a pressure limiting valve at the meter, typically set around 500 kPa.
• A failed or missing cold water expansion control valve. Where a non-return valve or pressure limiter stops expansion pushing back into the main, an expansion control valve is meant to take it instead. When that valve fails, the TPR valve inherits the job.
• A dying thermostat letting the water overheat. Rarer, and more serious, because the valve is then relieving genuine over-temperature. Scalding hot water at the discharge pipe is a same-day call.

Manufacturers instruct owners to lift the relief valve lever briefly every 6 months to flush grit off the seat. That advice is sound on a maintained unit and slightly risky on a neglected unit, because a valve that has not moved in 9 years sometimes fails to reseat afterwards and then runs continuously. If your valve has never been touched, know that outcome is possible before you lift the lever on a Sunday night.

When it is the tank itself, the system is finished

A domestic storage cylinder is mild steel with a vitreous enamel lining fired onto the inside, plus a sacrificial anode hanging down the middle. The enamel is glass, so it always has microscopic flaws, and the anode exists to corrode in place of the steel exposed at those flaws. When the anode is consumed, the steel starts corroding instead. Perforation happens from the inside out, which is why there is no visible warning until water appears on the floor.

By the time you can see water coming from the tank body, the steel has failed in more than 1 place. It cannot be welded, because the enamel around any weld is destroyed and corrosion accelerates from there. It cannot be sealed with epoxy, putty or a bottled tank sealer, because pressure and thermal cycling defeat all of them. Nobody in the trade repairs a perforated cylinder.

The honest silver lining is that a failed cylinder is a decision rather than a diagnosis problem. Nothing needs investigating and there is no diagnosis left to make. The only question left is what replaces it, and that deserves 20 minutes of thought rather than a panic purchase. Our guide on whether a hot water system is worth repairing covers it.

Shut it down safely, in this order

1. Turn off the cold water supply at the isolation valve on the cold inlet, normally a lever or a small stop tap at the top of the unit. If it will not move, or there is no valve fitted, shut the water off at the meter instead.
2. Electric units: switch the hot water circuit off at the switchboard, not at a wall switch you are unsure about. Never handle a leaking electric unit while the circuit is live.
3. Gas storage units: turn the gas isolation valve at the unit to off, and set the control knob to off according to the label on the unit.
4. Continuous flow gas units: there is no tank to drain. Isolate the water and the gas and stop there.
5. Solar systems with a roof-mounted collector: isolate the cold feed, then get advice before draining anything. Draining a collector on a hot day can cause stagnation and thermal shock damage.
6. If the leak is inside the house or above a living area, drain the unit. Connect a hose to the drain valve at the base, run it to the garden, and open a hot tap somewhere in the house to break the vacuum so the tank will empty.

If you cannot find the isolation valve, or the leak is running across the floor, shut the whole property off at the water meter. Our guide to turning off the water mains covers where the meter sits on a typical Adelaide allotment and which way the tap turns, and it is worth reading before you need it rather than in torchlight.

Leaks that are not the hot water system at all

A surprising number of hot water leaks turn out to be something else discharging into the same corner of the yard or the same floor waste. Rule these out before spending money on the unit.

• Heat pump condensate. A heat pump pulls heat from the air and the moisture in it condenses, so several litres of clear water a day in humid weather is correct operation, not a fault.
• Air conditioner condensate lines, frequently plumbed to the same drain point as the hot water discharge and dripping through summer for the same reason.
• A leaking flexible hose under a nearby vanity or laundry tub, tracking along the slab and surfacing somewhere unexpected. Flexible hoses have a finite life and are among the most common causes of internal water damage in Australian homes.
• A rooftop solar collector relieving through its own valve on a hot afternoon. Occasional discharge in high summer is normal, constant discharge is a valve or controller fault.
• A slab leak on the hot water line, showing up as an unexplained warm patch on the floor and a water meter that keeps turning with everything shut off.

The meter test settles most of these for free. Turn off every tap and appliance, note the water meter reading including the small red dials, wait 30 minutes without using water, and read it again. Movement means water is escaping somewhere on your side of the meter. No movement means you are looking at valve discharge rather than a leak, which is a completely different repair. Our post on investigating a high water bill takes that test further.

Why Adelaide units fail when they do

Storage hot water systems in Adelaide generally give 8 to 12 years, and there are local reasons for the spread inside that range. Water hardness varies across the metropolitan area depending on whether your supply zone is fed from the Mount Lofty Ranges reservoirs or from River Murray water, and SA Water publishes that data by suburb. Harder water scales elements and consumes the sacrificial anode faster, which shortens cylinder life.

• External installations. Adelaide overwhelmingly puts hot water units outside, often wedged into a narrow side passage. Valves seize, casings rust and lagging degrades in the sun, none of which the same unit in a garage would suffer.
• Coastal corrosion. Around Glenelg, Somerton Park, Hove and Brighton, salt air attacks casings, brass fittings and relief valve springs on a shorter clock than it does 8km inland.
• Hills exposure. Above Belair and through Crafers, Stirling and Aldgate, exposed pipework and discharge lines see frost, and many properties run LPG rather than reticulated natural gas.
• The anode nobody replaces. Checking the sacrificial anode at around 5 years is a small job and materially extends cylinder life. Almost nobody does it, which is why so many units die at 10 rather than 15.
• Age clustering in postwar suburbs. Where a street of 1960s homes was converted at the same time, the units tend to fail within a few years of each other.

None of that helps once a cylinder has perforated, and all of it matters on the unit you buy next. An anode check every 5 years is about the simplest maintenance available on any appliance in the house.

So do you repair it or replace it

• Tank body leaking: replace. There is no version of this where repair is the answer.
• Under 8 years old with a valve, element, thermostat or pilot fault: repair, and check the warranty first, because cylinder cover commonly runs 5 to 10 years.
• Between 8 and 12 years with a consumable part fault: repair, and have the anode checked while the unit is open.
• Over 12 years and needing a major component: replace.
• Any age, still under warranty: get the model and serial number off the compliance plate before anyone touches it. Plenty of units get thrown out while still covered.

The reassuring part is that valves and connections are consumable items designed to be replaced, while cylinders are meant to last the life of the system. A leak at a valve is ordinary maintenance. A leak at the tank is the end of the road. The whole purpose of this page is that you can work out which of the 2 you have, standing in your own side passage, before anyone quotes you.

Allmat Plumbing covers hot water repairs and replacements across the inner south, the western suburbs, the coast and the Adelaide Hills. Marc is a licensed plumber and gas fitter with 20 years of experience, he does the work himself, pricing is agreed up front, and he texts an arrival time before setting off. Where the honest answer is that a valve needs replacing and the unit has years left in it, that is what you will be told.

Questions we get asked

References

• SA Water: household water supply and water quality by area
• Energy Rating (Australian Government): water heaters
• Standards Australia: AS/NZS 3500 Plumbing and drainage