Why Hot Water Systems Stop Working: The Seven Real Causes

Knowing what makes systems fail is valuable trivia; it helps you understand that some warnings are worth paying attention to, some maintenance is worth doing, and when your system fails, whether you’re seeing a quick part change or the end of the day. The following are the seven actual reasons for the hot water problems in Sydney, in order of discovery, with the common denominator in front of them all.

1. Element burnout: the electric workhorse retiring

There are two ways an electric tank’s heating element can reach the end of its life; first, it undergoes thousands of heating cycles, causing stress on the internal wiring and insulation, and second, it undergoes thermal stress during each of those cycles. The silent burnout: the element just stops operating, the tank is cold, the breaker is not tripped, and it is an easy diagnosis. And the dramatic failure to earth, the element’s insulation fails, current leaks where it shouldn’t and the circuit breaker trips, and re-trips on every reset, as it is doing the right thing to reject a faulty element. The stealth version turns up in tandem element tanks, where one element dies, the other half heats the tank and everyone suffers months of lukewarm mediocrity before anyone realises the connection. Element replacement is one of the more common types of plumbing repairs; the question is what did it, which leads us to the next causes.

2. Thermostat and control drift: the brain ageing

All systems have a part to determine when to heat, a thermostat on electric tanks, a gas control valve on gas storage, electronics on continuous flow, and these parts don’t fail in dramatic or sudden fashion, but rather as they creep off the temperature setting. A drifting thermostat provides water slowly colder (or hotter) than set, and a dying thermostat doesn’t call for water at all, just like a failed element on a working thermostat; a gas control valve can maintain its pilot without ever operating the main burner. The tell that distinguishes control faults from heating faults: the heating equipment is performing well, but it simply isn’t being asked to perform. Controls are service parts to be replaced on a regular basis and should not be ignored; an erratic thermostat is a loophole that wastes energy and increases scald risk, and should be fixed as soon as it is found.

3. Sediment: the accelerant under everything

Across the decade of a tank’s service, slowly in Sydney’s relatively soft water, but relentlessly, the little layer of silt that settles out of the water commits three crimes: it leaves the water and settles on the tank base, and then it grows, and grows, and grows. It buries the element (or blankets the base above a gas burner), making the heat source work much harder to compensate, driving the insulation to premature failure, sediment is the biggest element killer. It consumes space, forcing litres of stored hot water to be displaced from the tank, and thus reducing the time it will remain hot. And it makes noise: the rumbling, popping, kettling noise of water boiling under the silt layer is sediment’s signature, and a tank that talks during heating is confessing. The answer is straightforward, although time-sensitive: an early and mid-stage tank that has a sediment problem, a professional flush is the answer; a veteran tank, with a lot of puckering on the bottom, is beyond the point of no return, and flushing will not improve the situation.

4. The anode-to-tank timeline: the one terminal cause

There are six causes on this list that end with repairs. This one is the one that ends with replacement, which is why it deserves to be told in its entirety. A steel tank is made to endure for decades of hot water thanks to a sacrificial anode, a magnesium or aluminium rod that hangs inside the cylinder, is more prone to electrochemical corrosion than steel, and suffers the corrosion instead of the cylinder. The timeline is set the day the tank is installed: the anode is used up in about three to five years (closer to three to four in hard water; about four in coastal salt air); after that, the cylinder begins to rust, followed by rusty hot water and finally, a “weep” or leak from the cylinder body. The leak is intractable, there’s no patching up the corroded steel, and it’s an optional item, a check and replacement at year 5 will reset the clock, and it’s definitely one of the most cost-effective maintenance tasks around a hot water tank, routinely adding years to a tank at a small fraction of the cost of replacement. Almost no one does it these days. The story is in the tanks.

5. Valve failures: the small parts with big portfolios

There are three valves and all of them fail with a different characteristic. The TPR (temperature and pressure relief) valve protects the tank from over-pressure: if it dribbles when it is heated, it is normal; if it dribbles continuously, the valve or system pressure has failed; if it is bone dry and scaled up, it might be seized, which is a problem, because it will not protect anything when the tank needs it. (Its maintenance is quite a real thing: push the lever for a few seconds a few times a year.) The tempering valve mixes stored 60°C water with cold water to the legal 50°C delivery to the taps, and when it wears out after five to eight years of operation, it tends to go further toward cold, and the “system running, taps lukewarm” paradox is what it explains in this series of articles. The pressure one: houses that are not fitted with a limiting valve and have high mains pressure put pressure on every part above, valves, elements, fittings, the tank itself, hence the ubiquitous presence of the humble PLV in failure post-mortems.

6. Gas component wear: the flame-side service items

Gas systems bring a wear list of their own. The old faithfuls are all the same: the thermocouple (the safety sensor that keeps the gas valve open while it is sensing the heat from the pilot burner) gets worn, and it stops sensing and the safety system closes the gas valve, and if you are a faithful relighter, the pilot won’t stay lit. Igniters go off by clicking without spark or without sound. Sooty, yellow and weak-heating (overdue-service) flames that have drifted from a healthy blue roar are a problem that occurs over the years as burners corrode and clog, along with combustion itself and flue problems. The periodic gas service, burner cleaned, pilot assembly checked, flue inspected, is more effective in preventing the winter-morning surprise than the big parts, because the small parts of the gas system are where the failures occur.

7. Supply and settings: the failures where nothing is broken

The last and most depressing reason: the issue was upstream or administrative and all components passed all tests, a meaningful percentage of “hot water system stopped working” calls. The roll call: off-peak tariff arithmetic (a controlled-load tank that emptied on a heavy day and can’t legally reheat until tonight); tripped breakers from storms and surges; gas supply interruptions, empty LPG bottles, and closed meter valves (the cooktop test settles it in seconds); scrambled timers after blackouts, heating quietly at the wrong hours; and various switched-off states, boosters off since summer, systems set to pilot before holidays, isolation valves closed during work. Not a single one of them requires a repair, but each of them requires the proper check in the proper order, hence the diagnostic guides in this series begin with checking your clocks, your boards and your windows before checking any component.

The multiplier and the stress test

All seven causes are controlled by two forces. Age multiplies everything: every cause mentioned above is a slow process, so for any system, the practical question of risk boils down to just how far along the stories are for any particular symptom in the system: at year 4, it is a repair, at year 11, it is an announcement, at year 13, it is a repair plus an announcement for several of the stories, and so on. Winter is the examiner, and when demand is greatest, those elements that are just below standard, those worn-out thermostats, those failing burners and the burned-out tanks all fail for the first time in the first cold snap of the season. The failures were not caused by June, but made apparent by it, and it’s the households that listen to autumn’s warnings who are showering hot while the queue is building up.

The prevention that actually moves the needle

If you know the causes, you know what to add to the maintenance list, and it won’t take up much room. The anode check at year 5 (3 to 4 coastal): the single item of the highest value, as it’s the only terminal cause, and the flush that goes with it clears the element and restores capacity. The TPR ease every six months: thirty seconds, and the safety valve keeps honest. Flame-side systems: a gas service every other year. The autumn once-over before the winter test: age counted off the compliance plate, warning signs fairly evaluated, marginal systems fixed or replaced on your timetable, plus a limiting valve if the gauge shows high mains pressure. That list, kept, beats most of this article, and that’s the point of the article.

Questions we’re often asked

Mine really did fail with no warning at all. Which cause does that?

Usually it’s the one whose warnings weren’t absent, just not heard: the pilot that needed a flame relit every so often, or the hot water that ran out a few minutes earlier every winter. Controls can fail to give warning at any time, which is a legitimate zero-warn failure, but the number of these is small. Some of the work of this article is to make next time’s “no warning” into “oh, that’s what that was.”

What’s the single most common cause?

It is broken up by fuel: elements and their breaker dramas are before and after electric; thermocouples before and after gas; boosters before and after solar; sediment is the universal ending. And across all types, the “nothing broken” category of tariffs, trips and switched-off states takes a share, which would embarrass all of us if it were published.

Is the year-five anode service genuinely worth it, or plumber upselling?

There is only one failure that can’t be fixed, and it is the one intervention that targets it. The numbers: a modest service now, against the flood risk of a leaking tank a year or more in the future. Disclaimer: if you have a symptom-bearing tank in its second or third decade, the anode moment is over, and you will hear it from the honest plumber and not the seller.

My system is twelve years old and just failed. Is any repair worth it?

Ask what failed. A tank-body leak: nope, that’s the end of the story, and the call has been made. An inexpensive addition to a quiet, dry tank: at times a bridge of borrowed time, though with an anode that’s long gone, that bridge is short. As this is the twelfth-year repair quote, every repair quote has to have its replacement quote next to it, including the rebate; picking with both numbers visible is the game.

The bottom line

Hot water systems don’t just “stop,” they finish stories, elements burn out, controls drift, sediment accumulates, anodes give their lives, tanks give theirs, valves wear, gas parts age, and sometimes nothing breaks, except the setting. The stories are based on age (how far along they are) and they are graded in the winter. Learn the short maintenance list, and you’ll see why when your own day does arrive: the seven causes will be legible, and it will be obvious if you’re buying a part or planning a retirement, and that’s the position you want to be in when your day does arrive.

The information on this page was created by the Premium Sydney Plumbers team, who are licensed plumbers servicing all types of hot water systems throughout Sydney, and available for same-day work where possible. When your hot water system has failed, it’s just a phone call away, any time, on 1300 026 452.