Water heater faults split cleanly by type: electric heaters fail mostly at the thermostat or heating element, gas heaters fail mostly at the pilot, thermocouple, or gas valve, and both fail from sediment build-up over time regardless of fuel type. Knowing your heater type before you start narrows the list considerably.
Electric or gas: check this first
Look at the bottom of the tank. A gas water heater has a burner compartment and usually a visible flue running up through the top. An electric water heater has no flue and no gas connection, just electrical conduit entering the top or side. This single check determines which half of this guide actually applies to your fault.
Find your fault by symptom
| Symptom | Most likely cause | Full guide |
|---|---|---|
| No hot water at all | Tripped breaker (electric) or pilot out (gas) | No hot water at all |
| Water leaking from the tank | Failed pressure relief valve, drain valve, or tank corrosion | Water heater leaking |
| Pilot light will not stay lit | Weak or failing thermocouple, or a draft | Pilot light will not stay lit |
| Hot water smells like rotten eggs | Bacteria reacting with the anode rod | Smelly hot water |
| Popping or rumbling noises | Sediment build-up on the tank bottom | Related problems below |
| Runs out of hot water quickly | Undersized tank, sediment reducing capacity, or a failing element | Related problems below |
| Water only lukewarm, never fully hot | Failed lower heating element, or a thermostat set too low | Related problems below |
| Water heater trips the breaker | Failed heating element or thermostat | See circuit breaker guide |
Sediment: the slow, universal cause
Minerals in tap water settle to the bottom of every tank-style water heater over months and years, regardless of fuel type. This layer insulates the heating element or burner from the water above it, forcing the heater to work harder for less effect, and it is directly responsible for popping and rumbling noises as water boils underneath the sediment layer. Left long enough, it also reduces effective tank capacity, which shows up as running out of hot water sooner than the tank’s rated size suggests.
Flushing the tank once a year clears sediment before it builds up enough to cause noise, reduced capacity, or accelerated element wear. It is a straightforward job — worth doing on a schedule rather than waiting for symptoms to appear.
The safety line for gas water heaters
Checking whether a pilot light is lit, and relighting it following the manufacturer’s printed instructions on the unit, is generally considered acceptable for a homeowner. Anything beyond that — adjusting gas valves, working on burner assemblies, disconnecting or reconnecting gas lines, or diagnosing a thermocouple or gas control replacement — belongs with a licensed plumber or gas engineer. If you ever smell gas near the unit, do not attempt to relight anything: leave the area if you can do so safely and contact your gas utility’s emergency line.
How age changes the diagnosis
Most tank water heaters are rated for eight to twelve years of service. Beyond that age, faults become more likely to be the tank itself — internal corrosion, a failing anode rod that can no longer protect the tank, or a lining breakdown — rather than a replaceable component. A leak or persistent rust-coloured water on a heater past this age is a stronger signal to consider replacement than to keep repairing.
When to call a professional
Call a plumber for any leak from the tank itself, for element or thermostat replacement on electric units, or for general installation and venting questions. Call a licensed gas engineer for anything beyond checking and relighting a pilot per the unit’s instructions — gas valve, burner, thermocouple, and venting work all sit outside safe DIY territory.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
