Why my gas water heater pilot light keeps going out

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Constantly working pilot light is essential for the operation of a gas water heater and here are a few of the most common issues that cause the pilot light to go out. Be sure to shut off gas supply before attempting to troubleshoot or repair any part of the water heater! If you are not comfortable working with gas, please call a professional. If you smell gas at any point in trying to find the problem, do not light the pilot. Repair the gas leak or call your utility company first!

1. Thermocouple
2. Pilot light assembly needs cleaning
3. Gas valve (gas thermostat) not working
4. Strong downdraft
5. Flue Gas Spill Switch has tripped
6. Main burner partially clogged and causes a blow back which blows out the pilot.
7. Condensation or small leak in the tank causing water to drip on the pilot light.

1. Thermocouple [/B]
Thermocouple is a device that consists of two joint wires made of dissimilar alloys that produces slight voltage proportional to the temperature difference between the hot and the cold ends. It is used in the heater to sense the presence of the pilot flame. If the flame goes out, the thermocouple cools off and the gas valve which it is connected to, shuts the gas – a safety feature.
Thermocouples are arguably the most common cause of water heater pilot light going out either immediately or shortly after being lit. The thermocouples are constantly inside a flame, they can literally burn out in a few years and need replacement. These are an inexpensive part (less than $10) and are easy to install for a DIY homeowner. Measure the length of the original thermocouple to get a suitable replacement. Be careful when buying the thermocouple - check for the thread on the gas valve. Some older models had left handed threads, a newer right-handed replacement thermocouple will not work. Also, check the type of gas the thermocouple is intended for – propane burns hotter than natural gas and so the thermocouple's design should accommodate for that.
When installing the new thermocouple, take care positioning it in the flame in such a way that the flame of the pilot circles the thermocouple. The tip of the flame is its hottest part and will soon burn the thermocouple out if it touches.
2. Pilot light assembly needs cleaning [/B]
If you have trouble lighting the pilot or it keeps going out, the pilot light orifice may be partially clogged and needs to be cleaned. You will need a stiff brass wire scratch brush. The pilot and the thermocouple are usually mounted on the same bracket and so the thermocouple will need to be disconnected from the valve and removed first. Remove the pilot light assembly and take it apart. Give a good scrubbing to the pilot burned and around the orifice. The orifice may need to be cleaned with a thin wire. Blow air through the pilot supply tube to be sure it's not clogged. When putting the pilot light and the thermocouple back, pay close attention to their respective positions as described in the previous paragraph about the thermocouples.
3. Gas valve (gas thermostat) not working[/B]
If you have cleaned the pilot assembly, it produces a good stable flame, you have a new thermocouple and yet the pilot light still goes out as soon as you release the knob of the valve, you may need a new gas valve. It is a more expensive part (anywhere from $60-$300 depending on the model), it cannot be repaired and has to be replaced, and we strongly recommend you seek a professional HVAC contractor's help in installing a new gas valve.

4. Strong downdraft
A strong downdraft from the chimney can blow out the pilot light. If this is an intermittent problem and happens only on very windy days, check if the chimney cap is properly installed, not rusted and otherwise intact. If this is a continuous problem that also manifests in smell of burnt gas near the water heater, installation of a powered draft inducer may be required. Other causes of downdraft include inadequate combustion air supply due to small utility room with no air supply or negative pressure in the building – there may be a strong enough fan working in the building, such as a whole house fan, that can overcome the natural hot air draft.

5. Flue Gas Spill Switch has tripped
This is another issue that has to do with downdraft. Some water heaters are equipped with sensors installed on the draft hood that detect presence of combustion gases outside of the flue. The switch may have gotten tripped and needs to be reset manually. If you find yourself resetting it too often, you will have to seriously investigate the reason. Sometimes the switch malfunctions and needs to be replaced. A proper installation of a flue gas spill switch is not simple and is better done by an HVAC professional.

6. Main burner partially clogged and causes a blow back which blows out the pilot.
Remove and clean the burner
7. Condensation or small leak in the tank causing water to drip on the pilot light.[/B]
Condensation naturally occurs when moist air comes in contact with cold tank. It should normally go away when the water warms up and should not be a problem unless you completely empty the tank by filling a hot bath for example and it has to refill with cold water. If the problem persist, it is most likely not condensation and rather a leak, probably due to rust of a manufacturing defect. A leak in the tank is not repairable. The water heater will need to be replaced. Investigate a possibility that it may still be on the manufacturer's warranty and if that's the case, make sure to involve the manufacturer.

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