Why Won’t The Pilot Light Stay Lit On My Gas Water Heater?

There are a number of reasons why pilot lights go out on gas water heaters. These are in the order of most often to least often.

Defective Thermocouple

Thermocouples do go bad from time to time. The thermocouple is a device that generates about 36 thousandths of a volt of DC electricity. They are in the flame 24/7 and occasionally they burn out. A couple of years ago there were a string of problems from thermocouples going bad from some of the big box retailers. The majority of water heater calls we got were a certain brand failing each week. Unfortunately the thermocouples came with reverse threads and not a single wholesaler or local supply house stocked them. They were specially made and after a little while we just started asking the homeowners calling what type of water heater they had and we ended having to refer them to the manufacturer as they were the only ones carrying this type of thermocouple. A lot of people went without hot water for about a week waiting for a replacement.
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Pilot Light Too Small

If the pilot light is too small or is a soft flame, like a Tiki torch, it will not generate enough heat to produce the needed electricity. A thermocouple generates electricity from the difference in temperature between the tip (up to 25% of the length of the thermocouple) and the base. A soft flame is cooler and does not produce as much electricity plus a small flame is much more likely to be wicked away from the thermocouple by the drafting action caused by the flames. It is not unusual to have a pilot able to hold a flame until the burners kick on and it is not unusual for the pilot to hold a flame until the burners kick off. Both of these situations change the patterns of air movement in the burner chamber and a poor pilot can drift away from its intended target area.

Pilot Light Too Big

Too big or too strong a pilot is every bit as bad as too big. If the pilot gas pressure is too high the flames can leave the top of the pilot assembly and ignite past the thermocouple. This has the flame of the pilot lit, but it is unable to heat the thermocouple up enough to generate enough electricity to hold the flame in place.

Pilot Light Not Touching Thermocouple Correctly

This typically comes from the plumber replacing the thermocouple and not installing it correctly. Only the top 25% of the thermocouple should be in the pilot flame and it should be fully engulfed in flame. When the thermocouple is pushed too far up into the pilot assembly the pilot flame tends to heat up the middle of the thermocouple. This means that there is very little temperature difference between the tip of the thermocouple and the base of the thermocouple. You can translate this into very little electrical generation.

Not Venting Properly

If the vent is not drafting properly the products of combustion, CO2 and many other things you shouldn’t be breathing, cool off and drop down to the bottom of the water heater. This means a lack of oxygen for the flames to burn and ultimately causes the pilot light to snuff out or the burners to not light evenly. If the burners do not light evening the result can be a buildup of natural gas in the combustion chamber resulting in sudden ignition causing minor explosions. This is called delayed ignition and the explosions from these delayed ignitions can blow the pilot light out. I have seen 2 plumbers over my long career go to the hospital when diagnosing this problem. Both were looking into the burner chamber when it exploded and both suffer very painful, not permanent, eye injuries from the flames. One of them I took to the hospital which was, lucky, within a mile of the home we were working on. After a dose of morphine he calmed down. Not a very pretty sight. I have been very careful ever since after experiencing that.

Defective Gas Valve

When gas valves die they malfunction in two ways. The first, most often way, is that they fail to allow the pilot to stay lit. This is a function of the safety in the valve. The valve contains a high limit control that is a onetime safety shut off. Once activated the valve is bad and must be replaced. It is the last ditch effort by the manufacturer to protect the home from a runaway water heater on fire. Once the water hits a high temperature the gas valve has a thermal device embedded in the temperature probe that separates (opens) and doesn’t allow electricity from the thermocouple to hold the pilot valve open. Once the pilot valve won’t stay open the main gas valve fails. It is a safety function. I have seen this many times. I have only seen a runaway fire in a water heater a couple of times. Perhaps the reason is that the safety works and allows the valve to fail. The runaway valves were whistling with steam and turning on a faucet in the house literally had steam flowing out of it.

“According to the Consumer Product Safety Commission, about 3,800 injuries and 34 deaths occur in homes across the county each year due to scalding from excessively hot tap water. Most involve children under the age of five years old and the elderly.” Thin about this. If steam is coming out of the tap you touch it you will have a third degree before you feel it. I have seen happen once. It happen to my brother when we teenagers. I grew up in Oregon and my parent owned a laundry and drycleaners. My brother was leaning on one of the shirt presses when a neighbor friend stepped on the bottom steam foot peddle fooling around. My brother lost many layers of skin from his palm to his elbow. It was an ugly site.

Inadequate Gas Supply

On tank type water heaters the inadequate gas supply is normally the result of water in the gas line to the water heater or in the gas meter itself. These water heaters are typically a maximum of 50,000 btu’s (100 gallon can be 75,000 btu) and this only requires a minimum of a ½” gas supply. The supply lines are nearly always large enough, but if they are not installed correctly water can condense and collect in the line blocking the gas from reaching the gas valve. The gas lines should be installed as to allow drainage of condensing water. Natural gas has a considerable amount of water an every year we arrive at homes in the low elevations parts of the count, the beach cities, and at the bases of hills. Such a Palo Verdes, Fullerton, Whittier and Anaheim that the gas meters are full of water. There is not a lot you can do about this yourself, so give us a call and we can resolve the problem for you.


Most often this is a problem with a door next to the water heater in the garge. The house door is fashioned in such a way that it catches the wind. The wind then blows the pilot light out. If this is happening to you there is a solution. It is highly unlikely that the new water heaters with the flame shield and the ultra low nox burners (every tank type water heater sold here in Southern California now) will have this problem. Change your water heater out to the new type and this problem goes away due to the new design of the burner chambers.

The vent through the roof can be a source of problems when it comes to wind. If the vent cap is missing wind can rush down the flue and snuff out the pilot light. If the vent is not installed properly the drafting during a mild wind can cause the pilot light to go out. If the vent termination is within 10 feet of the peak of the roof the vent must extend beyond the top of the roof. The vent cannot terminate within 10 feet of a vertical wall.

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