Gas Water Heater Repair

How a gas water heater works

Gas flows through a shutoff valve to the gas control valve on the front of the tank. The control valve feeds a small pilot flame. A thermocouple sitting in that flame proves the pilot is lit and holds the valve open. When the thermostat calls for heat, the valve sends gas to the main burner under the tank, the pilot lights it, and the exhaust rises through a flue in the middle of the tank and out the vent.

Each step in that chain is a repair. Find the step that's failing and you've found the job.

What fails

1. 1

   Cause 1: Pilot and thermocouple

   If the pilot won't stay lit when you release the knob, the thermocouple is the usual cause. Bradford White's checks are to make sure the thermocouple nut is finger tight plus ¼ turn at the valve, that the thermocouple is seated in its bracket, and that the pilot flame fully covers its tip. A good thermocouple reads 20 to 30 millivolts on Bradford White's open-circuit test.

   Thermocouple replacement is the one gas repair many homeowners take on. Details are on how to replace a water heater thermocouple. If the pilot won't light at all, see pilot light won't light.

2. 2

   Cause 2: Gas control valve

   The gas control valve regulates gas to the burner, and a faulty one causes weak heating or failed ignition. A.O. Smith says it needs precise calibration, so a professional should inspect or replace it. Bradford White checks the valve's magnet and ECO (the energy cut-off, a high-temperature safety switch) and replaces the whole valve if either fails. On some A.O. Smith models the ECO is single-use, and once it trips the gas control must be replaced. See gas control valve problems.

3. 3

   Cause 3: Burner and air supply

   A burner clogged with dirt, debris or rust burns poorly. A.O. Smith suggests cleaning it about once a year with the gas and water shut off, and having a pro deal with heavy buildup or damage. Blocked air screens or filters starve the burner of combustion air and should be cleared.

   Look at the flame through the sight glass. Bradford White says the main burner should light smoothly off the pilot and burn blue with minimal yellow tips. Lazy yellow flames mean poor combustion. Don't keep running it. Call a plumber.

4. 4

   Cause 4: Venting and flue

   This is the repair you shouldn't skip and shouldn't do yourself. Bradford White's troubleshooting checks the vent for proper sizing, operation and blockages. A.O. Smith's manual has a simple draft test: run the burner for five minutes, then hold a just-extinguished match about one inch from the draft hood opening, and the smoke should be drawn in. If it drifts away or blows out, turn the gas control off and have the vent inspected. HomeGuide prices flue or vent pipe repair at $100 to $350.

5. 5

   Cause 5: Gas supply

   If the pilot struggles and other gas appliances misbehave too, the problem is upstream. Bradford White lists manifold pressure for natural gas at 4.0 inches of water column, give or take 0.3. Measuring that takes a manometer and a technician. Your job is to confirm the valves are open and, on propane, that the tank isn't empty.

Jobs to leave to a pro

Some of these repairs are fine for a careful homeowner: relighting, cleaning lint and dust from the air openings, and swapping a thermocouple. Leave these to a licensed plumber:

• replacing the gas control valve
• anything involving the gas line or fittings upstream of the heater
• vent pipe repair or replacement
• pulling the burner on a sealed-combustion (FVIR) heater
• any heater that's been flooded or exposed to fumes, which A.O. Smith says must be replaced

A.O. Smith's manual also says not to disable or modify the flammable vapor system's thermal switch. It's there for a reason.

What it costs

HomeGuide's 2026 figures for gas water heater repairs:

Overall, HomeGuide puts most gas water heater repairs between $50 and $600. Many plumbers add a $50 to $200 service call fee. If the heater is more than 10 years old or the repair would cost more than half of a new heater, HomeGuide suggests considering replacement. Compare on water heater repair cost.