Unitrol Water Heater Gas Valve Troubleshooting
On this page (8 sections)
The Short Answer: A Robertshaw Unitrol is an older standing-pilot gas control with an OFF/PILOT/ON knob and a temperature dial. If the pilot dies when you release the knob, suspect the thermocouple, a weak pilot flame, or an open high-limit (ECO). If the pilot stays lit but the burner never comes on, the valve’s thermostat section or the gas supply is more likely.
Is it a Unitrol?
Unitrol controls appear mostly on older tank heaters. They have a round temperature dial and a gas knob marked OFF, PILOT and ON. You light the pilot by hand, either with a match or a push-button igniter, while holding the knob down. They have no status light and give no flash codes.
If your gas control has a blinking LED, it is an electronic control. Use water heater gas valve electronic fault detected or Richmond water heater gas valve problems instead.
Match the symptom to the part of the valve
| What happens | Likely cause | Who handles it |
|---|---|---|
| No pilot flame at all while holding the knob | Supply valve closed, gas service off, clogged pilot orifice | Owner checks supply; technician for the orifice |
| Pilot lights but goes out when the knob is released | Thermocouple worn or not in the flame, open ECO, weak magnet in the valve | Technician |
| Pilot was steady for years, now goes out every few days | Thermocouple wearing out, draft, or dirty pilot | Technician |
| Pilot stays lit, main burner never comes on | Dial set too low, failed thermostat side of the valve, low supply pressure | Owner checks dial; technician for the rest |
| Burner lights, then goes out with the pilot | Possible overheating (ECO), venting or draft problem | Technician; check for CO alarm |
| Smell of gas at the valve | Leak at a fitting or the valve body | Leave and call from outside |
Why the pilot won’t stay lit
A thermocouple sits in the pilot flame and makes a very small voltage when it is hot. That voltage holds a magnet in the Unitrol open, which keeps gas flowing to the pilot. On these valves, the ECO (an over-temperature cutoff in the tank) is wired into the same circuit. So a worn thermocouple, a pilot flame that is small or yellow and doesn’t reach the thermocouple tip, an ECO that has opened, or a magnet that has weakened with age all cause the same symptom: the pilot goes out as soon as you let go.
An ECO opens when the water overheats, which can happen if the thermostat side of the valve fails. If the ECO is the cause, replacing only the thermocouple won’t fix it. Water heater thermocouple problems and pilot won’t stay lit after replacing the thermocouple explain how to tell these apart.
When the pilot is fine but the burner isn’t
Turning the knob to ON and the dial up should open the main burner gas within a few seconds if the water is below the setting. If it doesn’t, the dial may be on its lowest setting or on VACATION, or the thermostat section of the valve may have failed. If the supply can feed the small pilot but not the burner, there may also be a gas pressure problem at the meter or regulator.
What you can check safely
- If you smell gas anywhere, stop and leave the building.
- Make sure the gas supply valve on the line to the heater is open (the handle runs along the pipe) and that other gas appliances work.
- Look at the knob position and the temperature dial. Turn the dial to a normal setting if it was turned down.
- Relight the pilot only by following the lighting instructions printed on the heater’s label, including the waiting period after turning it off.
- Look through the sight glass to see whether the pilot is blue and steady or small and yellow.
- If the pilot won’t stay lit after you follow the label instructions twice, leave the knob on OFF and call for service.
What the technician will check
A gas technician will look at the pilot flame and its position on the thermocouple and measure the thermocouple’s output while the valve is working. They will also check whether the ECO circuit is closed and whether the magnet holds. If the burner won’t light, they test gas pressure at the inlet and burner against the rating plate and check the thermostat section of the valve. If they replace the valve, they match it to the gas type and the heater, leak-test all connections, and check the burner and venting once it is running.
Repair or replace
A thermocouple or pilot cleaning is a minor repair. A Unitrol with a weak magnet, a failed thermostat section, or a leak at the body has to be replaced as a whole valve, because these controls are not serviceable inside. On many old tanks, original replacement controls are no longer available. Then the decision becomes whether a new valve is worth putting on an aging tank. If the tank is old, check its age before you approve the work.
Frequently Asked Questions
Q: Can I replace a Unitrol gas valve myself? A: Replacing a gas control means opening fuel-gas connections and leak-testing them, so it is work for a licensed gas fitter or plumber in most areas. A bad connection can leak gas or cause poor combustion.
Q: Why does my pilot go out when the heater gets hot? A: If the pilot stays lit while the water is cool but goes out after a long heating cycle, the ECO may be opening because the water is overheating, or the burner area may not be drafting properly. A technician needs to find out which. Don’t just keep relighting it.
Q: Is “air in the line” why my Unitrol won’t light? A: Only right after the gas supply was off or the heater was newly installed. A heater that worked yesterday doesn’t suddenly get air in the line. Don’t loosen fittings to bleed it.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
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