No Hot Water? 7 Causes and How to Fix Them

Losing hot water is one of those problems that always seems to happen at the worst moment. The good news is that most causes are repairs rather than replacements, and a couple of them you can sort out yourself before anyone charges you a cent.

Here's how to work out which one you're dealing with.

First, check whether it's actually the hot water system

Before anything else, turn on a hot tap somewhere else in the house. If one tap is cold and the rest are fine, the problem is that tap or its supply line — not the hot water unit.

Then check whether you have water pressure at all. No water anywhere means a supply problem, not a hot water problem.

1. The pilot light or ignition has gone out (gas)

This is the single most common cause on gas storage systems. A gust of wind, a brief gas supply interruption, or a tired thermocouple will put the pilot out.

Most units have relighting instructions printed on a label near the burner. If it lights and then goes out again within a minute, the thermocouple has failed — that's a genuine repair, and it's a safety device doing exactly what it's meant to do. Don't try to bypass it.

2. The heating element has failed (electric)

Electric storage systems have one or two elements. If the lower element fails you'll get a small amount of hot water that runs out almost immediately. If the upper one fails, you'll usually get nothing.

Elements are a consumable — they fail eventually on every unit. Replacing one is a straightforward job and costs a fraction of a new system.

3. The thermostat has drifted or failed

If your water is lukewarm rather than hot, the thermostat is the usual suspect. They drift over time, and they can fail outright.

It's worth knowing that a storage tank should be held at 60°C to prevent legionella growth, with a tempering valve bringing it down to a safe 50°C at the tap. If someone has simply turned the thermostat down to "save money", that's a health risk rather than a saving.

4. The off-peak circuit isn't energised (electric)

A lot of electric storage systems in NSW run on a controlled load tariff, which only energises overnight. If your household's usage has crept up — teenagers, a new dishwasher, more people working from home — you can genuinely run out before the tank reheats.

If you're consistently running out in the evening, that's a sizing or tariff problem, not a fault.

5. The tank is leaking

Look underneath the unit. Water pooling there means the tank itself has corroded through, and a corroded tank cannot be repaired — it has to be replaced.

Turn it off at the isolation valve to stop it emptying into your yard or under your house, and call a plumber. If you catch this early you avoid the water damage that usually comes with it.

6. Sediment build-up in the tank

If your unit rumbles, bangs or knocks while heating, sediment has built up on the base and the burner or element is heating through a layer of it. That's inefficient and it shortens the tank's life considerably.

A flush often fixes it, and it's a cheap job relative to what it prevents.

7. The sacrificial anode is spent

Every storage tank has an anode rod that corrodes so the tank doesn't. Once it's gone, the tank starts corroding — and rusty or discoloured hot water is the warning sign.

Replacing an anode every five years genuinely extends a tank's life by years. Almost nobody does it, which is why so many tanks fail at twelve when they were rated for fifteen.

What you can safely check yourself

• Whether other taps are affected
• Whether the pilot light is out, and relighting it once using the printed instructions
• Whether the circuit breaker for the hot water unit has tripped
• Whether there's water pooling under the tank

What needs a licensed plumber

Anything involving the gas valve, the thermocouple, elements, thermostats, the tempering valve, the pressure relief valve, or the tank itself. Gas work in particular is licensed for good reason — an incorrect repair produces carbon monoxide.

Repair or replace?

The honest rule of thumb: if the tank is leaking, it's a replacement. If it's an element, thermostat, valve or pilot assembly, it's almost always worth repairing — provided the unit is under about twelve years old.

If you're stuck without hot water in the Hills District, Hornsby or the Upper North Shore, we can usually diagnose it the same day and often have you back in hot water that afternoon. Have a look at our hot water system repairs and installation for what's involved and what it costs.