Most often a safety device is doing its job: a worn thermocouple, a tripped thermal cutoff from poor air supply or venting, a flame arrestor clogged with lint, or flammable vapors near the burner. Low gas supply and a failing gas control are the other causes. Relight it once, then get repeat shutoffs checked.
How do you tell which problem you have?
Start with what the heater does. That one detail narrows things down.
- The pilot goes out as soon as you let go of the knob. A. O. Smith's guide says this usually points to an electrical problem. Press the thermal switch reset button. If it clicks, the switch tripped. If it didn't trip, the thermocouple is next. Our page on why a pilot light won't stay lit covers this case in detail.
- The pilot stays lit but the main burner shuts off. The gas control, the thermal cutoff, a blocked air path or a vapor shutdown are the suspects.
- It shuts off only when something else is running. The Department of Energy's Building America guideline warns that whole-house fans and other high-volume exhaust systems can cause a naturally vented water heater to backdraft, which is a fire hazard. Tell the technician what else was running.
Why do the safety devices shut a gas heater off?
### The thermal switch
It's a safety cutoff. A. O. Smith's guide says that if the thermal switch tripped, it is likely an air supply problem. The steps it lists are to clean the air filter and flame arrestor and confirm the room supplies enough air. It also describes a "draft test": turn the main burner on, and after five minutes of operation, smoke held near the draft hood should be drawn into it. If it isn't, there's an air supply or venting problem.
That test tells you whether exhaust is leaving the house. If gases aren't being drawn up the vent, they can spill into the room. The Consumer Product Safety Commission lists malfunctioning water heaters among the sources of carbon monoxide in homes. Carbon monoxide has no smell, and early symptoms look like the flu: headache, fatigue, shortness of breath, nausea and dizziness. CPSC recommends alarms on every level and outside sleeping areas, and a professional inspection of the heating system every year. Our page on whether a gas water heater can cause carbon monoxide has more. If soot, venting, or the gas control is the problem, it's also worth checking whether your water heater has a recall, since some recalls cover exactly those parts.
### The flame arrestor
Modern gas water heaters are built to resist igniting flammable vapors, a standard called FVIR. Rheem explains that the heater must not ignite flammable vapors outside the water heater from gasoline spilling onto the floor. Air reaches the burner through a flame arrestor, and A. O. Smith's guide says to clean it when the thermal switch trips. If the arrestor is clogged with lint and dust, the burner can starve for air and shut down.
Some brands use a different design. Rheem says its Guardian system needs no filter service, so follow your heater's own manual for whether the arrestor is serviceable. If the manual doesn't say how to clean it, leave it to a technician.
### Flammable vapors near the heater
Rheem's manual warns that natural air movement can carry vapors from gasoline, paint, thinners, solvents or adhesives some distance across a room. When the burner ignites them, the heater goes into a shutdown condition and won't relight. This matters most in garages and utility rooms, where those products tend to be stored. If you store any of those products near your heater, move them outside the room. It's also a fire hazard on its own, whether or not the heater shuts off.
### Blocked vents on power vent and direct vent models
If your heater has a fan and plastic vent pipes, the shutoff may be a pressure switch. Rheem's troubleshooting chart for its power vent model says blocked inlet or outlet vent systems will cause the pressure switch to shut the unit off, and that a kinked or disconnected pressure switch hose can do it too. It says to contact a qualified service technician to evaluate the vent pipes.
What else can cause a gas heater to shut off?
### Gas supply
A. O. Smith's guide says that if the pilot won't light but the igniter sparks, it's likely a gas supply problem. It lists checking that the main gas valve is on, that the control knob is fully pushed in, and the gas pressure, which requires a gauge. Rheem's troubleshooting chart also lists a manual gas shutoff that isn't fully open and air in the gas line, and A. O. Smith says to bleed air from the gas line on a new installation.
### A weak thermocouple or a failing gas control
A. O. Smith's guide says to measure a thermocouple's output in millivolts with the pilot lit. If it reads below 10 millivolts, the guide says to replace it. If the reading is above that and the wiring is secure, it points to the gas valve. Deciding between a thermocouple and a gas control is a call for someone with a meter.
### Electronic gas valves and status lights
Some heaters use a self-powered electronic gas valve with a status light. A. O. Smith's guide says the pilot must be lit for the light to flash, and that the light must flash once per second for the heater to operate. If it doesn't flash with the pilot lit, the guide points to an electrical problem, and for a thermopile it gives a 350 millivolt threshold. Other flash patterns are codes. Our pages on Rheem blinking light codes and A. O. Smith blinking light codes explain them.
What can you do yourself, and what shouldn't you?
Rheem's manual is direct: don't attempt to repair gas piping, the gas control, the burner, vent connectors or other safety devices, and refer those repairs to qualified service personnel. Don't jumper or bypass a thermal switch to keep the heater running. A repeatedly tripping safety device is telling you something.
Here's what is safe:
- 1Relight it once using the instructions on the tank label.
- 2Clear flammables away from the heater.
- 3Check the CO alarm near the bedrooms. Test it, and replace the batteries if needed.
- 4Look at the air path. Make sure nothing is stacked against the heater and blocking its air inlet or the room's vents.
- 5Write down what happened, including whether the pilot or the main burner went out, and what else was running. It saves time.
When should you call a plumber?
If it shuts off again, turn the gas control to off or pilot and call. HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before work starts. If the heater is old, our page on whether to repair or replace a water heater can help you weigh it. If the shutoff is happening with a rotten-egg smell, see what to do if the house smells like gas.