Water heater gas control valve: settings, flash codes and when it needs replacing

What the valve does

The gas control valve, often called the gas control valve/thermostat, controls the gas to both the pilot and the main burner. It reads the water temperature through a sensor in the tank and opens the burner when the water cools. On A. O. Smith's FVIR models with a Honeywell valve, it's powered by a thermopile that the pilot heats, which A. O. Smith says generates 750 to 900 millivolts with no load. There's no plug: no pilot, no power.

If the water gets too hot, the main burner and pilot shut off automatically.

The dial settings

A. O. Smith lists these settings for its gas control knob:

• Low (vacation): about 55°F
• Hot: about 120°F, its recommended setting
• A: about 128°F
• B: about 138°F
• C: about 147°F
• Very Hot: about 160°F

It warns that 140°F water can burn in under five seconds. For trips, Rheem suggests vacation mode for a week or so and pilot mode, which shuts the main burner off but keeps the pilot lit, for longer ones. See what temperature a water heater should be and how to turn off a water heater.

Reading the status light

A. O. Smith's service handbook for its Honeywell-valve models lists these codes. Each code repeats after a 3-second pause:

• Not lit: the control is off or the pilot is out. Relight following the label.
• Solid on: the pilot recently went out and the thermopile is cooling. Turn the knob off and wait 10 minutes before relighting; the valve won't allow a quicker retry.
• 1 flash: normal operation.
• 2 flashes: thermopile voltage low. Check the wiring; A. O. Smith says to replace the thermopile if it reads under 350 millivolts.
• 4 flashes: temperature exceeded, meaning the high-limit (ECO) tripped. Replace the valve.
• 5 flashes: sensor failure. Replace the valve.
• 7 flashes: internal control failure. Replace the valve.
• 8 flashes: power-off failure. If the pilot stays lit with the knob off, replace the valve.

After lighting, it can take up to 90 seconds of pilot flame before the light starts blinking. Other brands use different valves and codes, so check the label on your heater. If the pilot won't stay lit at all, see pilot light keeps going out; on older valves with a thermocouple, see our thermocouple guide.

Replace, don't repair

Every valve fault in A. O. Smith's handbook ends the same way: turn the knob off, turn the main gas supply off and replace the gas control valve/thermostat. The U.S. Consumer Product Safety Commission is even firmer about valves that have been underwater: they can't be salvaged and must be replaced, because silt and corrosion can stop them working and let gas leak (see water heater after a flood).

If you smell gas, A. O. Smith says to turn off the gas supply valve and call a professional rather than keep troubleshooting. See water heater smells like gas.

Valve or new heater?

A valve is a covered part under most tank warranties during the parts term, which is 6, 8 or 10 years on A. O. Smith's residential models, though labor isn't covered (see water heater warranty). On a heater near the end of its life, the cost of a valve plus labor often points to replacement instead. Check how old yours is.

Gas tanks are common where homes run on gas. About 69% of Caldwell, Idaho homes heat with gas, with Intermountain Gas holding the city franchise (see our Caldwell page), and in Marshfield, Massachusetts about 61% heat with gas (see our Marshfield page). Our gas water heater repair page covers valve replacements.

Local pages mentioned

• Caldwell, ID
• Marshfield, MA