Gas water heater not heating
No hot water from a gas water heater is almost always a pilot light issue, thermocouple failure, or gas supply problem. Some diagnostics are safe DIY; anything involving gas valve work is professional-only.
Quick answer
For a standing pilot: check that the pilot is still lit through the sight glass; if out, relight it following the label instructions on the tank. If it won't stay lit, the thermocouple has likely failed — a $20 part you can replace yourself. For newer electronic ignition water heaters: check the diagnostic light code and consult the manual. If you smell gas, leave and call your gas utility.
Key facts
| Difficulty | Medium |
|---|---|
| Time | 30–90 min |
| Parts cost | $0–$40 |
| Tools needed | Flashlight, small wrench |
| Parts needed | Thermocouple ($10–$25), thermopile ($20–$40) for newer units |
What's happening
Gas water heaters work by burning natural gas or propane to heat water in a tank. The system needs three things: gas supply (from your utility), ignition (pilot light or electronic igniter), and a working control valve that opens the main burner when the water gets cold.
The most common failure mode is the pilot light going out or the thermocouple failing. The thermocouple is a small sensor that detects the pilot flame — if it doesn't sense flame, the gas valve closes as a safety measure. When thermocouples wear out (typically 5–15 years), the pilot won't stay lit even after you relight it.
Newer water heaters (post-2005) use electronic ignition and a thermopile instead of a thermocouple. Same principle, different replacement part.
Common causes (ranked by frequency)
- Pilot light out 40% of cases
Simple: pilot got extinguished by a draft, brief gas interruption, or vibration. Relight per label instructions.
- Thermocouple failed 35% of cases
Pilot lights but goes out when you release the pilot button. Thermocouple no longer generating enough current to keep the gas valve open. $20 part, DIY-doable.
- Gas supply issue 15% of cases
Gas valve to water heater closed, main gas issue, or pilot orifice clogged. Check that the gas valve at the tank is fully open (handle parallel to pipe).
- Gas control valve failure 8% of cases
The main control valve at the base has failed. Requires professional replacement — often approaching cost of new water heater.
- Vent/exhaust issue 2% of cases
Blocked flue or negative-pressure situation shutting off the burner via safety switch. Ensure vent pipe is clear.
How to diagnose
Step 1 — smell test: If you smell gas near the water heater, STOP. Leave the house and call your gas utility. Do not use switches, phones, or lighters.
Step 2 — check the pilot: Most gas water heaters have a small sight window at the bottom. Look for a small blue flame. If dark, the pilot is out.
Step 3 — verify gas supply: The gas shutoff valve near the water heater should have its handle parallel to the pipe (open). If perpendicular, it's closed. Also confirm other gas appliances in the house are working (stove, furnace) — rules out house-wide gas issue.
Step 4 — try to relight (if standing pilot): Follow the label on your water heater exactly. Typical: turn control to OFF, wait 5 minutes for gas to clear, turn to PILOT, press and hold down while igniting pilot with striker, hold 30–60 seconds after pilot lights, release. If pilot goes out when you release the button: thermocouple failure.
Step 5 — for electronic ignition: Look for a diagnostic LED on the gas valve. Count flashes per cycle and consult your manual — the code identifies the specific problem.
The fixes
Relight the pilot TRY FIRST
Tools
- Long-reach lighter or the built-in igniter
Parts
- None
- Turn control knob to OFF. Wait 5 full minutes for any residual gas to dissipate.
- Turn control to PILOT.
- Press and hold the pilot button (usually red or labeled PILOT).
- Ignite the pilot using either the built-in igniter (press the sparker button) or a long lighter through the pilot access.
- Continue holding the pilot button for 30–60 seconds after the pilot lights. This lets the thermocouple heat up and start generating current.
- Slowly release the pilot button. If the pilot stays lit, turn the control to ON and set the temperature (120°F recommended).
- If the pilot goes out immediately: Try the relight process one more time, holding the button 90 seconds. If it still goes out, the thermocouple needs replacement (see next fix).
Replace the thermocouple IF PILOT WON'T STAY LIT
Tools
- Adjustable wrench or small wrench
- Flashlight
Parts
- Universal thermocouple ($10–$25)
- Turn gas control valve to OFF and turn off gas supply valve. Wait 15 minutes for cooling.
- Remove the burner assembly. Two or three connections at the gas control valve (thermocouple, pilot tube, main burner tube — all with small hex nuts).
- Slide the burner assembly out. Note orientation for reassembly.
- Remove the old thermocouple. It's clipped in place next to the pilot flame position, with the copper wire running back to the gas valve.
- Install the new thermocouple in the same position and orientation. The tip should be in the pilot flame when it burns.
- Reconnect at the gas valve — snug but do NOT overtighten. Overtightening damages the connection.
- Reinstall the burner assembly in exact reverse.
- Turn on gas supply. Relight pilot per instructions above.
Prevention
Gas water heater maintenance:
- Flush the tank annually — sediment buildup reduces efficiency and shortens life.
- Check the anode rod every 3–5 years — replace when heavily corroded. This alone can double tank life.
- Test the T&P (temperature and pressure relief) valve annually.
- Keep the area around the water heater clear of flammables — no boxes, paint cans, or gasoline within 3 feet.
- Set temperature at 120°F — hotter increases scald risk and wastes energy.
- Consider tankless when replacement time comes — 20+ year life vs 10–15 for tank units.
When to call a pro
- You smell gas at any point — leave the house and call the gas utility immediately
- Gas control valve replacement needed
- Vent/flue pipe repair or replacement
- Water heater is 12+ years old and failing — often better to replace than repair
- Any leak from the tank itself (not from fittings)
- You're not comfortable with the diagnostic steps or safety requirements
Plumber cost: $150–$300 for thermocouple replacement (though DIY saves this easily). Gas valve replacement: $250–$500. New standard gas water heater installed: $1,200–$2,500.
Cost estimate
Frequently asked questions
10–15 years typically. Signs of end-of-life: sediment noises, rust in hot water, recurring pilot issues, drips from fittings. Beyond age 12, weigh repair cost against replacement — often replacement makes more sense.
Weak thermocouple (most common), dirty pilot orifice, draft near the water heater, or bad gas control valve. Try replacing the thermocouple first — cheapest and most likely fix.
Yes, and often recommended — turn the temperature control to VACATION or lowest setting rather than turning off pilot. Keeps the tank slightly heated (prevents standing water issues) and avoids relighting hassle.
Consider it. Tankless units cost more upfront ($3,000–$4,500 installed vs $1,500 for tank) but last 20+ years, save 20-30% on energy, and provide unlimited hot water. Best for households with high hot water demand or space constraints.