How to Replace a Water Heater Gas Valve – DIY Tips

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. Exercise caution when handling the gas valve; avoid using a pipe wrench or similar tools to grip the valve body, and do not apply excessive force, as this could damage the unit. Both removal and replacement require disconnecting gas piping and electrical wiring.

1. Turn Off the Gas Supply: Locate the manual shut-off valve on the gas line, usually found near the water heater, and turn it off to prevent gas flow.
2. Adjust the Thermostat: Set the thermostat on the control valve to the lowest setting to minimize any residual gas in the lines.
3. Disconnect Electrical Power: Ensure that all electrical power to the unit is turned off at the circuit breaker to avoid any electrical hazards.
4. Set Control Knob to OFF: Turn the control knob on the water heater to the OFF position to stop any gas flow to the burner.
5. Drain the Water Heater (if necessary): If required by your model, drain the water heater to reduce pressure and prevent any water spillage during the repair.
6. Disconnect Gas Line Components: Carefully disconnect the thermocouple, pilot tube, main burner supply tube, and piezo igniter. Make sure to label or take photos of the connections for easy reassembly.
7. Unscrew the Gas Valve: Using the correct tools, disconnect the ground joint union and any other connections that are attached to the old gas valve. Remove the gas valve assembly from the tank.
8. Prepare the New Gas Valve: Compare the new valve with the old one to ensure correct specifications such as model number and ratings. Apply approved Teflon tape or pipe sealant on the threads of the new gas valve to prevent leaks. It’s crucial to wrap the tape clockwise around the threads to ensure a tight seal.
9. Install the New Gas Valve: Carefully screw the new gas valve into place. Ensure it is securely fastened but avoid overtightening, which might damage the valve.
10. Reconnect Gas Line Components: Reattach the thermocouple, pilot tube, main burner supply tube, and piezo igniter. Refer to your labels or photos to ensure correct reassembly.
11. Test for Leaks: Once everything is reconnected, turn the gas and power back on. Apply a soap-water solution to the new connections and observe for any bubbles that might indicate a gas leak. If you see any bubbles, turn off the gas and tighten the connections before testing again.
12. Final Checks: Once you confirm there are no leaks, set the control knob to the ‘Pilot’ position and follow your water heater’s instructions to relight the pilot light. Set the thermostat to your preferred temperature and check the operation of the water heater.

Safety Note: If at any point you are unsure or uncomfortable with performing these tasks, it is advisable to contact a qualified professional. Replacing a gas valve involves working with gas lines and potentially hazardous conditions if not handled properly.