Pilot Light Is Lit but No Hot Water

How to tell what's happening

Take the outer cover off the burner compartment if your heater has a removable one, and look through the sight glass. Then turn on a hot tap and leave it running for a few minutes so the tank calls for heat.

• Burner lights and runs: the heater is working. If the water is still lukewarm, the problem is how much heat reaches the taps, not ignition. Go to water heater not hot enough.
• Burner tries to light, then goes out: usually an air supply or burner problem.
• Burner never lights: usually the thermostat setting or the gas control valve.

The likely causes

1. 1

   Cause 1: The temperature dial is turned down

   Check this first. The thermostat on a gas tank is the dial on the gas control valve near the bottom. Many valves have a "Pilot" or "Vacation" position that keeps the pilot burning but never fires the burner. Someone may have set it there while traveling, or bumped it.

   Some dials show "Low," "Medium" or "Hot" instead of degrees, and the manual explains the marks. Turn the dial up to the normal setting. A.O. Smith says to wait 30 to 60 minutes after a change, then test at a faucet.

2. 2

   Cause 2: The tank just hasn't recovered

   If a few people showered back to back, the burner may be running and simply not caught up yet. A.O. Smith gives 30 to 40 minutes of recovery time for a gas tank. If the burner is lit, wait it out.

3. 3

   Cause 3: Blocked air intake

   The burner needs air as well as gas. Newer gas tanks draw combustion air through a screen or flame arrestor at the base, and lint, dust and pet hair clog it. A.O. Smith says blocked air screens and filters restrict combustion air and affect burner performance, and says to clear them. A starved burner may light and drop out, or never light.

   Use a vacuum with a brush on the outside of the screen. Don't poke into the combustion chamber. If the heater sits in a laundry room, this is the first place to look.

4. 4

   Cause 4: A dirty or clogged burner

   Rust flakes and debris fall onto the burner over the years and block the ports. A.O. Smith suggests cleaning the burner assembly once a year, with gas and water off first, and says heavy buildup or damage needs a pro. A.O. Smith notes that a clogged burner affects flame quality and heating.

5. 5

   Cause 5: Gas supply problem

   The pilot uses very little gas, so it can stay lit on a supply that can't feed the burner. A.O. Smith lists gas supply problems among the causes of ignition failure, and says to make sure the gas valve is open and check for supply issues. On propane, check the tank level. On natural gas, see whether other appliances are running weak. If they are, call the gas utility.

6. 6

   Cause 6: A failed gas control valve

   When the dial is set right, the air screen is clear and the supply is good, but the burner won't come on, the gas control valve (also called the gas valve or thermostat) is the usual suspect. Its built-in thermostat may not be sensing the tank temperature, or the valve itself has failed. A.O. Smith says a faulty gas control valve needs professional inspection and replacement. It's not a part to take apart yourself.

Burner runs but the water is still cold

If you can see the burner running and the hot water is still poor, the gas side is fine. Two things to suspect:

• Sediment. A thick layer on the tank bottom keeps heat from reaching the water, and it usually pops or rumbles while the burner runs. See our noises page.
• A broken dip tube. Cold water dumps into the top of the tank and mixes with the hot. Rheem lists damaged dip tubes as a cause of reduced hot water.

Electronic ignition and power-vent models

Some gas heaters don't have a standing pilot at all. They use an igniter and electronic control, often with a status light that blinks a code. If yours has a status light, the code chart is on the label or in the manual, and it will tell you more than any generic checklist. Power-vent models also need electricity, so check that the unit is plugged in and the outlet works.

Boilers and combi units

If the pilot belongs to a boiler or a combination boiler that also heats your water, this page doesn't fit. The controls are different, and a heating technician is the right call.

When to call a plumber, and what it costs

Call if you smell gas, if the dial is set correctly and the burner still won't fire, or if the burner flame is yellow or rolling out of the chamber. Those all point to the gas control, the burner or venting, which are pro jobs.

HomeGuide's cost data for the repairs behind this symptom:

A gas control valve on an older tank is where the math gets tight. HomeGuide suggests replacing instead of repairing once a heater passes ten years or the repair passes half the cost of a new one. Compare with water heater cost.