If your gas water heater is not heating, check three things first: that the gas supply valve is open, that the control knob is on and not set to "pilot" or "vacation," and that the pilot or burner is lit. If the burner is lit and the water is still lukewarm, the likely causes are a faulty gas control valve, a dirty burner, blocked combustion air screens or heavy sediment, and those are jobs for a licensed plumber.
Start With the Gas Supply and the Control Setting
The simplest causes are the most common. A.O. Smith lists gas supply problems first among gas water heater faults: they prevent "burner ignition and can also cause the pilot to extinguish," and the fix is to "ensure the gas valve is open and check for supply issues."
Check the shutoff valve on the gas line next to the heater. The handle should run in line with the pipe. Then check whether your other gas appliances, such as the stove or furnace, are working. If nothing gas-fired in the house works, the problem is upstream of the water heater and it is a call to your gas utility, not a repair.
Next, look at the control knob. After a power cut, a relight or someone bumping it during laundry, it can end up on "pilot" or "vacation," which keeps the pilot lit but will not heat the tank.
Pilot Out? That Is a Separate Problem
If the sight glass is dark, the pilot is out and the burner cannot light. Relight it following the instructions on the label on your heater, and only if you are comfortable doing so.
If it lights and then goes out again, or will not stay lit at all, that is usually a thermocouple, a draft or the heater's flammable vapor safety system doing its job. We cover all three in why your water heater pilot light keeps going out, so we will not repeat it here.
Burner Lit, Water Still Lukewarm
This is where it gets harder to diagnose without tools. A.O. Smith describes three common causes.
- Faulty gas control valve. It "regulates the gas flow to the burner. A malfunction can lead to inadequate heating or failure to ignite." A.O. Smith says it "requires precise calibration and should be inspected and replaced by a professional if found to be defective."
- Clogged burner assembly. "Dirt, debris, or rust can clog the burner assembly, affecting the flame quality and heating efficiency." A.O. Smith says the burner "should be inspected and cleaned of any debris once a year."
- Blocked air screens or filters. These "restrict combustion air, affecting burner performance." Many newer gas heaters draw combustion air through a screen near the base, and lint and dust from a nearby dryer can clog it.
A healthy burner flame is mostly blue. A lazy, yellow or sooty flame is a sign the burner is not getting the air or gas it needs. Do not try to adjust gas pressure or take apart the burner yourself. Our water heater repair technicians test the control valve and clean or replace the burner on site.
Sediment Makes the Burner Work Harder
On a gas tank heater, the burner sits under the bottom of the tank, and that is exactly where sediment settles. A.O. Smith explains that "hard water minerals accumulate at the bottom, causing popping or rumbling noises as the water heats," and the fix is to "drain and flush the tank to remove sediment."
A thick layer of sediment insulates the water from the flame. The burner runs longer, the tank heats more slowly, and the bottom of the tank runs hotter than it should. If your heater rumbles and your water is lukewarm, sediment is a strong suspect. Our guide to preventing water heater sediment buildup explains why Delaware and Tri-State water makes this worse.
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Is It Broken, or Just Recovering?
Sometimes nothing is wrong. After a run of showers, the tank has to reheat, and A.O. Smith gives the guideline that "gas tanks need 30-40 minutes, electric tanks need 60-90 minutes." If hot water comes back after about half an hour, the heater is working, but it may be too small for how your household uses it now. Our guide on what size water heater you need covers that.
If you have an electric heater rather than gas, the checks are different. See electric water heater not heating.
If You Smell Gas, Stop
Every gas water heater check ends at the same line. A.O. Smith's advice is: "If you smell rotten eggs or suspect a gas leak for any reason, turn off the gas supply valve immediately and call a professional—don't continue to troubleshoot and don't try to diagnose the problem yourself." Leave the house and call your gas utility from outside.
Get It Fixed Before the Cold Months
Water heater problems cluster in winter across Wilmington and New Castle County. The incoming water is colder, the burner runs longer, and a weak component that got by in September gives out in January.
Water Heater Now provides same-day gas water heater repair in Wilmington and across New Castle County, Chester County and Cecil County, with the price confirmed before work starts. Call (302) 240-2818. We answer 24 hours a day.
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Source, verified September 30, 2026: A.O. Smith, Is Your Hot Water Not Getting Hot? Common Causes & What To Do About Them.
