How to Replace a Water Heater Gas Valve - DIY Tips

Water heater gas control valve

How to remove and replace a Robertshaw and White-Rodgers gas valve in gas water heaters. Features explained and reviewed.

A water heater gas valve or a control gas valve is the small device located inside the tankless water heater, or outside, in the lower part of the tank-type heater. It is designed to open or close the gas flow to the pilot or other ignition device and the main burner.

The gas control valve is constructed with a built-in safety shut-off device that closes the gas supply to the burner and pilot light if the pilot light is extinguished for any reason. What sends the signal is the electrode or a thermocouple.

Water heater gas valves: Types and features

Some water heater manufacturers like AO Smith, Bradford White, or the American Water Heater company are using White-Rodgers and Robertshaw gas control valves, and some manufacturers Honeywell gas valves.

Features of the White-Rodgers water heater gas valve

• Gas valve assembly with the safety valve, thermostat valve, and electromagnet
• Valve control knob with ON-OFF-Pilot settings
• Water temperature dial and setting indicator
• Inlet and outlet
• High limit or temperature probe
• Pilot tube threaded port
• Thermocouple threaded port
• Thermal switch

Features of the Robertshaw gas control valve

• Gas valve with the safety valve, thermostat valve, and electromagnet
• Control knob with ON-OFF-Pilot settings
• Reset button
• Water temperature dial and setting indicator
• Inlet and outlet
• High limit or temperature probe
• Pilot tube threaded port
• Thermocouple threaded port
• Thermal switch

Gas control valve anatomy

Some gas valves are equipped with the manifold gas pressure tap where you can measure the pressure on the line supplying the main burner. You just have to remove the plug and attach the gas gage.

The recommended pressure is marked on the water heater gas valve.

Both the pilot light tube and the tube from the main burner are connected to the gas valve, also a thermocouple - used as the safety element. The unit works on 24 VAC but has to be able also to work without the outside electricity.

Temperature setting indicator

A temperature setting indicator, installed on the front of the gas valve, allows you to select the right hot water temperature. The temperature is factory set on its lowest setting to comply with the safety regulations. On some control valves (Robertshaw), there is a "HOT" mark which shows where the 120 F temperature or scalding water range starts.

Ignition

The pilot burner is lit by switching the control knob to the pilot setting (or pressing the reset button on Robertshaw gas valves) and pressing the piezo igniter button. There are some old water heaters where you have to use a match to ignite the pilot manually.

Thermocouple

When the standing pilot is lit, the flame will heat the thermocouple, which will generate a small electric current sufficient to keep the gas valve open, and once you release the knob, the gas will continue to flow to the pilot.

Thermostat

Then, the incoming cold water temperature will activate the thermostat that initiates the water heater gas valve to open to the burner. The pilot light ignites the gas on the main burner. Hot water temperature rises, and once it reaches the set temperature, the thermostat interrupts the main flow.

Energy Cut Off - ECO 

Two metal probes are mounted onto the back of the gas valve and immersed inside the tank.

The longer temperature probe contains a metal rod or Energy Cut Off (ECO) switch that expands and contracts as the water changes its temperature. The probe acts as the temperature high limit. If water gets excessively hot, a sensor within the probe opens interrupts the small electrical current to the gas valve, interrupting the flow.

Thermal Cut Off - TCO

A thermal Cut Off Switch or TCO is a resettable thermal switch designed to shut off the gas supply if there is a high temperature inside the combustion chamber.

Note: If subjected to excessive pressure (over 0.5 psi), the control valve gets damaged, resulting in gas leaks or, even worse - explosion.

A manual shut-off valve in the gas supply line should be installed together with the drip leg or sediment trap to prevent foreign material like dirt from coming into the control valve. A flexible pipe or ground joint union between the shut-off and water heater gas valve should be installed to allow easier service.

Note: If for any reason the heater fails to shut down, i.e., due to overheating, always manually turn the water heater gas control valve off.

How to replace a water heater gas valve

When replacing a water heater gas valve, use the factory-authorized replacement parts only. Be very careful when working on the gas valve, do not use a pipe wrench or similar tool to grip the body, and do not apply too much force, as it might damage the element. The removal and replacement involve gas piping and electrical wiring disconnection:

• Turn off the manual shut-off valve on the gas line. It is typically located beside the water heater.
• On the control valve, turn the thermostat to its lowest setting.
• Turn the electrical power off.
• Turn the control knob to the OFF position.
• If it is needed, drain the water heater.
• Disconnect the thermocouple, pilot tube, main tube, piezo...
• Disconnect the ground joint union and other connections from the control valve.
• Remove the gas valve assembly.

When replacing the water heater gas valve, make sure to match the part with the one with the same characteristics, and follow the steps above but in reverse order. Use the approved Teflon tape or compound on all connections.

Check for gas valve leaks using the soap-water solution.