Gas Water Heater Repair

Why isn’t there any hot water? This Fix-It Guide on gas water heater repair tells how a gas water heater works, what often goes wrong, how to identify a gas water heater problem, and what parts and tools you will need to fix it. It then gives simple step-by-step instructions for how to light a gas water heater pilot light, how to  clean the pilot orifice, how to service a gas water heater thermocouple, how to replace a thermocouple, how to test and replace a gas water heater relief valve, how to replace a drain valve, how to test and maintain the flue and vent, and how to drain a gas water heater tank.

How Does a Gas Water Heater Work?

Gas Water Heater Repair

A natural or propane gas water heater uses a burner to heat water in the tank for distribution throughout the home.

A water heater is an apparatus for heating and storing hot water. A water heater typically warms water to a temperature between 120° and 140°F. When a hot-water faucet is opened, hot water flows from the top of the tank toward the faucet, and cold water enters via the dip tube to replace it. The fuel for heating the water heater can be natural gas or propane, or it can be electricity (see Electric Water Heater Fix-It Guide).

As the water temperature inside the tank drops below the setpoint, the thermostat opens a valve that sends gas to the burner, where it is ignited by a pilot flame or electric spark. Combustion gases are vented from the burner chamber through the flue and its heat-retaining baffle, then out the draft hood and vent. If the temperature or water pressure rises too high inside the water heater, a relief valve opens to prevent the tank from exploding.

What Can Go Wrong with a Gas Water Heater?

The pilot light may be out or may not stay lit. There may not be enough hot water, or the water may be too hot. The water heater may leak or be noisy, or the hot water may be dirty.

Fix-It Tip

A problem with your water heater may be due to overwork, not mechanics. If your water heater holds less than 15 gallons per family member (tank volume is stamped on a metal plate affixed to most water heaters), consider a larger unit or staggering your use of hot water.

How Can I Identify a Gas Water Heater Problem?

  • If there is no hot water, check to see if the pilot light is out and re-light it if it is (see below).
  • If the pilot light won’t stay lit, clean the pilot orifice (see below); tighten the thermocouple connections (see below), and replace a faulty thermocouple (see below).
  • If there is not enough hot water, check and reset the temperature control if necessary.
  • If the water is too hot, check and reset the gas water heater temperature control. If it is set too high, have a faulty thermostat serviced professionally.
  • If the heater leaks, operate the pressure relief valve and if the leak persists and the temperature is okay, have the valve replaced.
  • If the gas water heater tank is rusted, replace the water heater.
  • If the heater is noisy, drain and flush the tank (see below) and refill.
  • If the hot water is dirty, drain and flush the tank and refill (see below).
  • If the gas water heater relief valve or the drain valve leaks, test and replace as needed (see below) Regular maintenance of the flue and vent (see below), and regular draining of the tank (see below) will prolong the life of your gas water heater.
  • If you have hard water, check the anode rod yearly (see below) and replace if necessary.

Fix-It Tip

Do you live in an area with hard water? If so, you know that minerals in the water supply can build up, creating a layer of sediment on the bottom of a water tank, hindering performance and shortening the system’s life. Besides softening the water (see Water Softener) fed to the heater, you can minimize damage by lowering the temperature setpoint to 130°F or less.

What Do I Need for Gas Water Heater Repair?

You can get replacement parts–and even a replacement gas water heater–from plumbing suppliers, large hardware stores, and home centers. The tools you’ll need to fix your water heater include these:

  • Pipe wrench
  • Open-end wrenches
  • Thin copper wire
  • Plumber’s pipe tape
  • Soft brush
  • Vacuum cleaner

What Are the Steps to Gas Water Heater Repair?


If you smell a gas odor for more than a couple of minutes, close the gas-shutoff valve supplying the water heater, open a door or window to ventilate the room, and call the gas company for service.

Light a gas water heater pilot light:

  1. Remove the burner access panel. Read and follow lighting directions on or near the cover.
  2. To light a pilot that has blown out, turn the temperature-control dial to its lowest setting and the gas-control knob to off. Wait at least five minutes for the gas to clear.
  3. Tightly twist a piece of paper and light it. Turn the gas-control knob to pilot. Depress the reset button (or the gas-control knob if there is no reset button) while holding the burning paper near the pilot burner. The pilot may be hard to reach.
  4. If the pilot fails to light after a few seconds, close the gas-shutoff valve and call the gas company. If it lights, continue pressing the reset button or knob for one minute, then release it.
  5. If the pilot stays lit, turn the gas-control knob to on; the main burner should light when the temperature-control dial is set above 120°.
  6. If the pilot does not stay lit, turn the gas-control knob to off. Check that the thermocouple’s tip is positioned so the pilot’s flame touches it, and tighten the nut holding it in place. Try lighting it once more. If the flame goes out again, replace the thermocouple.

Clean a gas water heater pilot orifice:

  1. Shut the gas cock (refer to the heater’s owner’s manual for specific instructions).
  2. Disconnect the burner assembly lines at the gas control valve and remove the burner assembly.
  3. Carefully probe the pilot light orifice with a thin wire; do not use a needle or paper clip that can damage the orifice. Vacuum out any dislodged debris.

Service a gas water heater thermocouple:

  1. Check the thermocouple for loose connections at the control valve. The thermocouple is located near the pilot flame.
  2. Make sure that the thermocouple’s bulb tip is held in the pilot flame. If necessary, carefully tighten the nut, but don’t over-tighten it.

Fix-It Tip

When you buy replacements, take the old water-heater parts, as well as the brand and model number, with you.

Replace a gas water heater thermocouple:

  1. Turn the gas-control knot to off and close the gas-shutoff valve.
  2. Loosen the nut that secures the thermocouple lead to the control unit, then unscrew it by hand.
  3. Remove the old thermocouple from the control unit.
  4. Push the tip of the new thermocouple into the pilot bracket clip as far as it will go and tighten the fitting.
  5. Screw the nut at the end of the lead to the control unit.
  6. Open the gas-shutoff valve and relight the pilot. If the pilot light goes out, close the gas-shutoff valve and call a professional.

Test and replace a gas water heater relief valve:


The relief valve spews hot water. Make sure you use caution when working on this component. Before replacing it, turn off the hot water system and let it cool down.

  1. Lift the spring lever on the valve for a few seconds to clear the valve of mineral scale.
  2. If no water spurts out, or if water continues to drip after the valve is released, replace the valve.
  3. Turn the gas-control knob to off and close the gas-shutoff valve. Close the cold-water supply valve.
  4. Drain a gallon or more of water from the tank.
  5. Unscrew and remove the discharge pipe if there is one.
  6. Unscrew the relief valve from the tank with a pipe wrench.
  7. Apply pipe tape to the threads of the new valve.
  8. Screw the relief valve into the tank by hand, then tighten with a pipe wrench.
  9. Screw the discharge pipe (if any) into the valve outlet.
  10. Refill the water heater and light the pilot. If the valve leaks, have a plumber check for high water pressure in the house.

Replace a gas water heater drain valve:

  1. Turn the gas-control knob to off and close the gas-shutoff valve.
  2. Close the cold-water supply valve and drain the water heater completely (see above).
  3. Use a pipe wrench to unscrew the base of the drain valve.
  4. Purchase an exact or better replacement drain valve.
  5. Wrap pipe tape around the threaded end of the valve and screw it into the coupling.
  6. Apply pipe tape to the pipe threads that emerge from the water heater.
  7. Screw the coupling and valve onto the nipple and tighten.
  8. Tighten the valve so that it faces down toward the floor.
  9. Refill the tank and relight the pilot.

Test and maintain a gas water heater flue and vent:

  1. Set the water heater at a high temperature to light the burner and wait 10 minutes.
  2. Hold a lighted match at the edge of the draft hood. A properly working vent will draw the flame under the edge of the hood. When you blow out the match, the hood should suck up the smoke. If the flame or smoke is blown away from the hood, the vent may be blocked.
  3. To disassemble a blocked vent, first turn off the water heater, close the gas-shutoff valve; let the burner, draft hood, and vent cool.
  4. Remove the burner access panels and cover the burner and floor with paper to catch soot and debris.
  5. Mark the vent sections for reassembly.
  6. Remove the draft hood from the top of the tank and carefully clean it with a wire brush.
  7. Replace any rusted or damaged ductwork.
  8. With the vent removed, lift the baffle from the flue and clean it of soot with a wire brush.
  9. Reinstall the baffle, draft hood, and vent, then vacuum the inside of the combustion chamber.
  10. Clean the burner and its ports with a small brush (such as an old toothbrush).
  11. Relight the pilot. Test the vent with a match as in Steps 1 and 2. If the flame or smoke is not drawn up, there may be a blockage in the main chimney; call for service.

Drain a gas water heater tank:

  1. Turn the gas-control knob to off and close the gas-shutoff valve.
  2. Close the cold-water supply valve and open a hot-water faucet in the house to speed draining.
  3. Attach a garden hose to the drain valve and run it outside to a drain. If the heater is in the basement you may need to run the hose to or through a sump pump.
  4. Open the drain valve and allow the tank to drain.
  5. Once done, close the drain valve, open the cold-water supply valve, and open any nearby hot-water faucet. When a steady stream of water flows from that faucet, the tank is full; close the hot-water faucet.
  6. Once the tank is full, turn on the gas and relight the pilot.