A.O. Smith Water Heater Water Too Hot - Causes & Fix

A.O. Smith Water Heater Water Too Hot — What’s Happening

When an A.O. Smith water heater delivers water that is too hot, it means the temperature-control system is allowing the tank to heat beyond the desired setpoint or beyond safe levels. On electric storage models, this typically points to one or both thermostats being set too high, miscalibrated, or stuck closed so the heating elements continue to run past the target temperature. On gas storage models, the issue is usually an incorrect gas control dial setting or a failing gas control valve and thermostat assembly that does not regulate correctly.

If the water at your fixtures is dangerously hot but the heater’s own tank temperature is normal, the problem may actually be a missing or failed thermostatic mixing valve in your plumbing rather than the heater itself. A.O. Smith recommends verifying the actual delivered temperature at a faucet with a thermometer rather than relying only on the dial position, since setting and outlet temperature can differ.

Jump to Fix

Most Likely Causes

• Thermostat set too high The dial is above the recommended 120°F safe setting, causing hotter-than-desired water at fixtures.
• Electric thermostat mismatch A.O. Smith electric heaters have two thermostats and both must be set identically or the tank will overheat.
• Failed thermostat (electric) A stuck or miscalibrated thermostat does not open the heating circuit at setpoint, so the elements run continuously.
• Failed gas control valve/thermostat assembly (gas) The gas control does not regulate to the selected temperature and allows the burner to overheat the tank.
• No or failed mixing/tempering valve When tank temperature is correct but fixture water is scalding, a missing or broken mixing valve at the point of use is the cause.
• Improper temperature measurement Relying on the dial position alone rather than measuring actual outlet water temperature with a thermometer can misidentify the problem.

How to Diagnose and Fix

1. Measure the actual water temperature at a faucet with a thermometer to verify the complaint and establish a baseline.
2. Identify whether you have an electric or gas A.O. Smith heater, then shut off power at the breaker for electric units or turn off the gas supply for gas models before any service work.
3. For electric heaters, remove both access covers and insulation and check that both thermostats are set to the same temperature (A.O. Smith recommends 120°F as the safe default).
4. For gas heaters, inspect the gas control dial and verify it is set to the desired temperature and that the pilot or status light is functioning normally for your model.
5. If the setting is too high, lower both thermostats (electric) or the gas control dial (gas) to 120°F, wait for the tank to stabilize (1–2 hours), then recheck the faucet temperature.
6. If water remains too hot at the lowest reasonable setting, replace the thermostat(s) and high-limit switch on electric models or replace the gas control valve/thermostat assembly on gas models.
7. If the tank temperature is acceptable but fixture water is dangerously hot, inspect and replace any thermostatic mixing valve in the distribution system or at the point of use.
8. Reinstall all access covers and insulation (electric) or restore any gas-control covers (gas), then restore power or relight the pilot per A.O. Smith instructions and verify safe outlet temperature.

Parts You Might Need

Related Error Codes

If your appliance also shows a code on the display, these match this problem:

• A O Smith Water Heater E2 error code
• A O Smith Water Heater E4 error code
• A O Smith Water Heater E6 error code

When to Call a Pro

Call a licensed plumber or water-heater technician if you are uncomfortable working with electrical circuits at the breaker or inside the heater, or if you have a gas model and are unsure about shutting off gas supply, testing controls, or relighting the pilot safely. Gas control valve replacement and any work involving gas lines or burner components should be handled by a qualified professional to prevent gas leaks and carbon-monoxide hazards. If the heater continues to overheat after you have verified correct thermostat settings, or if you see signs of tank corrosion, leaking, or pressure-relief valve discharge, professional diagnosis and possible tank replacement are recommended. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.