Water Heater Not Heating? A Plumber's Gas Checklist
I'm Dan Reeves, a licensed master plumber with 20 years in residential work and more than 4,000 jobs behind me. A gas water heater that stops heating is not one problem; it is a ladder of four or five problems, and they have to be checked in order, because each one hides the ones below it. Most no-hot-water calls I run end at the first or second rung and cost under $325. The expensive mistakes happen when someone skips the ladder and authorizes a gas valve, or a whole new tank, before anyone has looked at a $15 thermocouple. If your heater is electric, stop here: the ladder is completely different, and I wrote it out in my guide on [electric water heater not heating](/articles/electric-water-heater-not-heating/).
I’m Dan Reeves, a licensed master plumber with 20 years in residential work and more than 4,000 jobs behind me. A gas water heater that stops heating is not one problem; it is a ladder of four or five problems, and they have to be checked in order, because each one hides the ones below it. Most no-hot-water calls I run end at the first or second rung and cost under $325. The expensive mistakes happen when someone skips the ladder and authorizes a gas valve, or a whole new tank, before anyone has looked at a $15 thermocouple. If your heater is electric, stop here: the ladder is completely different, and I wrote it out in my guide on electric water heater not heating.
What should you check first when a gas water heater has no hot water?
Start at the pilot viewing window. Is the pilot lit? A standing-pilot heater with the pilot out has no heat at all, full stop, and everything else on the unit will look dead. If the pilot is out, check the obvious before you blame parts: is the control knob actually set to a temperature, or did it get bumped to Pilot or Vacation while someone was cleaning around the unit? I find that more often than I can politely say.
Next, rule out the supply. Are your other gas appliances working, a furnace, a stove burner? If the whole house has no gas, the heater is not broken; the utility has a problem or a valve is closed. If the heater is your only dead appliance, the failure is inside the unit, and the table below is your map. One more check on newer electronic-ignition models: a power outage or a tripped outlet can leave them dark even with gas available, because their controls run on electricity.
How do you diagnose a gas water heater that is not heating?
| Symptom | Likely cause | The fix | DIY or pro |
|---|---|---|---|
| Pilot will not light at all | Closed gas valve, air in the line after gas work, failed igniter | Verify supply, relight following the label steps | DIY checks; pro if supply is dead |
| Pilot lights but dies when you release the knob | Worn thermocouple, or a dirty pilot orifice starving the flame | Clean the orifice, replace the thermocouple | Handy DIY; pro $100–$325 |
| Pilot stays lit, burner never fires | Failed gas control valve or its temperature sensor | Replace the gas control valve | Pro, $150–$550 |
| Burner fires, water only lukewarm | Sediment insulating the bottom, failed dip tube, or control set too low | Flush the tank; test and replace the dip tube | Flush DIY; dip tube pro $150–$300 |
| Hot water gives out within minutes | Broken dip tube or heavy sediment eating tank volume | Replace dip tube; flush the tank | Pro or advanced DIY |
| Burner shuts off early, rumbling from the tank | Sediment overheating the tank bottom | Full flush | DIY or pro $150–$300 |
| Soot, scorch marks, flames rolling out of the door | Blocked flue or failed draft, a combustion danger | Shut the unit down, do not relight | Pro immediately |
Read the table top to bottom the way the failures stack. Pilot problems mask everything: you cannot judge the control valve until the pilot holds, and you cannot judge heating capacity until the burner runs a full cycle. Anyone who quotes you a valve replacement over the phone, without a lit pilot in front of them, is guessing with your money.
Why does the pilot light control everything?
A standing pilot is not just a little flame waiting to light the burner. It is the heater’s safety brain. The flame heats the thermocouple, which generates a tiny electrical current, millivolts, that holds a magnetic safety valve open inside the gas control. As long as that current flows, gas is allowed to reach the burner when the thermostat calls. The second the flame dies, the current dies, the magnet lets go, and gas flow stops. That is why a heater with a dead pilot cannot heat at all, and why the design is safe: no flame, no gas, by physics rather than by electronics.
Because the whole chain depends on that flame, pilot behavior is the most information-dense symptom on the unit. A pilot that will not stay lit has its own set of causes beyond the thermocouple: drafts from a failing vent, a partly clogged pilot orifice making a weak flame, a thermopile on newer millivolt systems getting tired. I gave that problem its own full treatment in my article on pilot light keeps going out, because it is the single most searched gas heater problem I deal with.
What does a failing thermocouple look like?
The classic pattern: you hold the control knob down, light the pilot, it lights easily, and the moment you release the knob after a proper 30 to 60 second hold, the flame dies. Over days the hold time gets longer and the failure gets more frequent, until it will not hold at all. That progression is a thermocouple wearing out. It produces less and less millivoltage as its junction degrades, until it can no longer hold the safety magnet against its spring.
The part costs $10 to $20. Professionally replaced, the visit runs $100 to $325, and the work takes 30 to 45 minutes including relighting and verifying a full burner cycle. On a service call this is the first part I swap on a pilot complaint, and it resolves the majority of them. If a new thermocouple does not hold the pilot, the problem is upstream: a weak pilot flame from a dirty orifice, or a failing control valve magnet. Both are findable in the same visit by anyone who tests instead of guessing.
What if the pilot stays lit but the burner never fires?
Then the safety chain is fine and the failure is in the control itself. The gas control valve contains the thermostat: a sensor in the tank water tells it the temperature, and it opens the main gas path when the water is below setpoint. When that sensor or the valve mechanism fails, the pilot burns cheerfully forever and the burner never gets gas. Check the dial first, because a knob sitting at the low end of Warm fools someone every season. A dial set correctly with cold water in the tank and no burner response is a dead control.
Replacing the valve means draining the tank, breaking gas connections, remaking them, leak-testing every joint, and verifying combustion and draft. That is licensed-plumber work, and it runs $150 to $550 depending on the model. It is also the repair where age math matters most, which brings us to the cost table in a moment.
Why is the water only lukewarm when the burner runs fine?
Two different failures produce the same complaint. The first is the dip tube. Incoming cold water enters at the top of the tank and is carried to the bottom through a long plastic tube, so it can be heated as it rises and the hot water at the top leaves undisturbed. When that tube cracks or slowly dissolves, cold water dumps straight into the top and mixes with the outgoing hot water. The shower starts acceptable and fades to lukewarm, and it gets worse over months. My field tell: I pull a faucet aerator. White or blue plastic crumbs in the screen are pieces of the dead tube. Replacement runs $150 to $300.
The second is sediment. A thick mineral layer on the tank bottom insulates the steel, so the burner heats crust instead of water and the effective hot volume shrinks. The fix is a flush, and the full procedure, including the refill mistake that kills heaters, is in my guide on how to flush a water heater.
How much does it cost to fix a gas water heater that is not heating?
| Repair | DIY cost | Pro cost | Notes |
|---|---|---|---|
| Relight pilot, basic diagnostic | $0 | $50–$150 | Free fix if it stays lit and heats |
| Thermocouple replacement | $10–$20 part | $100–$325 | The most common no-heat repair |
| Pilot assembly or igniter repair | $20–$60 parts | $75–$350 | Electronic-ignition models |
| Gas control valve replacement | Not recommended | $150–$550 | Includes leak and combustion checks |
| Dip tube replacement | $15–$30 part | $150–$300 | For lukewarm water and short showers |
| Tank flush for sediment | $0 with a hose | $150–$300 | Also restores usable capacity |
| Full replacement, tank failed or too old | — | $1,200–$2,500 installed | Standard 40–50 gallon, varies with code work |
The age rule I give customers is simple. Under 8 years old, repair anything on the ladder; the tank has life left to protect. From 8 to 12 years, compare the repair against half the cost of replacement, and weigh the tank’s visible condition. Over 12 years, with rust at the base or a valve-level repair on the quote, my honest recommendation is usually replacement. A $500 valve in a tank that leaks next spring bought you nothing.
When should you call a pro about a gas water heater with no hot water?
Call immediately, and leave the house first, if you smell gas anywhere near the unit. That is a utility call from outside, not a plumbing appointment. Call a pro the same day for soot or scorch marks around the burner door, flames rolling out instead of drawing in, or a flue pipe that is disconnected or rusted through. Those are combustion and carbon monoxide problems, not heating problems.
Book a pro for the gas control valve, every time, and for any pilot that still will not stay lit after a fresh thermocouple, because the remaining causes are draft, venting, and the valve itself. And if you are not comfortable relighting a pilot by the label instructions, do not improvise: a $50 to $150 relight visit is cheap tuition, and you get the rest of the unit inspected while the plumber is standing there.
Frequently asked questions
Why is my gas water heater not making hot water?
Work the ladder in order. If the pilot light is out, nothing downstream matters, and the pilot is out most often because the thermocouple has worn out and can no longer hold the safety valve open. If the pilot stays lit but the burner never fires, the gas control valve and its temperature sensor have failed. If the burner fires but the water is only lukewarm or runs out fast, the problem is usually sediment insulating the tank bottom or a broken dip tube mixing cold water at the top. In 20 years of calls, most end at one of those four, usually the first two.
How much does it cost to replace a thermocouple on a water heater?
The thermocouple itself is a $10 to $20 part. A professional replacement runs $100 to $325 total in most markets, and nearly all of that is the service visit and labor, not the part. The job takes 30 to 45 minutes: kill the gas, swap the sensor, relight, and verify the pilot holds and the burner cycles. It is the most common repair on gas tank heaters and the first one I try on a pilot that will not stay lit. If a company quotes you far above that range for a thermocouple alone, get a second quote before you authorize it.
Can I replace a gas control valve myself?
This is where I draw the line for homeowners. A gas control valve replacement means breaking and remaking gas connections, then leak-testing every joint and verifying combustion and draft before the unit goes back into service. Done wrong, the failure mode is a gas leak, not a drip. The professional job runs $150 to $550 depending on the model, and it includes those safety checks. The valve itself is also model-specific, and the wrong one is a return trip and a cold house. Relight a pilot yourself, yes. Swap a thermocouple if you are handy, fine. Leave the valve to a licensed plumber.
Why does my water heater make the water lukewarm instead of hot?
Two causes explain nearly every case. The first is a failed dip tube, the plastic pipe carrying cold water to the tank bottom. When it cracks or disintegrates, cold water dumps in at the top and mixes with the hot water leaving, so the first minutes feel warm and then fade. Tiny white plastic bits caught in your faucet aerators are the giveaway. The second is a heavy sediment layer insulating the tank bottom so the burner heats crust instead of water. A dip tube replacement runs $150 to $300; a flush is $150 to $300 from a pro or free with your own hose.
Is it worth fixing a 12-year-old gas water heater?
It depends entirely on which part failed. A thermocouple at $100 to $325 on a 12-year-old tank is worth it, because the job is cheap and can buy you the tank's remaining years. A gas control valve at $400 to $550 is a harder call: a gas tank's design life is roughly 8 to 12 years, so that money goes into a tank that may leak next year, against $1,200 to $2,500 for a full replacement with a new warranty. My rule: compare the repair to half the replacement cost, and weigh the tank's age and condition. On a rusting 12-year-old, I recommend replacement and say so.