Water Heater Gas Control Valve: Problems and Replacement

What the gas control valve does

It does three jobs at once:

• Pilot safety. The thermocouple or thermopile powers the pilot safety valve inside the control. No flame, no power, no gas.
• Gas flow to the burner. A.O. Smith says the gas control valve regulates gas flow to the burner.
• Thermostat. A sensor in the tank tells the valve when to open. A.O. Smith's manuals call the part the gas control valve/thermostat for that reason.

On heaters with a status light, the control also reports what it sees. On the ProMax, a flash once every three seconds means the pilot is lit, and a solid light means it was lit and has gone out.

Signs the gas control valve is failing

Common signs, with the cheaper causes covered in the next section:

1. Pilot won't stay lit with a known-good thermocouple. If the thermocouple tests fine and the pilot still drops out, the magnet in the valve may be weak. A.O. Smith says a failed pilot control valve usually needs a service visit and likely replacement.
2. Pilot lit, burner won't come on. The pilot is fine, the knob is turned up, and the water stays cold. The valve may not be opening.
3. Water temperature all over the place. A.O. Smith notes this valve needs precise calibration, and a faulty one can cause poor heating.
4. Status light flashing an error. On heaters with a status light, extra flashes point to a fault. A.O. Smith's manual sends you to its status light chart for anything faster than one flash every three seconds.
5. Gas smell at the valve. Rare, but it's the one to act on right away.

What to check before blaming the valve

Several cheaper problems can look like a bad gas valve. Work through these first.

The gas supply. Make sure the gas supply valve on the line to the heater is on. A.O. Smith's guide says supply problems can stop the burner and put out the pilot.

The knob setting. It has to be turned from PILOT to ON or a temperature setting. The vacation setting on the ProMax holds the water at about 55°F, so a heater left on VAC feels broken.

The thermocouple. Test it with a multimeter; see how to test a water heater thermocouple. Replacing a bad one is cheap, covered on how to replace a water heater thermocouple.

The connection at the valve. A thermocouple nut that's backed off loses voltage. Honeywell's spec is finger tight plus 1/4 turn.

Air and dirt. A clogged air filter or flame arrestor starves the burner and can let the pilot go out. A.O. Smith says to inspect the filter at least once a year. A dirty burner gives poor flame and poor heating, and A.O. Smith recommends a yearly inspection and cleaning.

The FVIR thermal switch. Many heaters have a resettable thermal switch as part of their flammable vapor protection. If it has tripped, the heater won't run, and you need to find out why it tripped before you reset it.

If all of that checks out, the valve is the likely culprit. Our pilot light lit but no hot water page covers the cold-water version of this problem in more detail.

Replacing the gas control valve

This is where we say call a plumber, and here's why. The job means shutting off and draining the tank (the valve's sensor sits in the water on most models), unthreading the old valve from the tank, threading in the new one with the right sealant, reconnecting the gas line, pilot tube, burner line and thermocouple or thermopile, refilling, and leak-testing every gas joint. A.O. Smith's manual covers removing the valve and warns not to overtighten the fittings because that can damage them. Its general rule is that installation and service must be done by a qualified installer, service agency or the gas supplier.

The replacement has to match your heater. Buy by the model and serial number on the rating plate, not by what the old valve looks like. Natural gas and propane valves are not interchangeable.

Honeywell / Resideo valves. Many tank makers use a Honeywell or Resideo gas control. Even so, the right replacement is the one the water heater maker lists for your model, because the sensor length, gas type and settings are matched to the tank.

On an old heater, think twice. If the tank is old, the valve is one of the pricier repairs on a gas heater. Compare the quote with replacing the whole water heater before you approve it.

What it costs

HomeGuide's numbers:

HomeGuide also says to shut off the gas supply right away and call a plumber if the gas control valve malfunctions.