A.O. Smith Water Heater No Hot Water - Causes & Fix

A.O. Smith Water Heater No Hot Water — What’s Happening

On A.O. Smith electric storage water heaters, no hot water means the unit is not converting electrical power into heat. The most common reasons are loss of power to the heater, a tripped ECO safety reset, a burned-out upper heating element, or a failed thermostat.

On A.O. Smith gas tankless models, a no-hot-water complaint often corresponds to a burner fault such as Error 111, which indicates flame failure during ignition. The burner did not light or lost flame after starting.

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Most Likely Causes

• Tripped ECO reset button (electric) The upper limit safety device opened because the tank overheated, cutting all power to the elements until manually reset.
• Burned-out upper heating element (electric) A.O. Smith identifies a failed upper element as one of the most common reasons for total loss of hot water on electric tank models.
• No electrical power to the heater A tripped breaker, miswired circuit, or lack of voltage at the unit prevents the elements from heating.
• Faulty thermostat (electric) A failed upper or lower thermostat will not signal the elements to heat, even when power is present.
• Gas supply or ignition failure (gas/tankless) Closed gas valve, lack of fuel, ignition component failure, or flame-proving problems prevent the burner from lighting.
• Sediment buildup (tank models) Sediment at the bottom of the tank insulates water from the heat source and reduces heating efficiency.
• Undersized system or plumbing-side leak A defective thermostatic mixing valve or hot-side leak can make the heater appear to fail even when it is operating correctly.

How to Diagnose and Fix

1. Identify your model type (electric tank, gas tank, or tankless gas) because the diagnostic path differs for each.
2. On electric models, confirm the breaker is on and measure actual incoming voltage at the heater; do not rely only on breaker position.
3. Turn off power, remove the upper access panel, and press the red ECO reset button; if it clicks, the safety tripped due to overheating and you have reset it.
4. If the ECO did not trip or the heater still has no hot water after reset, turn power off, isolate the upper heating element leads, and measure resistance with a meter; an open or infinite reading means the element is failed and must be replaced.
5. Check both thermostats for continuity and correct operation if power and elements test good.
6. On gas models, verify the gas valve is open, fuel supply is present, and the pilot or ignition system is working; if the unit is tankless and shows a flame-failure error, have a technician inspect ignition components, gas pressure, and combustion air supply.
7. Flush the tank or descale the heat exchanger to remove sediment or scale buildup, especially in hard-water areas.
8. Inspect the hot-water plumbing for leaks or a faulty mixing valve if the heater itself tests good but hot water is still unavailable at fixtures.

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 electrician if you are not comfortable working with 240-volt circuits, if the ECO keeps tripping after replacement of the thermostat and element, or if incoming voltage is absent and you need the electrical panel inspected. For gas or tankless models showing ignition or flame-failure faults, have a qualified gas technician inspect the burner, gas valve, ignition components, and venting before attempting repairs. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.