Cold showers and a gas water heater that will not light turn a normal morning into a service call. The unit burns natural gas to heat the water feeding every tap in the house, so when it stops, bathing, dishwashing, and laundry stop with it. Most failures trace back to a short list: the pilot light, the gas supply, the thermostat, sediment in the tank, or internal corrosion. Working through that list in order separates a ten-minute fix from a replacement decision.
Troubleshooting starts with knowing the hardware. Gas water heaters come as tank-type, tankless, and heat pump models, and the selection and installation differences matter because each type fails in its own pattern. A tank-type unit stores heated water and relights its burner on demand; a tankless unit heats water only when a tap opens; a heat pump unit moves heat from the surrounding air. The fixes below focus on the tank-type gas unit, the most common residential setup.
Safety First: Gas Supply, Pressure, and the Relief Valve
Water heaters hold scalding water under pressure, and a damaged tank can release both. Before any troubleshooting, shut off the gas at the unit’s supply valve if you smell gas or hear hissing, open a window, and call a professional. Do not relight a pilot after a suspected leak; let the space air out first.
The temperature and pressure relief valve is the unit’s last line of defense against overpressure, and it deserves a check before anything else. A valve that weeps water, or one that is seized and will not lift, needs replacement. Temperature and pressure relief valve maintenance keeps the tank from becoming a hazard, and the test takes one minute: lift the lever briefly, let it snap shut, and confirm water stops.
Confirm the gas supply is actually on
A gas water heater with the supply valve closed will not fire, and the fix is a quarter turn of the handle. If the range and furnace are also dead, the problem is upstream, possibly a tripped regulator or an outage.
Listen and look before you touch
- A rotten-egg smell means a gas leak; leave the area and call your utility.
- Hissing near the base of the tank points to a leaky fitting.
- Water under the unit can come from the relief valve, drain valve, or tank.
- The burner compartment should show a clean blue flame.
| Symptom | Likely cause | First check |
|---|---|---|
| No hot water at all | Pilot out, gas valve closed, failed thermocouple | Pilot assembly and gas supply |
| Lukewarm water | Thermostat set low, sediment layer, burner issue | Thermostat dial and flame |
| Water too hot | Thermostat set high, failed thermostat | Thermostat dial |
| Rumbling or popping | Sediment on the burner or tank floor | Flush the tank |
| Rusty or smelly water | Corroded tank or anode, bacteria | Check anode and tank age |
| Relief valve leaking | Overpressure, failed valve | Test and replace valve |
Pilot Light and Ignition Problems
The pilot is a small flame that ignites the main burner when the thermostat calls for heat. When it goes out, the unit cannot fire until the pilot is relit. Drafts, a dirty pilot orifice, and a failed thermocouple are the usual causes.
- Turn the gas control knob to off and wait five minutes for any gas to clear.
- Switch the knob to pilot and press and hold the reset button.
- Light the pilot with a long lighter or match while holding the button.
- Keep holding for about a minute so the thermocouple heats up.
- Release the button, confirm the flame stays lit, and turn the knob to on.
- Set the thermostat and check for hot water.
The thermocouple: a safety device that wears out
The thermocouple sits in the pilot flame and generates a small voltage that keeps the gas valve open. When it fails, the valve closes even though the pilot looks fine, and the flame dies moments after you release the reset button. A failing thermocouple is a common replacement, but the burner assembly must be cool and the gas off before you swap it.
When to stop troubleshooting and call a pro
Repeated pilot failures, a gas smell, or a valve that will not respond point past the homeowner level. If you cannot restore hot water safely within an hour, a same-day hot water service can diagnose and repair the unit, and the wait beats the risk of working on live gas.
Thermostat Settings and Burner Problems
A water heater that runs but delivers lukewarm water often has a thermostat problem or a burner that is not getting full gas. The thermostat dial sits near the base of the tank, usually marked in temperature settings. The recommended setting is 120 degrees Fahrenheit, balancing comfort, energy use, and scald risk.
Set the dial, then wait
If the dial is set low, turn it up and wait; recovery takes time because the tank holds a large volume of water. For a family-sized unit, expect 30 to 60 minutes to bring a full tank up to temperature. If the dial is already at 120 and the water is still lukewarm, the problem sits below the dial.
Burner and gas valve checks
A burner that lights weakly, sputters, or shows a yellow flame is not burning cleanly. Dust on the burner ports, a clogged orifice, or low gas pressure all reduce heat output. The burner assembly sits behind a small access panel, but cleaning ports and replacing orifices is best left to a professional.
Flame color tells the story
A healthy gas flame is blue with a distinct inner cone. Yellow tips mean incomplete combustion and soot; orange flickers often mean dust. Either way, the burner needs cleaning before carbon builds up.
Electric backup elements appear on hybrid and dual-fuel units, and they fail with their own signature: the water goes warm but never hot. Replacing an immersion heater element in a hot water cylinder follows a drain-and-swap procedure an electrician can finish in an hour, and it is worth checking before condemning the unit.
Rusty Water, Leaks, and Corrosion
Discolored water is the clearest sign of internal damage. A tank-type heater uses a sacrificial anode rod to attract corrosion, and once that rod is consumed, the tank itself starts rusting from the inside out. Rust-colored water from the hot tap, a metallic smell, or persistent leaks all point to a tank near the end of its life.
Read the color and the smell
A little rusty water after a long vacation is usually harmless stirred-up sediment; rusty water that returns every morning is not. Black or rotten-egg-smelling water can come from sulfate-reducing bacteria in the tank, treated by replacing the anode and sanitizing the tank. Either way, the tank’s internal condition decides whether the unit is worth repairing.
Leaks: find the source before replacing the tank
Water under the unit is not always a failed tank. The relief valve, the drain valve, and the inlet and outlet fittings all leak independently, and each is replaceable. Dry the area, run the heater for a day, and trace where the water reappears. A leak from the tank body itself, especially a rusted seam or bottom, means replacement; corrosion cannot be patched.
A corroded tank is also the moment to reconsider the system, not just swap like for like. Combined hydronic heat and hot water systems pair space heating with domestic hot water in a single appliance, and for a house already running radiators or in-floor heat, that consolidation can beat a standalone tank. The trade-off is complexity: combo systems need professional sizing and annual service.
Sediment, Noises, and Tank Flushing
Hard water leaves calcium and magnesium behind as it heats, and the minerals settle on the tank floor and around the burner. The layer insulates the water, so the unit runs longer and hotter to deliver the same result. Rumbling or popping sounds are the sediment heating and cracking, the classic call to flush.
- Turn the thermostat to off and shut off the gas supply to the unit.
- Attach a garden hose to the drain valve at the bottom of the tank.
- Run the hose to a floor drain or outside; open a hot tap upstairs to vent the tank.
- Open the drain valve and let the tank empty completely.
- Open the cold water supply briefly to churn the sediment, then drain again.
- Close the drain valve, remove the hose, refill the tank, and relight the pilot.
How often to flush
Once a year is the standard for a tank-type unit in most water conditions, and twice a year with very hard water. A flush that runs clear in under a minute means the unit has been maintained; one that sputters chunks of mineral shows years of neglect. After the flush, check the anode rod, which needs replacement every three to five years.
Ignition problems follow the same pattern across gas appliances. The causes that keep a gas water heater pilot from lighting, a dirty orifice, a worn thermocouple, or a failing gas valve, show up in gas fireplaces too, and common ignition problems in a gas fireplace follow the same diagnosis order. Owning both means learning the shared checklist once.
Repair, Replace, or Upgrade: Sizing the Next Unit
When a tank leaks, the burner will not stay lit, or the unit passes fifteen years, replacement beats repair. Repair if the tank is sound and the fix costs less than a third of a new unit; replace if the tank itself is damaged, the unit is old, or repairs are stacking up.
Compare lifetime costs, not sticker prices
A new tank-type unit costs less up front, while tankless units cost more but last longer and cut standby losses. Energy factor ratings show how efficiently each model converts gas into hot water, and utility rebates often shift the math toward efficiency. Labor is a fixed cost either way, so the fuel type and expected life drive the decision.
Size the replacement for the household
An undersized unit runs constantly and still runs out of hot water; an oversized unit short-cycles and wastes fuel. For an electric or hybrid replacement, choosing the right size heat pump water heater for your home depends on the number of occupants, the incoming water temperature, and the peak demand of the busiest morning, and the same first-hour math applies to gas tanks.
Most gas water heater failures fit one of six patterns: a pilot that will not light, a failed thermocouple, thermostat error, sediment, corrosion, or a tank at the end of its life. Running the checks in order catches the cheap fixes first. When the diagnosis points to replacement, size the new unit for the household, not for the old tank.