Most repairs to a gas water heater are made at the burner control. If your heater leaks, the tank is rusted through and the entire unit must be replaced. Here's a quick troubleshooting guide to gas water heater problems:
-- If the unit suddenly stops heating water, try relighting the pilot. If it does not stay lit, the thermocouple probably needs to be replaced. If that does not solve the problem, you may need to clean the burner.
-- If the flame is mostly yellow rather than mostly blue, clean the burner.
-- If water isn't heating sufficiently even though the thermostat is turned up and the flames are blue, drain the tank and refill it.
-- If you have hard water, unscrew and remove the anode rod once a year. Look for a 1-inch nut in the top of the heater. Loosen the nut; the anode rod will be attached. If it's encrusted with minerals, replace it to get more efficient operation.
About an hour to replace a thermocouple; two to three hours to service a burner
Long matches or a barbecue lighter, screwdriver, adjustable wrench, thin wire, soft metal brush, vacuum cleaner
No special skills are needed
Clear the area; you may need to shut off the gas
New thermocouple of the correct length, new pressure-relief valve
If a pilot light won't stay lit after you've followed the lighting directions, turn the gas control knob to OFF. It's often easiest to detach the thermocouple and pilot supply tube at the control and, using a stubby screwdriver, disconnect them from the burner. Pull out the thermocouple.
Purchase a thermocouple the same length as the old one. Unroll it carefully to avoid kinking. Insert the tip into the pilot bracket so the pilot flame will heat it. Reinstall the thermocouple and the supply tube and attach them at the control. Turn the gas back on and light the pilot.
If flames are yellow or erratic, remove and clean the burner. Turn the gas control knob to OFF. Unscrew and pull the gas tube, burner tube, and thermocouple from the control. Reach inside the chamber and pull the burner unit out or down until it unclips. Carefully pull the unit out.
Turn the burner upside down and shake out any debris. Vacuum the jets to remove all dust. Use a thin wire to make sure the opening in the gas tube is clear and vacuum out any debris. Reassemble, reinstall, and relight. If the burner still malfunctions, call for service.
The flue on a gas water heater should pull all fumes up and send them out of the house. To check that a flue is working, light a match, blow it out, and hold it near the flue. Smoke should be sucked up into the flue. If it isn't, check that all the connections are tight. Have the gas company inspect to be sure.