Is the pilot currently lit?

Look through the sight glass or observation port on the equipment. A standing pilot should be burning continuously. If the pilot is out, this is where most standing pilot calls begin. Proceed to relight before diagnosing further.

Is there a call for heat?

The pilot is lit and healthy. Now determine if the main burner should be firing. For 24V systems: • Check for 24 VAC between R and W at the equipment • Thermostat must be calling for heat For millivolt systems: • The thermostat completes the TH–TH circuit on the gas valve • Measure continuity or very low resistance across the thermostat terminals when calling for heat If there's no call for heat, the main burner won't fire — this is expected behavior, not a fault.

Does the main burner light?

With the pilot lit and a call for heat confirmed, the main gas valve should open and the main burner should ignite from the pilot flame. If the main burner does not light: • Listen for the gas valve click when the thermostat calls • Check for 24 VAC at the main valve operator (24V systems) • On millivolt systems, verify the thermostat circuit is completing through the TH terminals on the valve • If the valve clicks but no gas flows to the main burner, the valve may be partially stuck or the main burner orifices may be blocked

Does the system complete a normal heating cycle?

With the main burner running, observe the full cycle: • Does the blower come on after the appropriate fan-on delay? • Does the system run until the thermostat is satisfied? • Does the main burner shut off cleanly when the call ends? • Does the blower run on fan-off delay then stop? • Does the pilot remain lit after the main burner shuts off? If the system completes a full cycle normally, the issue may have been a simple pilot outage or a corrected condition.

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Gas Furnace / Boiler / Water Heater with Standing Pilot (millivolt or 24V)

Standing Pilot Troubleshooting

Step-by-step diagnostic for standing pilot systems — from pilot outage through thermocouple testing, flame quality, gas valve operation, and main burner response.

Safety Notice

Gas-fired equipment produces open flame, combustion gases, and hot surfaces. Only trained, qualified technicians should perform these procedures. Follow NFPA 54 and local codes. Never bypass safety controls or force a gas valve open.

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Standing Pilot Troubleshooting Overview

This workflow covers standing (continuous) pilot systems. The pilot burns constantly and generates millivolts via a thermocouple to hold the gas valve open. On a call for heat, the main gas valve opens using either the thermocouple millivolt circuit or a separate 24 VAC signal. Common symptoms: • Pilot is out • Pilot won't stay lit after releasing the knob • Pilot is lit but main burner won't fire • System cycles erratically Before starting: • Confirm gas supply is on at the meter and equipment shutoff • Note any unusual odors — if you smell gas strongly, follow gas leak procedures first

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Standing Pilot Troubleshooting

Standing Pilot Troubleshooting Overview

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