Water heater repair, before the tank decides for you

No hot water: gas tanks

On a gas water heater the chain is short: gas arrives, a pilot or igniter lights the burner, the burner heats the tank, a thermostat decides when to stop. A fault anywhere in that chain gives the same symptom.

• Pilot out. On older units with a standing pilot, relighting per the instructions on the tank is a reasonable homeowner step. If it will not stay lit, stop, that is a thermocouple or gas valve problem, not something to keep attempting.
• Thermocouple or flame sensor. The safety device that proves a flame is present and shuts the gas off if it is not. They degrade with age and are a common, inexpensive failure.
• Gas supply. Check the shut-off valve at the appliance is open, and whether other gas appliances in the house are working. If nothing gas-fired works, the problem is upstream.
• Gas control valve. A failed thermostat or control valve assembly. More expensive, and on an older tank this is often the point where replacement enters the conversation.
• Venting. Some units have a blocked-vent safety switch that shuts the appliance down. A blocked flue is a genuine safety issue and needs resolving before the heater is used again.

No hot water: electric tanks

Electric heaters usually have two elements, an upper and a lower, each with its own thermostat, plus a high-limit cutout.

Tripped high-limit. There is a red reset button behind the upper access panel. If it has tripped, something caused an overheat, so resetting it once is reasonable but a repeat trip means a failed thermostat or element and needs diagnosis rather than another reset.

Failed element. The classic pattern is diagnostic: no hot water at all usually means the upper element or upper thermostat; a small amount of hot water that runs out very quickly usually means the lower element has failed and only the top of the tank is being heated. Elements burn out routinely, especially where sediment has built up around the lower one, and replacement is straightforward.

Breaker. Check the panel. A repeatedly tripping breaker on a water heater circuit is not a nuisance to be reset, it usually means a shorted element or a wiring fault.

Failed thermostat. Either thermostat can fail, and the symptoms overlap with element failure, which is why testing rather than guessing matters.

Not enough hot water

Running short is a different fault pattern from having none.

Sediment. The commonest cause on any tank more than a few years old. Minerals settle to the bottom, occupying volume that should hold water and insulating the burner from the water above it on gas units. The tank is nominally forty gallons and functionally thirty. Flushing annually prevents it; flushing a badly neglected tank sometimes reveals a leak the sediment was plugging, which is worth knowing before you start.

A failed lower element on an electric unit produces exactly this symptom, a short burst of hot water, then cold.

Dip tube failure. The tube that delivers cold water to the bottom of the tank can crack or break, dumping incoming cold at the top where it mixes straight into the outgoing hot. Output goes lukewarm and capacity collapses.

Thermostat setting that has been turned down, sometimes years ago by someone else.

Genuine undersizing. If it has always run short and nothing has changed, the tank is too small for the household, no repair fixes that.

Crossover. A failed mixing valve or a faulty single-lever faucet can let cold bleed into the hot side, which feels like a heater problem and is not.

Water that is too hot

Scalding water is a safety issue, not a comfort one. At high delivery temperatures a serious burn takes seconds, and children and older adults are affected fastest.

The usual cause is simply a thermostat set too high, which is easily corrected. A thermostat that has failed and is no longer controlling is more serious, and a tank that is overheating to the point of the relief valve discharging is a genuine hazard that should be dealt with immediately.

The right long-term arrangement in most homes is to store at a temperature high enough to be microbiologically safe and fit a thermostatic mixing valve so the water arriving at taps and showers is safe to touch. That gives you both, plus more usable capacity, since hotter stored water blended down goes further.

Leaks and what they mean

Where the water is coming from determines everything.

From a fitting or connection at the top. Often repairable, a union, a flex connector, or a valve. Worth fixing promptly but not a crisis.

From the temperature and pressure relief valve. The T&P valve is a safety device that discharges if pressure or temperature exceeds safe limits. A dribbling T&P can mean a failed valve, but it can equally mean the system genuinely is over-pressure or overheating, which is a real hazard. Never plug or cap a T&P valve. Have it diagnosed.

From the drain valve. Usually a worn valve, often replaceable.

From the bottom of the tank itself. This is the one that ends the conversation. A tank leaking from the body has corroded through and cannot be repaired at any price. It needs replacing, and it needs replacing soon, because tank failures tend to progress from seep to flood without much warning.

Noises, rust and smells

Rumbling or popping on a gas tank is water flashing to steam under a sediment layer at the bottom. It is the sound of a tank working much harder than it should, and it is a flush-or-replace signal depending on age.

Rusty or brown hot water means corrosion. If it appears only on the hot side, the tank or its anode is the source; if both hot and cold run rusty, the problem is in your supply piping instead.

Rotten-egg smell in hot water is typically a reaction between the sacrificial anode and bacteria in the water rather than a fault. Changing anode material generally resolves it.

All three point back to the anode rod, which is the most neglected component on any storage water heater. It is a sacrificial rod that corrodes so the tank does not, it lasts a few years, and replacing it periodically is the single cheapest way to extend tank life substantially.

Repair or replace

Repair is the right answer for a tank under roughly eight to ten years old with a discrete component failure, thermocouple, element, thermostat, dip tube, drain valve. These are inexpensive parts and the tank has years left.

Replacement is the right answer when the tank body leaks, when the unit is past twelve years and needing repeated attention, when the repair cost approaches half a new installation, or when the tank has always been too small for the household.

Age matters more with water heaters than with most appliances, because the failure mode at end of life is a tank full of water letting go. Nursing a fifteen-year-old heater through one more winter is a gamble with a specific and expensive downside.

If you are replacing, that is the moment to reconsider type and capacity: see water heater installation and tankless installation. If your hot water comes from your boiler rather than a standalone unit, see indirect water heater service.