Hot Water Heater Not Working: Find the Fault in Order

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

Hot Water Heater Not Working: Find the Fault in Order

Short answer

Start with the two free checks. On an electric heater that is the breaker and the red reset button behind the upper panel. On a gas heater it is whether the pilot is still lit. Neither needs a tool or a part.

A hot water heater not working usually comes down to a few parts, and two of them cost nothing to check. The trick is doing them in the right order instead of buying the part somebody on a forum guessed at.

This page is the map. Each step says what to look at, and where the full test is when you need it.

Before you start

Gas or electric? Look at the bottom of the tank. A gas heater has a gas pipe going into a control on the front, a flue coming out of the top, and a small window near the floor. An electric heater has a thick cable going into the top and one or two metal panels on the side. The two are checked completely differently.

What to carry: a torch. That is all for the first two checks. Later on, a screwdriver and a multimeter.

How long has it been cold? A tank that ran out during a long shower and came back in an hour is not broken, it is just small. A tank that has been cold since yesterday is.

Stop if You smell gas, you see water near the electrical supply, or the tank is swollen. Shut the gas or the breaker off, and the cold water valve, and call somebody. Nothing below is worth a risk.

Work down one column

Electric on the left, gas on the right. The first two in either column are free.

Electric: the three checks in order

Check 1Free, thirty seconds

The breaker

Find the double breaker marked water heater. A tripped one often does not look off, it sits limply in the middle. Push it firmly all the way off, then back on.

If it trips again straight away, stop. That is an element shorted to the tank and it needs the meter test, not another flick of the switch.

Check 2Free, five minutes

The red reset button

Power off at the breaker. Take the upper access panel off the side, fold the insulation back, and you will see a small red button on the upper thermostat.

Press it firmly until it clicks. Put everything back, switch on, and give it 60 to 90 minutes, the time A.O. Smith gives an electric tank.

If that button pops again in a day or two, do not keep pressing it. Something is overheating the water, and that is a stuck thermostat or a shorted element.

Check 3

The thermostat, then the element

No hot water at all points at the upper thermostat or the upper element, because the upper thermostat runs the whole heater.

Hot water that runs out in a few minutes points at the lower element instead, because the top of the tank is heating and the bottom is not.

Both are ten minute tests: how to test a water heater thermostat, then how to test a water heater element.

Gas: the three checks in order

Check 1Free, ten seconds

Is the pilot still lit?

Look through the little window near the bottom of the tank with the room light off. A small steady blue flame means the pilot is fine and the fault is further on. Nothing at all means you have found it.

Check 2Free, fifteen minutes

Relight it properly

Gas control to OFF, wait ten minutes, then hold it on PILOT while you click the igniter, and keep holding for up to 90 seconds after the flame catches, as A.O. Smith says. The full steps and the reason for each are here: lighting a water heater pilot, and keeping it lit.

Check 3

The thermocouple, then the gas valve

A pilot that lights and dies the moment you let go of the knob points at the thermocouple first: four of the seven causes Bradford White lists are the thermocouple, its fit or the flame on it. It is the cheapest part on the heater.

A pilot that stays lit while the main burner never fires, with the dial turned up and cold water in the tank, points at the gas control valve. That is the expensive one, and on a heater over ten years old it is usually the moment to replace the whole thing.

When it is not really broken

It ran out and came back. Rheem lists water use that outran the tank as a cause, and says to wait for it to recover. A heater that recovers in an hour is working, it is just undersized for the day you had.

Somebody turned it down. Holiday mode, or a dial moved during a decorating job. A.O. Smith says to set both thermostats to the same number, and they leave the factory at about 120.

One tap is cold, the rest are fine. That is the tap or its cartridge, not the heater.

Warm, never hot. On a gas heater a dirty burner, on an electric one a dead lower element or a thermostat set too low.

Questions people ask

How long should the water take to come back?

A.O. Smith gives 60 to 90 minutes for a 40 to 50 gallon electric tank, and 30 to 40 minutes for a gas one. Judging it after ten minutes is why people think a good repair failed.

There is water on the floor as well.

Deal with that first, and it is a separate job: finding where a water heater is leaking.

It makes a rumbling noise and heats slowly.

Sediment on the bottom of the tank. It is boiling water underneath the crust, which is also what burns lower elements out early.

How old is too old?

A.O. Smith says eight to twelve years for a tank heater. At that age, keep repairs to cheap parts, because the tank is what runs out of life.

Do I need a plumber for any of this?

Not for the first two checks in either column. Gas valve work and anything involving the tank itself, yes.

Sources: A.O. Smith residential electric use and care guide 100264218 (no hot water, ECO, lower element), Rheem use and care manuals AP23124 (electric) and AP23698 (gas) troubleshooting charts, Bradford White service manual 238-51542 (pilot will not stay lit), A.O. Smith's status light support note, A.O. Smith "How To Relight A Water Heater's Pilot Light", "How Long Does It Take a Water Heater to Heat Up?" and "How Long Do Tankless Water Heaters Last?", and A.O. Smith's thermostat setting instructions