Gas Fireplace Pilot Light Won't Stay Lit

How the pilot is supposed to work

On a standing-pilot gas fireplace, a small flame burns continuously and wraps around a metal sensor. A thermocouple, or on many fireplaces a thermopile, turns that heat into a small electrical signal. While the signal is present, the gas valve keeps pilot gas flowing. When the signal drops, the valve closes. Holding the knob down while lighting simply overrides that check for the moment it takes the sensor to warm up.

So a pilot that dies when you let go is a safety feature working as designed. The useful question is why the sensor is not getting hot enough, or why its signal is not reaching the valve. Fireplaces with electronic ignition use a different system, an igniter and a flame sensor, but the logic is similar: no proven flame, no gas.

Watch the flame before anything else

Look at the pilot through the glass or the access panel after lighting it according to the label. A good pilot is a steady, mostly blue flame that covers the upper part of the sensor and, on many units, also reaches the burner. Its appearance tells you a great deal:

• Small, weak flame barely touching the sensor: partly blocked orifice or low gas supply
• Yellow, lazy or sooty flame: dirt in the pilot assembly or poor air mix
• Flame bending away from the sensor: a draft, or a pilot hood knocked out of position
• Flame that flickers and lifts off in gusts: wind or pressure changes reaching the firebox
• Strong blue flame on the sensor yet the pilot still dies: the sensor or its connection is the likely fault

Common causes, roughly in order

Most pilot complaints trace back to a short list. Dust, lint, pet hair and spider webs in the pilot orifice shrink the flame, which is especially common after a fireplace sits unused all summer. Thermocouples and thermopiles wear out with age and heat, and a tired one may produce enough signal on some days and not on others. The sensor's connection at the gas valve can loosen or corrode.

Less often, the gas supply valve to the fireplace has been left partly closed, or recent work on another gas appliance, such as a new range or boiler, has changed the supply. On vent-free units, an oxygen depletion sensor shuts the pilot off when it detects a problem with the air in the room, and it should be treated as a warning, not a nuisance.

When it only happens in wind or cold

A pilot that behaves all summer and fails on blustery winter nights is telling you something about air movement. Strong gusts during a nor'easter can push down a vented chimney or across a direct-vent termination on an exposed wall and blow the pilot sideways. A cold, oversized masonry flue draws poorly until it warms, and a missing or damaged cap lets wind drop straight down the flue.

The house itself plays a part. Kitchen and bathroom exhaust fans, a clothes dryer and a boiler all pull air out of the building. In a tightly sealed home, that can draw air down the fireplace vent, especially on vented gas logs in an open firebox. If the pilot fails mostly when the dryer or a range hood is running, mention it, because it points toward pressure and venting rather than the pilot parts.

Pilot stays lit, but the fire will not

If the pilot holds but the main burner does not come on, the trouble is usually in the control circuit. On units run by a wall switch or remote, check the batteries in both the handheld remote and the receiver, which is often tucked under the firebox. Then consider the wall switch and the thin wires running to the valve. On millivolt systems, a worn thermopile can make enough signal to keep the pilot lit but not enough to open the main valve, a classic sign that it is near the end of its life.

A burner that lights and then drops out after a few minutes is a different story. Many vented gas appliances have a spill switch, a heat-sensing device near the draft hood that cuts the gas if exhaust is spilling into the room instead of going up the vent. When that is what is happening, the problem is the vent or the chimney, and the fireplace should not be run until it is found.

Where the chimney or vent comes in

Gas logs in an older masonry fireplace and many gas inserts depend on the chimney for draft. A bird or squirrel nest, leaves or fallen mortar near the outlet, a liner that has separated, or an unlined clay flue with cracked tiles can disturb the pilot or trip a spill switch. Direct-vent units are sealed systems, but a damaged termination cap outside, a vent pipe pulled apart inside the wall, or a gasket that no longer seals the glass can do the same.

That vent path, from the firebox to the outlet, is what a chimney technician examines, along with the pilot area and burner for soot and debris. Gas valve replacement and supply line work are jobs for a qualified gas technician, and the appliance manual says who should service each part.

What you can safely do yourself

A few steps are within reach of any homeowner and often solve the problem:

• Relight using the instructions on the label inside the fireplace, which are written for that unit
• Confirm the gas shutoff valve for the fireplace is fully open
• Replace batteries in the remote, the receiver and any battery backup
• Turn off ceiling fans, exhaust fans and the dryer while you test, and note whether that changes anything
• Look for a pilot hood or log knocked out of place after cleaning, and compare with the manual's diagram

When to stop and call

Stop trying to relight it if the pilot keeps failing after these checks, if the flame is yellow and sooty, if you see soot on the glass or around the opening, or if the main burner shuts itself off repeatedly. Do not take the burner apart, tape over a sensor or bypass a safety switch. If you smell gas, leave the house without operating switches or the fireplace, and call your gas utility or 911 once you are outside. Carbon monoxide alarms on each level of the home are part of owning any gas appliance. A visit that covers the vent, cap and firebox, with upfront pricing before any repair, usually settles whether the fix is a part or the chimney.