Removing the gas burner assembly is not too difficult. Remember, turn the gas supply off to the unit before proceeding with any work.
See 'Gas Hot Water Heater Troubleshooting' if this is not your problem. Is it just the pilot light? See the article 'Gas Water Heater Pilot Light' for lighting instructions.
The first thing you need to locate is the gas valve and the burner assembly. They are near the bottom of the tank. The gas valve has the main gas supply connected to it and three other pipes connected at the bottom of it.
The larger tube in the middle is the main gas supply. The smaller tube is the gas supply to the pilot light. The other copper looking wire or tube is the thermocouple
Older water heaters have a cover plate that just snaps on and off. Newer ones may have some screws to remove. The difference will be the electric igniter that is found on newer models.
The main gas supply comes into the side of the gas valve. There should be a vertical gas pipe that tees into it. You will find the shut off valve for the hot water heater on the vertical gas line. Before you do anything, shut off the gas.
When the handle to the valve is perpendicular to the pipe, the gas is off. When it is parallel, the gas is on. Simple right? Look at the picture and make sure the gas is off.
Then using open end or adjustable wrenches you can loosen the connections below the gas valve. First loosen the thermocouple fitting. There are some gas water heater thermocouples that are threaded left hand. When it won't loosen try going the other way.
Next to remove would be the pilot light supply line. This should turn the normal way. Last unhook the main gas supply. Remove the wire that goes to the igniter if you have one for you hot water tank.
At this point the burner assembly should be free. Remove it from the chamber. The thermocouple might be threaded through the tubing for the gas lines. If it is bent in a special way you will want to preserve this to serve as a model for the new one. Pull the thermocouple out of the pilot assembly.
WARNING!! Natural Gas Can Explode! Make Sure the Gas Is OFF Before Working On Your Water Heater!!! WARNING!!!
With the burner assembly out of the way it is a good time to do some maintenance. Get a small brush on a long handle the clean the debris up off the bottom of the chamber. Use a vacuum cleaner to suck up the loose material.
Replacing a gas water heater thermocouple provides the ideal circumstances for a clean up and maintenance job. Dirt and debris build up over time that can interfere with the proper functioning of the burner.
Another task you can do when the burner out is to 'Replace the Thermocouple'.