Unitrol Water Heater Gas Valve Problems

The Short Answer: A Robertshaw Unitrol is an old standing-pilot combination valve — OFF / PILOT / ON and a temperature dial — not a Richmond electronic control and not an ICON “electronic fault.” Pilot that dies when you let go is thermocouple (~20–30 mV), ECO, or a weak magnet. Pilot that holds but the main never opens is the operator or gas supply. Diagnosis only. Valve replacement is licensed leak-test work.

Search Intent & The Real Problem

You are holding the knob on PILOT, you see a flame, you let go, it goes dark — or the pilot lives and the main burner never whooshes. The tank is often 15–25 years old with a Unitrol 110-series (110-202, 110-206 and cousins) or a later Robertshaw combination control. Forums sell a $12 thermocouple and a “universal valve.” The thermocouple is sometimes the part. The magnet, the ECO in series with that thermocouple, and a valve that is leaking at the body are not DIY.

This is not Richmond / Rheem electronic and not ICON electronic fault detected. Those have status LEDs. Unitrol does not Morse-code you. General stay-lit path: will not stay lit.

What other guides get wrong

They treat Unitrol like a millivolt fireplace valve or a Honeywell WV electronic control. Water-heater Unitrols are thermocouple-powered magnets, typically 30 mV class — not a 750 mV powerpile. They tell you to crack the union to bleed air on a tank that heated yesterday — do not purge it. And they skip the ECO: an open high-limit in the thermocouple circuit drops the magnet the same way a dead thermocouple does.

Step-by-Step Troubleshooting

Step 1: Confirm it is a Unitrol — then stop if you smell fuel

• What to look for: Rating plate (NG vs LP). Lighting card on the jacket. Pilot that is a small standing flame, not a spark trial.
• Required tools: Flashlight, phone photo of the valve face. No wrenches.

A wet tank is not a gas-valve job. A cold tank with a live smell of gas is leave-the-building.

Step 2: Pilot will not light

• What to look for: Gas cock parallel to the pipe (open). Knob in PILOT, reset/magnet button held per the card. No flame at all after a long hold can be a closed supply, a clogged pilot orifice, or a valve that is not passing pilot gas — all licensed if the supply cock is already open. A lazy yellow pilot is dirt, wrong gas, or starving pressure, not “turn it up from the flex.”
• Required tools: Eyes at the combustion opening with the door positioned as the manufacturer allows. Do not poke the orifice with the gas on.

Manifold / inlet pressure is a licensed manometer job: about 3.5–7 in. w.c. natural gas / 10–11 in. w.c. LP at the appliance — confirm the rating plate.

Step 3: Pilot lights, dies when you let go — millivolts, ECO, magnet

• What to look for: Blue pilot wrapping the top 3/8 in. of the thermocouple tip. Open-circuit thermocouple output is typically ~20–30 mV after a few minutes of heat. Closed-circuit (adapter in the valve) often needs ~13 mV or better to hold. Below that: thermocouple or a pilot that is not on the tip. At ~20–30 mV and it still drops: magnet or ECO.
• Required tools: Millivolt meter if you are qualified. Lighting the pilot per the jacket card is the homeowner limit. Unscrew-and-test of the ECO through the valve body is a tech procedure.

The ECO is in series with the thermocouple on these valves. An open ECO (overheat, dry-fire, failed limit) drops the magnet with a perfect thermocouple. Never jumper it. Magnet drop-out that will not hold a 5 mV fall after a good thermocouple means the valve — replacement, licensed.

Step 4: Pilot holds, main burner will not open

• What to look for: Knob in ON, dial calling for heat, pilot still there, no main. Temperature bulb/capillary damaged, operator not opening, or supply that will feed a pilot but not a burner (meter, lockup regulator). This is not a thermocouple problem. A burner that lights and dies after seconds is more often draft, FVIR lint on later jackets, or a valve that will not stay open — still not a DIY union-crack.
• Required tools: Eyes. Dial not left on the lowest stop. Do not hold the knob in ON to force a burner.

Step 5: Replacement is the end of the diagnosis

• What to look for: Proven millivolts, proven ECO, clean pilot, supply open — and the magnet still will not hold, or the main still will not open, or the valve body weeps fuel. Match gas type, inlet size, and thermocouple thread. A “universal” control that needs adapters on a 20-year tank is how leaks start.
• Required tools: On a licensed truck: manometer, leak-detection solution, combustion check after fire-up.

If a handyman offers to thread on a valve without a leak test, fire him.

When DIY Ends: Calling a Licensed Pro

It ended at the gas train. You may read the lighting card, confirm the shutoff is open, look at the pilot flame, and leave if you smell fuel. You may not loosen the flex, swap the Unitrol, jumper the ECO, or hold the knob in. A licensed plumber measures millivolts and manifold pressure, tests the magnet and ECO, and replaces the combination valve only after those pass — then leak-tests every joint.

Frequently Asked Questions

Q: Why won’t my Unitrol water heater gas valve stay on? A: Weak thermocouple (~20–30 mV open-circuit), pilot not on the tip, open ECO in the thermocouple circuit, or a magnet that will not hold. It is not “air in the line” on a tank that ran yesterday.

Q: Can I replace a Robertshaw Unitrol myself? A: In most cities that is licensed fuel-gas work. A wrong or leaking valve is a CO/fire event. Hire a plumber who will leak-test and match the control.

Q: Is a Unitrol the same as an electronic gas-valve fault? A: No. Unitrol is a standing-pilot magnet valve with no diagnostic LED. Electronic-fault LEDs are ICON / Honeywell / Rheem-family controls — different parts, different charts.