How can I tell if my water heater thermostat is bad?

What the thermostat does

A water heater thermostat reads the tank temperature and turns the heat on and off. Next to it sits a high-limit switch, usually called the ECO (energy cut off), which shuts the heat off if the water gets dangerously hot. The setup differs by fuel.

	Electric tank	Gas tank
Where it is	Behind upper and lower access panels, against the tank wall[3]	Inside the gas control valve, with a probe in the tank water[1]
How many	Usually two (upper and lower), one per element[3]	One, built into the valve
High-limit (ECO)	Part of the upper thermostat, with a red reset button[3][5]	Inside the gas control; on one A.O. Smith series an overheat trip means a new valve[7]
Replaceable alone?	Yes[1]	No, the whole gas control valve is replaced[1]

Electric tanks. Surface-mounted thermostats sense the tank's outside wall, so they must sit flat against it. Bradford White warns that a thermostat not in full contact with the tank leads to improper operation^[3]. On the usual two-element tank, the upper thermostat heats the top of the tank first, then hands power to the lower one^[3].

Gas tanks. The dial on the front of the gas control is the thermostat. On one A.O. Smith series with a Honeywell control, the dial covers roughly 55 F to 155 F^[7]. When the thermostat side of the valve fails, the fix is a new gas control valve. See gas control valve.

Signs of a bad thermostat

Symptom	Electric tank, likely cause	Gas tank, likely cause
No hot water	Failed upper thermostat or tripped ECO[3]	Valve not calling for heat, or a lockout
Lukewarm water or hot water that runs out	Lower thermostat, or setting too low[1][3]	Dial set low
Water too hot	Thermostat stuck closed, or not touching the tank[3]	Thermostat side of the valve failing[6]
Red reset button keeps tripping	Overheating; often a grounded element rather than the thermostat[3]	n/a
No change when the setting is raised	Thermostat not responding[1]	Same

HomeGuide's rule of thumb: if raising the setting does not raise the water temperature, the thermostat may be faulty. On a two-element tank, a failed upper thermostat means no hot water, and a failed lower one means a limited supply^[1].

Not every hot-water problem is the thermostat. Bradford White's troubleshooting chart lists elements, loose wires, sediment, a damaged dip tube, reversed connections, very cold inlet water and an undersized heater as causes of similar symptoms^[3]. A plumber checks power, the ECO and the elements before blaming the thermostat.

Settings and scald risk

A.O. Smith factory sets its electric thermostats to about 120 F to reduce scald risk, and its gas controls to their lowest setting^[4][6]. Its burn chart shows why^[6]:

Water temperature	Time to a serious burn
120 F	More than 5 minutes
130 F	About 30 seconds
140 F	Less than 5 seconds
150 F	About 1.5 seconds

A.O. Smith also notes that short, repeated draws can make the top of the tank as much as thirty degrees hotter than the setting, and that under the ASSE 1070 standard and many local codes the heater's own control should not be the only means of preventing scalds. Its advice is a thermostatic mixing valve at each point of use if you want the tank set higher^[6].

What a homeowner can safely check

• The breaker on an electric tank. Reset once; if it trips again, call.
• The dial on a gas tank. Make sure it is not on vacation or the lowest mark.
• Whether the water is too hot. Run a tap and feel carefully, or use a kitchen thermometer.
• Water near panels or wiring. A.O. Smith says to shut the heater off and have it inspected if the wiring or thermostats have been exposed to water, and to replace the whole heater if the thermostats were submerged in a flood^[4][6].

Do not open electric access panels or press the red reset button on your own. It sits next to live 240-volt terminals, and Rheem says the cause of the high-temperature condition must be investigated by a qualified service technician before the heater goes back in service^[5].

What it costs

Source (date)	Item	Range
HomeGuide (Aug 2025)	Electric thermostat replacement, parts and labor	$100 to $350[1]
Modernize (Aug 2026)	Electric thermostat replacement, one or both, with a heating cycle test	$100 to $300[2]
HomeGuide (Aug 2025)	Gas control valve, which includes the gas thermostat	$150 to $550[1]

HomeGuide notes minimum service call fees of $50 to $200 may apply^[1]. Ranges are national, not quotes.

When to call a pro

• Water hotter than normal, or a red reset button that keeps tripping.
• No hot water or lukewarm water with the breaker on and the dial set normally.
• Any work behind electric access panels, including testing or replacing a thermostat.
• Any gas tank thermostat problem, since the fix is a gas control valve.