No Hot Water: Diagnosing Gas and Electric Water Heaters

Start by identifying which kind you have, because the failure modes share almost nothing. Gas heaters have a flue out the top and a burner assembly at the bottom; electric heaters have a thick cable and one or two access panels on the side.

Electric: the pattern tells you the fault

Electric tank heaters have two elements — upper and lower — wired so only one runs at a time.

Symptom	Almost certainly
No hot water at all	Upper element or upper thermostat
Hot water, but very little	Lower element failed
Lukewarm water, never hot	Lower element, or crossed plumbing
Runs out fast, recovers slowly	Lower element or heavy sediment

Check the breaker first, then the reset button — a small red button on the upper thermostat behind the access panel. It trips on overheat, and it trips for a reason, so if it trips again after resetting, stop and get it looked at.

To test an element you need the power off at the breaker, confirmed dead with a meter, then the element’s resistance measured across its terminals: typically 10–16 ohms. Open circuit means it has failed. Continuity from either terminal to the tank body means it has shorted and must be replaced.

Do not open the panel without switching off the breaker and verifying. These are 240 V circuits.

Gas: the pilot chain

If the burner will not light:

Is the pilot lit? Look through the sight glass. Newer heaters have electronic ignition and a status LED instead — count its flash pattern and check it against the label.
Pilot lights but goes out when you release the knob — the thermocouple is not producing enough voltage to hold the gas valve open. It is a cheap part and the most common gas failure. Sometimes it is only sooted up and cleans up with fine abrasive.
Pilot will not light at all — no gas (check other gas appliances), a blocked pilot orifice, or the thermal switch on the combustion chamber has tripped.
Pilot burns yellow and lazy rather than blue and sharp — combustion air problem. Clean the flame arrestor screen at the base and make sure nothing is stored against the heater.

Smell gas and you stop here. Leave the building, do not operate switches, and call the gas utility from outside.

Both types: sediment

Minerals settle out of the water and bake onto the tank bottom. On gas heaters this insulates the water from the burner; on electric it buries the lower element. Symptoms are slow recovery, popping or rumbling noises, and shrinking capacity.

Draining a few gallons from the bottom valve every six months prevents most of it. On a tank that has never been drained, the sediment may have solidified and draining can clog the valve — worth knowing before you start.

The failure that means replacement

Water on the floor beneath the tank, not from a fitting, means the inner tank has corroded through. There is no repair for that. Tanks last 8–12 years, and past year ten a leak is the expected ending.

If a leaking tank is what you have, shut off the cold supply at the valve above the heater and switch off the gas or breaker before doing anything else.

What is not DIY

Gas line work, replacing a gas control valve, anything involving the flue or venting, and any repair on a leaking tank. Combustion venting failures produce carbon monoxide, and that is the reason for the line here rather than squeamishness about the plumbing.

Sizing and recovery, if the tank is simply too small

“Not enough hot water” is sometimes not a fault. Two figures describe a tank’s real capacity:

First-hour rating — how much hot water it delivers in the first hour from full, combining tank volume with recovery. This is the number to compare, not gallons.

Recovery rate — gallons per hour it can reheat. Gas recovers far faster than electric, which is why a 40-gallon gas tank often out-delivers a 50-gallon electric one.

Rough demand: a shower is 10–20 gallons, a bath 30, a dishwasher 6, a washing machine 20 on a hot cycle. A household taking three consecutive showers needs a first-hour rating around 60 gallons.

If the tank is correctly sized and healthy but you still run out, the fix is usage sequencing or a bigger unit — not a repair.

Setting the temperature

Two competing considerations:

Below 50 °C (122 °F), Legionella bacteria can multiply in the tank.
Above 55 °C (131 °F), tap water scalds quickly — a few seconds at 60 °C is enough to burn a child.

The usual resolution is to store at 60 °C and fit thermostatic mixing valves at the taps, or to set 49–50 °C where scald risk outweighs the rest. Many tanks ship at 60 °C. If you have young children or elderly residents and no mixing valves, this is worth checking rather than assuming.

Every 10 °F of reduction cuts standby losses by a few percent, so there is a modest efficiency argument for the lower end too.

Maintenance that decides its lifespan

Drain a few gallons every six months. Attach a hose to the drain valve at the base, open it, and let it run until clear. This is the single most effective thing you can do, and skipping it is why tanks fail at eight years rather than fifteen.

Check the anode rod every 2–3 years. It is a sacrificial magnesium or aluminium rod that corrodes so the steel tank does not. When it is consumed, the tank starts rusting. Replacing a rod is inexpensive; replacing a tank is not. It usually sits under a cap on the top of the heater and needs a socket and some leverage.

Test the T&P valve annually. The temperature and pressure relief valve is the safety device that stops the tank becoming a pressure vessel failure. Lift the lever briefly and confirm water discharges and stops. If it does nothing, or weeps afterwards, replace it — and route its discharge pipe to a safe location if it is not already.

Insulate the first few feet of pipe off the tank, and the tank itself if it is older and uninsulated.

Recognising the end

Tanks give warning:

Rusty or metallic-tasting hot water, with cold water clear.
Rumbling or popping during heating that persists after draining.
Weeping at the base or around the fittings.
Age past ten years with no maintenance history.

Replacing a heater before it fails is much cheaper than replacing it plus a floor. If yours is over ten and shows any of the above, plan it rather than wait.

Gas safety, restated

Carbon monoxide is the reason gas water heaters get their own cautions. Signs of a venting problem: soot around the draft hood, a pilot that keeps going out, condensation on nearby windows, or a headache when the heater has been running.

Fit a CO alarm near the appliance if there is not one already. It costs very little and it is the only warning you will get.