Most newer gas tank water heaters have a reset button on a thermal switch behind the outer door, beside the burner. It trips from a blocked vent, poor air or flammable vapors. A. O. Smith says to press it after failed lighting attempts, and to stop and call a technician if the flame arrestor is discolored.
What trips the thermal switch?
Three things, all of them about air and vapors rather than the water. A. O. Smith's handbook names three events that expose the heater to high combustion chamber temperatures:
- Flammable vapors, such as spilled gasoline.
- Poor combustion caused by a blocked vent.
- Insufficient combustion air.
The handbook adds that a tripped switch is most likely caused by poor combustion, such as a blocked vent or a dirty filter or flame arrestor. It also says that if the switch has tripped, you should determine the cause and correct it before returning the heater to service, and make sure there are no flammable vapors present.
That last point is the reason to treat the button carefully. The switch is a safety device. Pressing it clears the lockout but does not fix whatever heated the chamber.
What do the manuals say to do, in order?
Start with safety, then the lighting checklist, then the switch itself. First, the gas smell rule. If you smell gas, do not light anything, do not touch a switch or phone inside, and leave. Southwest Gas takes emergency calls at 877-860-6020, and 911 works for any suspected leak.
Second, the lighting checklist. A. O. Smith's installation manual, in its status light chart, says that for a heater with no status light: turn the knob to OFF, wait 10 minutes, and try to relight by the label. If the status light does not blink after three lighting attempts, check that the unit is getting gas, remove the outer door, press the reset button, replace the door, turn the knob to OFF, wait another 10 minutes, and try again. It says to look through the view port for the pilot flame, and that if the pilot is not visible, the igniter or the pilot gas supply should be checked.
Third, listen for the click. A. O. Smith's handbook says that if the switch clicks when you press the reset button, it may have tripped. In that case:
- 1Do not light the pilot if flammable vapors are present.
- 2Check the flame arrestor for discoloration, which could be caused by flammable vapors.
- 3If the flame arrestor is discolored, do not try to relight the pilot. Have a qualified service technician inspect the water heater.
- 4A technician also checks the wiring connections from the thermopile and thermal switch to the gas control and makes sure they are tight.
- 5Replace the outer door, wait 10 minutes, and relight by the label.
If the switch does not click, the handbook treats it as not having tripped and moves on to other causes, such as the thermopile or the pilot.
### What is a flammable vapor resistant heater?
The thermal switch is part of a larger design. Bradford White's manual says its heaters have a Flammable Vapor Ignition Resistant System, and that if gasoline or another flammable material is used or stored near the heater, the technology will resist ignition of the vapors outside the heater. Its listed features are an advanced flame arrestor design, a re-settable thermal switch, a piezo igniter and a sight window for watching the pilot and burner. A. O. Smith's heaters use the same concept, with an air intake through a base ring filter and flame arrestor. Both manuals treat activation of the system as an event to investigate, not an ordinary nuisance trip.
### Is there a reset for a heater that got too hot?
That is a different device. A. O. Smith's installation manual says its high water temperature shut off system is built into the gas control valve, is non-resettable, and if it activates, the gas control valve must be replaced. In that case the manual says to turn off the entire gas supply and contact your gas supplier or service agency. A four flash code on the status light is how the valve reports it. Our pages on A. O. Smith blinking light codes and Bradford White flashing light codes explain the flash counts.
Why does the thermal switch keep tripping?
If the button clears the heater and the switch trips again, the cause is still there. Check the things the manuals name:
- A dirty base ring filter or flame arrestor. A. O. Smith says the flame arrestor and filter should be inspected and cleaned at each service call. Lint, dust and pet hair build up at the base of the heater in a laundry room or garage.
- A blocked or poorly working vent. The handbook names a blocked vent as a top cause.
- Not enough combustion air. Bradford White's manual warns against blocking airflow and against storing materials against the heater or its vent. Our page on water heater clearance and access code covers the spacing rules, and our page on the 18 inch rule for water heaters in a garage covers flammable vapors.
- Flammable vapors. Bradford White's manual is blunt: if flammable vapors are drawn into the heater and burn, do not light any appliance, do not touch switches, leave, and call the fire department. Move gasoline, solvents and aerosols out of the room.
Our page on why a water heater pilot light will not stay lit covers the thermopile and other reasons the pilot drops out. If the heater has an older style gas control and no status light, the checklist above still applies to the thermal switch.
When should I call a plumber?
Call if the flame arrestor is discolored, if the switch trips more than once, if you smell gas, if the vent is blocked in a way you cannot clear, or if the heater still will not light after the checklist. Removing the outer door puts your hands near the gas control and burner, so if you are unsure, leave the door on.
HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, gives free estimates and quotes the price upfront. If the heater is old, our page on whether to repair or replace a water heater can help you weigh it.