Water Heater Pilot Stays Lit Drops On Third Call Decision Tree

Why this matters

A pilot that holds steady but drops after two or three burner calls is one of the most frequently misdiagnosed water-heater faults. Techs replace the thermocouple or thermopile on the first visit, the pilot relights, and the unit fails the same way 48 hours later. The pattern points to a thermal-cycling failure rather than a static component failure. The component, the gas valve, or the venting passes a single cold-start test but fails after the burner has cycled and heat-soaked the surrounding hardware. Reading the cycle count, not just the steady-state, is what separates a one-call fix from a chronic callback.

Symptom presentation

Typical customer report: "It works for a day, then the hot water runs out and the pilot is dead." On arrival the pilot is often already out, OR it lights and holds for the diagnostic period. Diagnostic clues:

• Pilot drops 5 to 20 minutes after burner shutoff, never during a call.
• Pilot drops only after the second or third complete heat cycle of the day.
• Unit installed in a closet, alcove, or small mechanical room with the door closed.
• Combustion air opening sized for original lower-BTU unit, replaced with higher-BTU unit.
• Thermopile millivolt reading at the gas valve reads above 350 mV cold, drops to under 200 mV after two cycles.

Quick checks

1. Read thermopile output cold at the TH/TP terminals on the gas valve. Acceptable: 600 to 800 mV open circuit, 350 to 450 mV closed circuit (call for heat). Below 350 mV closed circuit fails the gas valve magnet.
2. Confirm pilot flame envelopes the thermopile tip by 3/8 to 1/2 inch. A pilot that licks the tip but does not envelope underperforms when the thermopile is heat-soaked.
3. Check combustion-air opening size against the manufacturer install instructions. NFPA 54 Section 9.3 requires 1 sq in per 1000 BTU input for upper plus lower air openings to outdoors.
4. Inspect the draft hood for spillage with a smoke pencil after a 10 minute call. Spillage indicates inadequate vent draft, which back-drafts on shutdown and cools the thermopile.
5. Verify the unit is not in a downwind alcove of the building's prevailing wind. Wind-induced backdraft is a common third-call killer in coastal and ridgetop installs.

Isolation tree

Step 1: Run three back-to-back cycles. Turn the thermostat to max, let the unit cycle to setpoint, draw 40 gallons of hot, let it cycle again. Repeat for a third cycle. Time pilot drop from each shutoff. If the pilot drops after cycle 3 within 15 minutes, the failure mode is thermal-cycling related.

Step 2: Thermopile under heat soak. Read mV at the gas valve immediately after the third cycle shutoff. If the closed-circuit reading drops below 300 mV within 5 minutes of shutoff while the pilot is still lit, the thermopile is degraded and cannot hold the valve open under heat-soaked conditions. Replace.

Step 3: Combustion air starvation. If the thermopile holds mV but the pilot still drops, the combustion-air supply is depleting after multiple cycles. A closet door closing changes the room from "open" to "confined" per NFPA 54. Verify upper and lower opening sizing. Common failure: a single 100 sq in opening at the top, no lower opening.

Step 4: Vent draft on shutdown. With a manometer at the draft hood relief opening, log pressure for 30 minutes after a 10 minute call. A draft that reverses (positive pressure at the relief opening) for more than 30 seconds backdrafts combustion gas across the pilot, cooling it. Common causes: shared vent with a furnace that just shut off, blocked vent terminal, rain cap missing.

Step 5: Gas-valve magnet weakness. If thermopile mV stays at 350+ closed circuit and combustion air and vent draft both check clean, the gas-valve magnet itself may be weak. Test by shorting the TP terminal direct to a 750 mV bench source. If the valve drops with the bench source supplying mV, the valve is the failure point. Replace gas valve.

Step 6: Wind-induced backdraft. Run the same three-cycle test on a calm day vs a windy day. A 4 mph differential is often the threshold. Wind backdraft requires a vent-cap upgrade (e.g., Field Controls WMO-5) or vent extension.

Confirming diagnosis

Component-level diagnosis confirmed when:

1. Thermopile replacement under matching cycle conditions restores stable hold to cycle 4 and beyond.
2. Adding code-compliant combustion air openings restores hold and the thermopile mV no longer drops on cycle 3.
3. Vent-cap or vent-extension change eliminates the draft reversal logged on the manometer trace.
4. Gas-valve replacement restores hold while the existing thermopile retains specified mV.

If the unit is over 12 years old and the thermopile is the third one in two years, replace the unit. Cumulative tank scale at the burner skirt reduces flame-to-thermopile coupling, and chasing components on an end-of-life unit erodes margin.

Remediation

By isolated cause:

• Degraded thermopile: Replace with OEM part number for the unit. Generic universal thermopiles often produce lower mV than spec. Verify mV at the gas valve, not just open-circuit.
• Combustion air starvation: Install upper and lower openings per NFPA 54 Section 9.3. For a confined space with a 40,000 BTU heater the minimum aggregate is 80 sq in (40 upper, 40 lower) for openings communicating with outdoors via vertical ducts.
• Vent backdraft: Install vacuum-relief or wind-resistant cap, extend vent above adjacent obstructions per NFPA 54 Section 12.7.2.
• Weak gas valve: Replace the gas valve assembly. Honeywell, Robertshaw, and White-Rodgers all have OEM-specific cross references.
• End-of-life unit with repeated thermopile failures: Recommend replacement. Document tank age via serial-number decode and present buy-vs-don't-buy framing.

References

• NFPA 54 National Fuel Gas Code 2024, Section 9.3, Combustion Air
• NFPA 54 National Fuel Gas Code 2024, Section 12.7.2, Vent Termination
• NFPA 54 National Fuel Gas Code 2024, Section 12.13, Equipment Discontinuance Following Vent Failure
• ANSI Z21.10.1, Gas Water Heaters Standard
• PHCC Plumbers Handbook of Trade Practice, Chapter 11, Water Heating Systems