Split this by fuel type immediately, because the checks are completely different. On an electric water heater, start at the breaker and the high-limit reset switch. On a gas water heater, start at the pilot light. Both faults are usually simple and inexpensive — a full loss of hot water is rarely the tank itself failing outright.
Electric water heaters
- Check the breaker
Electric water heaters typically run on a dedicated 240 V double-pole breaker. Switch it fully off and back on rather than glancing at its position — a partially tripped breaker can look on while cutting power.
- Find and press the high-limit reset button
Most electric water heaters have a red reset button behind an access panel, usually near the upper thermostat. This safety device trips if the water gets too hot, and a tripped reset with no other symptom is a common, simple cause of a completely cold tank. Press it firmly — you should feel or hear a click.
- Listen for the elements
With power confirmed and the reset pressed, run a hot tap briefly and listen at the tank. Silence where you would expect a faint hum can suggest a failed element, though this is harder to confirm without an electrician’s multimeter.
Gas water heaters
- Check whether the pilot is lit
Look through the sight glass or viewing window near the base of the unit. If there is no small blue flame, the pilot has gone out.
- Follow the printed relighting instructions on the unit
Every gas water heater has a label with the exact relighting sequence for that specific model. Follow it precisely rather than improvising or using a method from a different appliance.
- Check the gas supply is on
Confirm the shut-off valve on the gas line to the unit is open, and that other gas appliances in the house are working normally — if none of them are, the issue is with the gas supply itself, not the water heater.
- If it lights but will not stay lit
This points at a different fault — see our dedicated pilot light guide rather than repeatedly relighting it.
If you smell gas at any point, do not attempt to light or relight anything. Leave the area if you can do so safely and contact your gas utility’s emergency line.
Common causes
| Cause | Fuel type | How likely |
|---|---|---|
| Tripped breaker | Electric | Very common |
| Tripped high-limit reset switch | Electric | Very common |
| Pilot light out | Gas | Very common |
| Gas supply valve closed | Gas | Common, especially after other plumbing work |
| Failed heating element | Electric | Occasional |
| Failed thermocouple preventing pilot from staying lit | Gas | Occasional |
| Failed thermostat | Both | Occasional |
| Gas control valve failure | Gas | Uncommon |
Checks that apply to either type
- Confirm the thermostat setting
Check it has not been turned down or off, particularly if anyone else in the household might have adjusted it recently.
- Check for a leak causing a safety shutdown
Some units include safety features that respond to a detected leak. If you find water around the base of the tank, that is a separate and more urgent issue — see our water heater leaking guide.
- Give it time after any reset
A tank of cold water takes a while to reheat fully after any reset or relight — do not judge the fix within the first few minutes.
When to call a professional
Call an electrician or plumber if the breaker and reset switch are confirmed fine but an electric unit still produces no heat. Call a licensed gas engineer if a gas unit’s pilot will not stay lit after following the correct relighting procedure, if you smell gas at any point, or if the gas control valve is suspected to have failed.
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.
