Reset is a test. It is not a repair.
A breaker that has tripped has done its job: it detected current going somewhere it should not. Pressing it back gives you one piece of information — does the fault clear by itself, or is it still there? If the heater runs for a week, the trip was a one-off. If it trips again within minutes, the fault is permanent and it is inside the unit, most often a heating element whose insulation has failed. That is the case the rule exists for.
Because a water heater is a tank of water with a mains-powered element inside it, an element that has failed to earth can make the water live. The breaker is the only thing between that and the person in the shower. Hold it closed and you are holding a switch closed on a live fault — which is why every manufacturer, every electrician and every fire service says the same sentence: reset once; if it trips again, stop and get someone qualified. There is no version of this where trying a third time tells you something new.
Two more that need no judgement at all: if any water at the tap feels tingly or numbing, stop using it and cut the power at the breaker. If a gas unit has been failing to light and you can smell gas, do not touch a switch, open a window, leave, and call the gas emergency line. With a gas unit it also matters that you do not sit there clicking the ignition over and over: unburnt gas accumulates, and the correct sequence is off, ventilate, wait, then try once.
Three sides of the system — and that is what the faults are
Before anything else: is it electric or gas? The two have completely different fault logic and a completely different first move. A storage electric heater is a big cylinder on the wall with a mixing valve beside it, and its supply is a breaker and a plug with a test button. A gas heater is a thin box with a flue on top, its supply is a meter and a valve, and — critically — you can test the supply in ten seconds by trying the hob. Same pipe, so if the hob lights, there is gas in the house and the fault is the heater. If it does not, you have found the whole problem without touching the appliance.
- Nothing at all — no display, no fan, no clicking. The supply side. On electric: the breaker, then the reset button on the plug, which needs to click and light up to have actually reset. On gas: the meter's own battery (many have one, and a flat battery closes the valve), the emergency shut-off lever, credit, and then the hob test. Add the freebies — a closed inlet valve, and a panel someone has left in holiday or half-tank or eco mode, or a child lock, or a temperature turned down to forty. Those are not faults at all, and they are astonishingly common after a house full of guests.
- It runs but the hot water is weak, or goes cold in a minute. The water side. Close the cold side and run hot only, to see the true hot flow. Then try a different tap: if only one outlet is affected, the heater is innocent and the problem is that outlet's mixing valve or cartridge — no appliance work at all. If it is everywhere, the inlet strainer on the cold connection is the first suspect, and it lifts out for a rinse. Hard water, high floors and peak demand all show up here as well, because a gas unit will not fire below its minimum flow.
- A breaker is down, or the plug's button has popped, or the panel is showing a code. A safety device has already operated. Note down which one and what the code says — that is the single most useful thing you can hand a technician — then reset it once, and watch. Clear for ten minutes and still running: probably fine, watch it for a day. Trips again straight away: stop, and it is no longer your job.
The decision that actually costs money
Once it is a part rather than a supply problem, the money question is not "what is broken" but "is this machine worth repairing". Three tiers, and the order matters: free (reset, battery, valve, mode) covers most calls; a part (strainer, thermostat, element, solenoid) is tens of dollars and worth it on a unit with years left; the unit is where a repair stops making sense. The working life is about eight years by standard, eight to twelve in practice. Past that, a large repair is not fixing the problem, it is paying in advance for the next one.
And one exception overrides all of that: if water is coming from the tank itself, not from a fitting, the answer is a new heater. The tank is a pressure vessel with a glass or enamel lining, and a lining that has rusted through cannot be patched from outside by anybody. Water from a valve or a connection is a different story and often a cheap fix — so look at where it starts, not just that the floor is wet. A few drops from the relief valve while it is heating is normal expansion, not a fault; a steady stream is.