Gas water heater not heating: how to find the cause

If you smell gas, stop here

A. O. Smith's gas water heater manual says that if you smell gas: don't try to light any appliance, don't touch any electrical switch, and don't use any phone in the building. Call your gas supplier from a neighbor's phone and follow its instructions; if you can't reach the supplier, call the fire department. More in water heater smells like gas.

1. Read the status light

Many newer gas heaters have a gas control with a small status light. In the A. O. Smith manual, a light that flashes once every three seconds means the pilot is lit and the control hasn't found a problem. If that's what you see and there's still no hot water, the manual says to check that the control knob is turned to HOT, not left on PILOT or a low setting.

If the light flashes more often than that, it's giving a code. The chart is in the manual or on the heater's label. On A. O. Smith models with a Honeywell valve, for example, two flashes means low thermopile voltage, and four, five, seven or eight flashes point to faults the company fixes by replacing the gas control (see water heater gas valve). If the light isn't flashing at all, the pilot is out.

2. Relight the pilot the right way

Always follow the lighting label on your own heater; models differ. On the A. O. Smith heater the manual covers, the steps are:

1. Make sure the manual gas valve on the heater's supply line is on.
2. Push the gas control knob in fully and hold it.
3. Click the igniter button about once a second for up to 90 seconds, until the status light starts to flash.
4. If it doesn't flash after 90 seconds, release the knob and wait 10 minutes before trying again. Repeat at least two more times, waiting 10 minutes between attempts.
5. If the light still won't flash after three attempts, turn the knob off and move to troubleshooting.

The manual adds that a newly installed heater has air in the gas line, so it can take several attempts. On older heaters with a standing pilot and thermocouple, Lennox says that if the pilot won't stay lit after two or three tries, the thermocouple likely needs a technician. Full walkthrough: how to light a water heater pilot light.

3. Check the igniter spark

If the pilot won't light, A. O. Smith's manual says to dim the room lights and look through the viewport while clicking the igniter. No spark? Check that the igniter wire is pushed firmly into its connector. A spark but no pilot? Make sure the gas supply valve is on and try the lighting steps again. If the pilot still won't light, the manual says to call a qualified person.

4. Pilot lit, burner won't come on

If you can see the pilot but the main burner never fires, check the knob first. The control in the A. O. Smith manual has a vacation (VAC) setting that holds the tank at about 55°F, which keeps it from freezing but won't give you usable hot water. If the knob is on HOT and the burner still won't light, the gas control or its thermostat may have failed, which is a technician job.

Newer gas tanks also have a flammable vapor ignition resistant (FVIR) system. A. O. Smith says improper installation or an inadequate air supply can cause the FVIR system to disable the heater, and that its resettable thermal switch must never be disabled or modified, because that can lead to fire, explosion or excess carbon monoxide. A heater that has been through a flammable vapor incident shows discoloration on the flame arrestor and needs to be replaced entirely.

5. Clean the air filter or screen

The air intake at the bottom of many gas tanks has a filter or screen. A. O. Smith's manual says it must be cleaned from time to time, and that if it isn't, the pilot may go out. Its troubleshooting guide adds that blocked air screens restrict combustion air and affect the burner. The manual says heaters in spots with a lot of lint or dust need the filter cleaned more often.

6. Confirm the gas supply

A. O. Smith says gas supply problems keep the burner from lighting and can also put the pilot out. Check that the valve on the gas line to the heater is open. If your stove or furnace is also out, the problem is in the supply, not the heater, and your gas utility is the call. On propane, check the tank level.

7. Burner runs, but water is only lukewarm

If the burner lights and runs but the water never gets properly hot, A. O. Smith's manual lists these causes:

For a heater that's warm but never hot enough, our guide to water heater not hot enough goes deeper.

Sizzling after you relight it is normal

When a gas heater is first lit and the tank is full of cold water, A. O. Smith says condensation often forms and drips onto the burner, making a sizzling sound, and you may see a small puddle. It isn't a leak and goes away once the tank reaches normal temperature. A puddle that keeps growing is different (see water heater leaking from the bottom).

How long to wait, and when to call

Once the burner is running, A. O. Smith's troubleshooting guide says a gas tank takes about 30 to 40 minutes to recover. Its manual says that if you still have no hot water two hours after lighting, go to troubleshooting.

Call a technician if the pilot won't stay lit, the status light shows a fault code, the burner won't fire with the knob on HOT, or you smell gas. Lennox puts the typical lifespan of a tank heater at 8 to 12 years, so on an older heater it's worth weighing repair against replacement (see water heater repair cost).

If your home runs on gas, these checks are the ones that apply. Census estimates on our town pages show about 83% of homes in Amherst, Ohio and 80% in Orrville, Ohio heat with gas.

Local pages mentioned

• Orrville, OH
• Amherst, OH