How a gas water heater decides to heat
A gas unit combines a burner, a gas control valve and a safety device that proves the flame. Water temperature drives the valve, and the safety device keeps gas flowing only while a flame is present.
Older models use a standing pilot and a thermocouple. Newer models use an electronic igniter and a flame sensor. Both do the same job, and both fail in ways that look identical at the tap: no hot water and a unit that will not stay lit.
Anything past the gas control valve needs a licensed fitter. The parts are not expensive, but the consequences of a poor connection are.
Gas safety comes before diagnosis
Three situations override every troubleshooting step on this page.
- A gas smell. Leave, then call your gas utility or 911 from outside. Do not touch switches, thermostats or lighters.
- A yellow or sooty flame. That points to incomplete combustion. Shut the unit down if the control is reachable and book a combustion check.
- Headache, nausea or drowsiness in the house. Treat it as possible carbon monoxide, get everyone outside and call for help.
A carbon monoxide alarm on each level gives you a warning before symptoms appear. Test it on the manufacturer's schedule, and replace the unit when the label says to.
If the water is also leaking, the two problems need separate handling. Our emergency water heater repair page covers the water side, while gas work waits for a licensed fitter.
Eight symptoms and what they suggest
- The pilot will not stay lit. A weak thermocouple signal or a failing gas control.
- Clicking but no ignition. Igniter, flame sensor or gas supply at the burner.
- The burner lights, then cuts out. Flame proving, draft or gas pressure under load.
- Hot water runs out faster than it used to. Sediment in the tank or a dipping tube failure.
- Rumbling or popping from the tank. Scale and sediment trapping heat at the base.
- Water around the base. Tank failure or a leaking fitting, which is a different repair.
- Soot on the draft hood or the wall above. Combustion is not burning cleanly.
- A stuffy utility room or a backdraft. The vent is not carrying exhaust away.
The faults behind those symptoms
Thermocouple and thermopile failure
The thermocouple sits in the pilot flame and produces a small voltage that holds the gas valve open. When it weakens, the valve closes and the pilot goes out. The symptom is a pilot that lights and then dies the moment the button is released.
A thermopile does the same job on some standing-pilot systems and also powers the control. Testing either one means measuring voltage under load, which is why the part is rarely diagnosed from the outside.
Igniter and flame sensor faults
Electronic ignition systems spark or glow to light the burner, then rely on a flame sensor to prove it. A dirty sensor reports no flame and shuts the gas off within seconds. The unit may try three times and then lock out.
Cleaning the sensor sometimes restores it, and sometimes the sensor or the control board has failed. The test tells you which, and it takes minutes with a meter.
Gas control valve and pressure
The gas control valve regulates flow to the burner and holds the safety shut-off. A valve that sticks can leave the unit cold, or overheat the water when it fails open. Inlet pressure also has to be right while the burner runs, not just when it sits idle.
Pressure problems are a supply issue and may involve the meter, the regulator or the line size. A gas fitter measures at the appliance and works backward from there.
Venting, draft and combustion air
A gas unit needs a clear flue and enough air to burn. A blocked flue, a crushed vent connector or a utility room starved of air causes backdrafting, and exhaust then spills into the room instead of leaving through the roof.
Soot is the visible clue. A draft test with the unit running is the proof. This is the fault most worth catching early, because it affects the air people breathe rather than the temperature of the water.
Tank and sediment problems that look like gas faults
Cold water and a lit burner can still mean the tank has failed internally. A broken dip tube sends cold water to the outlet, and heavy sediment keeps the burner from heating the water properly. Both make the unit look underpowered when the burner is fine.
Once water is on the floor, the repair is no longer about gas. A tank that leaks at the shell is discussed on the leaking water heater repair page, and replacement is often the practical answer.
What happens on a gas repair visit
The visit is a sequence of safety checks before any part is quoted.
- Check for gas and carbon monoxide. The area is tested before tools come out.
- Observe the flame. Colour, shape and stability narrow the fault quickly.
- Test the safety device. Thermocouple, thermopile or flame sensor readings are measured under load.
- Measure gas pressure. Inlet and manifold pressure are checked while the burner fires.
- Test the vent. Draft and spillage are confirmed with the unit running.
- Quote and repair. Parts and labour are listed, and the unit is retested before the visit ends.
Ask for the pressure readings if a gas valve is being replaced. Numbers on a gauge explain the diagnosis better than a description does.
Repair or replace a gas unit
Thermocouples, igniters, sensors and vent connectors are routine repairs. A gas valve costs more but still leaves you with the same tank, which matters if the shell is sound and the venting is correct.
The calculation changes when the tank is leaking, the flue is unsafe or the unit has failed twice in a season. At that point a water heater replacement is usually cheaper than a second repair on a unit that is already compromised.
Gas replacements also involve venting and gas line checks, so the quote should list them. A like-for-like swap can still grow if the existing flue does not suit the new appliance.
Keeping a gas unit safe between visits
An annual service covers the parts that fail quietly: the anode rod, the relief valve, the flue and the burner. Sediment flushing protects the tank, and a combustion check protects the household.
Keep the area around the unit clear. Paint cans, boxes and stored furniture near the burner restrict combustion air and add fuel to any fire. A utility closet packed to the ceiling is a risk, not extra storage.
Homes in Lawnwood and across the older parts of the city often have units in tight closets or converted garages. Those positions are exactly where draft problems appear, and they are worth checking before symptoms start. Our flush and maintenance page covers the service routine.
Gas water heater questions
Why does the pilot light keep going out?
A pilot that will not stay lit usually means the thermocouple or thermopile has stopped sending its signal, or the gas control has failed. It can also be a draft pulling the flame away. Each cause has a different test, so the part is not swapped on a guess.
I smell gas. What should I do first?
Leave the building and call your gas utility or emergency services from outside. Do not switch anything on or off, and do not light a match to look for the source. A gas smell is a utility emergency before it is a repair job.
Is a yellow burner flame dangerous?
A steady yellow flame with soot suggests incomplete combustion, which can produce carbon monoxide. Turn the unit off if you can do so safely and book a combustion check. A clear blue flame is the normal sign.
Can a gas water heater be converted to propane?
Sometimes, but only with the manufacturer's conversion kit and a licensed fitter. An unconverted unit running on the wrong fuel is a serious hazard. Confirm the kit is listed for your model before buying anything.
Do gas units need a permit for repair?
A repair often does not, while a replacement or a new gas line usually does. The provider confirms what the local building department requires and includes any permit in the quote.
How do I know the vent is working?
You cannot confirm it by feel. A provider tests draft with the unit running and checks the flue for spillage and blockage. Soot around the draft hood, a backdraft or a stuffy utility room are all reasons to book that test.