Gas Water Heater Thermal Switch Won't Reset
On this page (6 sections)
The Short Answer: A thermal (rollout or spill) switch on a gas water heater opens when flame or hot exhaust reaches a place it should not. If it will not reset, it is still too hot, has failed open, or melted in a severe overheat. Keep the heater off until the reason it tripped, usually venting, combustion air, or the burner, has been found. Never bypass the switch.
What this switch does
Many gas tank heaters have a thermal switch in one of these places:
- At the burner access area (flame rollout switch): trips if flames roll out of the combustion chamber instead of going up the flue.
- At the draft hood (spill switch): trips if exhaust spills out of the draft hood instead of rising up the vent.
- On a power-vent blower or exhaust outlet: trips if the exhaust gets too hot.
Some are resettable buttons; others are one-time devices that must be replaced. Electronic gas controls may show a flash code instead of having a visible button. This page covers the case where the switch will not reset at all. If it resets but keeps tripping, see thermal switch keeps tripping.
Why it won’t reset
| What you see | Likely reason |
|---|---|
| Button won’t stay in shortly after the heater shut down | Switch is still above its reset temperature; it needs time to cool |
| Button is discolored, cracked, sunken, or wires look scorched | Switch overheated and failed; it must be replaced |
| Switch is a sealed disc with no button | One-time switch; replacement only |
| No button at all, status light flashing | The control reports the fault electronically; check the code chart on the label |
Why it tripped in the first place
A thermal switch that will not reset is less important than the reason it opened. Common causes:
- Blocked or disconnected vent, or a vent cap obstructed by a nest, ice or debris
- Too little combustion air: a closet door that was replaced with a solid door, or new weatherproofing in a small room
- Downdraft caused by an exhaust fan, clothes dryer or fireplace pulling air from the space
- A dirty or damaged burner, or a flame arrestor screen clogged with lint and dust on newer heaters
- On power-vent heaters, a blower that is failing or a vent that is too long or blocked
Replacing the switch without correcting any of these means it will trip again, and in the meantime exhaust may be entering the home.
What you can check safely
- Turn the gas control to OFF and let the heater cool.
- Look at the vent connector on top of the heater: is it attached to the draft hood and to the vent going up?
- From outside, check that the vent cap or termination is clear.
- Look for soot or scorching on the jacket around the burner access door or draft hood. Note it for the technician.
- Check whether the closet or room door, or the combustion air openings, have been changed or blocked.
Do not hold the switch in, tape it, jumper its wires, or remove the burner access door. If your carbon monoxide alarm went off, do not relight the heater until it has been inspected.
What the technician will check
They will test whether the switch is open after it cools, inspect the vent from heater to termination, check draft at the draft hood with the burner running, measure combustion and look for carbon monoxide, inspect the burner and flame arrestor, and check combustion air. On power-vent models they check the blower and vent length. Once the cause is fixed, they replace or reset the switch and confirm the heater vents properly. If the furnace shares the same vent and is also acting up, see water heater and furnace not working.
Frequently Asked Questions
Q: How long does a thermal switch take to cool down? A: It varies by switch and heater, but expect minutes to tens of minutes rather than seconds. If it still won’t reset after the heater is fully cool, the switch has likely failed.
Q: Is it safe to run the heater with the thermal switch bypassed for a day? A: No. The switch is the only thing stopping the heater from running while flames or exhaust escape. Bypassing it creates a real risk of fire and carbon monoxide poisoning.
Q: Is this the same as the reset button on an electric water heater? A: No. Electric heaters have a high-limit (ECO) on the upper thermostat that trips on water temperature. A gas thermal switch trips on flame or exhaust heat, which points to venting or combustion problems.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
Editorial standards and corrections · Prepare a useful service request