AO Smith Water Heater Not Heating

Quick Answer

An AO Smith water heater not heating is usually caused by a tripped high-limit reset button or a burnt-out heating element. Start by checking your home breaker panel and then inspect the red ECO reset button behind the upper access panel.

Cold water is a nightmare and honestly most of the time it's a $20 fix. Ignore it long enough and you're looking at a tank that can overheat and rupture. What I usually find when I show up is a lower element that's been slowly cooking itself to death for months because nobody ever flushed the sediment out. Don't let it get to that point.

Ao-smithWaterheaterSeverity: highDifficulty: intermediate92% DIY Success

Time to Fix

30–90 min

Difficulty

intermediate

Parts Cost

—

Tools Needed

Phillips #2 screwdriver, Flat head screwdriver

What Does the NO-HEAT Code Mean?

Most of the time when an AO Smith goes cold it's either a tripped ECO button or a dead lower element caked in calcium. Both are cheap fixes if you catch them early. The thing techs know that homeowners don't: the lower element fails way more often than the upper one because it sits in the sediment layer at the bottom. That gunk insulates it and it basically cooks itself.

Most Likely Causes

Based on aggregated repair data, here is the probability breakdown for this error code:

Heating Element Failure45%

Tripped Safety Switch25%

Thermostat Malfunction15%

Gas Valve or Pilot Issue10%

Wiring Issues5%

Symptoms You May Notice

You turn the hot tap all the way on and after 3-4 minutes it's still the same temperature as the cold line, not even slightly warm.
The shower starts out hot for about 5-8 minutes then goes ice cold mid-rinse with no warning.
There's a loud popping or crackling sound coming from the tank area while it's supposedly in a heating cycle.
You notice a faint burning or sulfur smell near the base of the unit, especially right after it tries to fire up.
Hot water pressure seems noticeably lower than usual at the start of the flow before it evens out.

Can you reset a Ao-smith waterheater to clear the NO-HEAT code?

Turn off the breaker first. Pop the upper access panel and push the insulation aside. Find the red button on the high-limit switch and press it firmly until you feel a click. Put the insulation and panel back. Restore power at the breaker. Give the tank 45-60 minutes to heat up fully. If the ECO trips again within a day, you've got a grounded element or a stuck thermostat that actually needs replacing, not just resetting.

Tools Required for Diagnosis

Phillips #2 screwdriverFlat head screwdriver1-1/2 inch element wrench or matching deep socketMultimeter (AC voltage and ohms modes)Non-contact voltage testerGarden hose with female garden fittingWork gloves (fiberglass insulation protection)Bucket or floor drain access

Diagnostic Checklist

Follow these steps in order. We start with the easiest external fixes before opening up the machine.

ComponentComponent Under Test

Expected Range10–18 ohms

ConditionIf Open (OL) or infinite, replace component.

Frequently Asked Questions

Why does my AO Smith reset button keep tripping?

That ECO trips when the water hits around 170°F. Nine times out of ten it's because a heating element has a tiny crack and is shorting current directly to the water, which pulls more amperage than the thermostat expects. The other common cause is a thermostat stuck closed that just keeps calling for heat past the safe limit. Quick test: do the ground check on your elements. Touch one multimeter probe to the element terminal and one to the bare tank metal. Anything other than OL means you've got a grounded element and that's your culprit.

How long do AO Smith heating elements usually last?

In soft water areas, easily 10-12 years. But I work in a hard water area and I'm swapping lower elements every 3-5 years on average. The lower element sits in the sediment layer at the bottom of the tank and basically slow-cooks itself. If you've never flushed your tank once in 5 years, there could be 2-3 inches of calcium at the bottom. That's the lower element's worst enemy. Flushing once a year adds years to element life, seriously.

Can I replace just one element or should I do both?

Do both. I know it's tempting to just swap the bad one, but if you're already draining a 50-gallon tank and doing the work, the second element is $15-20 more. The labor to come back and do it again in 6 months costs way more than that. Upper and lower elements wear pretty evenly since they don't run at the same time, so if one failed the other is close. Replace both and you're done for another 5-8 years.

Is it normal for the water heater to make noise while heating?

Popping and crackling is scale. That sound is water trapped under a crust of calcium on the element flashing to steam instantly. It's not dangerous but it means your element is working way harder than it should and failure is coming. Humming is usually a loose element vibrating slightly in its threads. Both sounds are your tank telling you something. Not an emergency today, but plan on flushing it and probably a new lower element within the next year or so.

What tools do I need to change an AO Smith element?

A 1-1/2 inch element wrench or a deep socket that actually fits, a Phillips and flathead screwdriver, a garden hose long enough to reach a floor drain, a multimeter, and work gloves because that fiberglass insulation will itch for hours. Elements can be really stuck in there after years of mineral buildup, so an actual element wrench gives you way better leverage than a regular socket wrench. Budget $30-50 for parts and give yourself a solid 2 hours if it's your first time doing this.

How do I know if it's the upper or lower element that failed?

Here's the clue: if you get some hot water but it runs out way faster than normal, maybe 10-15 minutes instead of your usual 30-40, it's almost always the lower element. The upper element heats the top portion of the tank, so if that's the only working one you'll still get a short burst of hot water before it's gone. No hot water at all, like zero, usually points to the upper element or the upper thermostat, since that thermostat controls whether power even reaches the lower element in the first place.

Models Known to Experience NO-HEAT Errors

This repair applies to most Ao-smith waterheaters with this error code. Common model numbers include:

ECT-52-240V, ESST-52-240V, GPVL-50-200, HPTU-50N, ENR-52-200, ECT-80-240V, GCRL-50-200

Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 17, 2026