AO Smith Water Heater Not Heating [11 Easy Solutions]

The main reasons behind your AO Smith water heater not heating water properly are:

  • Interrupted power supply
  • Tripped ECO
  • Defective thermostat
  • Burned-out heating element
  • Plumbing issues
AO smith water heater not heating

Those are the most common issues in AO Smith electric water heaters. But don’t worry, you’ll learn all about fixing the issues in gas heaters as well. So, let’s dive in!

Table of Contents

Quick Solutions For AO Smith Water Heater Isn’t Getting Hot

Here are the instant solutions to the most common problems that you may encounter frequently with your AO Smith water heater. Here are the ones for electric units:

Possible Causes/ReasonsHow to Fix? 
Power supply issues> Make sure the circuit breaker is on.
> Replace the wiring or the circuit breakers.
Tripped ECO> Reset the heater.
> Don’t set the temperature too high to prevent the ECO from tripping.
Defective Thermostat> Replace the thermostat.
Faulty heating element> Replace the heating element.
Plumbing issues> Make sure the inlet and outlet connections are in the right order.
> Seal all the plumbing leaks.

AO Smith Water Heater Not Heating [Electric Heaters]

Here are the main reasons why your AO Smith electric water heater won’t heat water properly along with their fixes:

Note: You can also read AO Smith hybrid water heater troubleshooting guide.

1. Power Supply Issues

The number one culprit behind your AO Smith heater not heating the water is not having electrical power. It’s a common issue in new installations that were poorly done. Here are the main reasons why you don’t get power from your water heater:

  • Faulty Installation
  • Bad Breaker
  • Damaged Wiring

You can’t know if there are issues with the power supply without testing it properly. Luckily, you can easily test it using a noncontact circuit tester. Follow these steps to do it:

  • Locate the circuit breaker of your heater and turn it to the off position.
  • Then, open the electric junction cover box that lies at the upper of your heater.
  • Next, get the two power wires [Black-black and black-red wires]. You will not notice that there is also green or copper wiring. Leave that wiring alone as it’s the ground wire.
  • Connect the circuit tester to the power wires and turn on the circuit breakers.

If both the wires had power when you turned the circuit breakers on, there are no power issues with your water. But if even one of them didn’t get power, something is wrong with either the wiring or the circuit breakers in your house.


If you have confirmed that there are power supply issues, the first thing you should do is turn off the circuit breakers. Don’t mess around with your heater. Contact an electrician to fix the issue so that you can get hot water from the heater ASAP.

2. Tripped ECO

The next most common reason behind your heater not turning on is a tripped ECO. The ECO is a safety device that shuts off the operation of the water heater when the water temperature reaches an exceedingly high level.

The purpose of this Electric Cut Off device is to protect you from scalding. When the button gets tripped, your heater will no longer have power and you won’t get any hot water from it.


When the ECO gets tripped, you have to reset it. Luckily, it’s an easy process and you can do it on your own. Follow these steps to do it:

  • Turn off the power to the heater by flipping the breakers or removing the fuses.
  • Remove the upper access panel using a screwdriver and fold the insulation in front of the thermostat.
  • Locate the red reset button and press it.
  • You should hear a clicking sound when you press the reset button. If you heard the sound, it means the ECO was tripped.
  • Put everything back together and turn the heater on.

That’s how you can reset the ECO. The bigger question is what caused the ECO to trip in the first place. Did you set the temperature too high?

Make sure you set the temperature between 120°F to 140°F to prevent the ECO from getting tripped. Otherwise, it could trip again.

If you didn’t hear any clicking sound, that means the ECO wasn’t tripped. In that case, there is another issue behind the no hot water issue. Keep on diagnosing to find out what it could be.

3. Defective Thermostat

When your AO Smith water heater isn’t heating enough, a bad thermostat is the next culprit behind the scene. Mechanically, an AO Smith water heater’s thermostat is a vital component that senses and regulates the water temperature accordingly. 

AO smith water heater thermostat issues

There are two thermostats in AO Smith electric water heaters. The one located at the bottom of the heater is called the lower thermostat and the one located at the top of the heater is called the upper thermostat.

If any of them turn out to be faulty, it can mess up the operation of your heater. A bad thermostat can read that the desired temperature has already been reached even when the temperature is lower than that in reality.

It won’t signal the heating element to keep heating the water and that’s why you get no hot water from your heater.


Before going to replace the thermostat, first, you should inspect the temperature setting. If it is too low, simply increase the thermostat temperature to a higher level and see if that solves the issue.

inspect the Ao smith water heater temperature setting

Your water isn’t hot enough even after adjusting the temp setting. Then, it is time to test whether your thermostat is burned out or shorted electronically.

test the ao smith water heater thermostat

You can’t determine whether the upper or lower part is at risk until you test them. Follow these steps to do it:

  • Turn off the power in the circuit breaker
  • Take a Phillips Head screwdriver
  • Remove the upper access panel and pull out the insulation as well as the plastic protective cover.
  • Disconnect the power from the thermostat terminals.

Now, to verify the upper part, keep one probe to the left and another probe to the right side terminal of the red reset button. In this case, you will get zero continuity for a good thermostat. But if it fails to display close to zero continuity, you should replace the thermostat.

Similarly, test the lower part of your thermostat for continuity. If it results in zero, the lower part is good. Otherwise, you will have to change your thermostat immediately. 

Follow these steps to replace the thermostat in your AO Smith electric water heater:

  • Shut off the power supply to the water heater and remove the access panel in front of the thermostat you want to replace.
  • Fold the insulation and remove the plastic thermostat cover to access the thermostat. Mark the wires with tape or take a picture of the wiring with your phone so that you can put it back together correctly later on.
  • Disconnect the wires from the faulty thermostat and remove the thermostat from the mounting clip.
  • Insert the new thermostat in place of the old one and make sure it fits tightly against the tank. Attach the wires correctly using the picture you took earlier as a guide.
  • Put everything back together and you’re good to go.

That’s how you can easily replace the thermostat. When you turn on the heater, give it at least a couple of hours to resume normal operation and start heating the water.

4. Burned-Out Heating Element

When the water heater thermostat is fine, most probably, a faulty heating element is a major listed reason for your AO Smith water heater not getting hot.

AO smith water heater defective heating element

As usual, the heater has two heating elements: the upper and lower heating elements. They contain an electric current which converts into heat and warms up the water.

And then the water temperature increases. But a defective heating element will stop providing heat even when the thermostat signals it to do so.


First, you should confirm whether you have a bad heating element or not before you think of replacing it. You can test the heating element with a multimeter. Follow these steps to do it:

First, Switch off the power of your heater from the main circuit breaker. Pull out the access panel and detach the insulation to get access to the elements.

Secondly, set your digital multimeter to the lowest Ohms setting and loosen the heating elements screws and detach the wire from it.

Thirdly, touch one of the multimeter probes to the loosened screw and keep the other probe to the screw face to the heating element.

If your multimeter shows a reading between 5-25 ohms, the heating element is fine. But if the reading is outside the range, it means you have a bad heating element.

If the test reveals that you have a bad heating element, you need to replace it. Follow these steps to do it:

replace the AO smith water heater heating elements

Step-1: Turn off the power supply to the heater and the cold water supply. Open a hot water faucet until you get cold water coming from it. Open the drain valve and drain the water by connecting a hose. 

Step-2: Remove the access cover in front of the heating element you wish to replace. Fold the insulation and remove the thermostat cover. Disconnect the wires from the heating element and pull it out using an element wrench.

Step-3: Verify the new heating element’s voltage and wattage with the heater’s data plate. Clean the tank threads and install the new heating element. Tighten it using the element wrench. Connect the power wires to the heating element.

Step-4: Ensure the drain valves are closed. Then, let the tank refill by opening the cold water supply. If there are no leaks, you did a good installation job. You can put everything back together and complete the installation.

If there are leaks, you need to install the heating element again. You can do it yourself or shut off the cold water supply and hire a qualified technician to do it.

Make sure you turn the power to the heater on only after the tank is completely filled with water. Otherwise, the heating element can get burned out again.

5. Plumbing issues

This problem occurs while installing a new water heater. If the inlet and outlet connections get reversed mistakenly, you will get no hot water.

Plus, your water heater won’t supply any hot water due to leakage of water pipes. Almost all leaks occur with the connections frequently.


To fix it, make sure all the connections are fine for working between the inlet and outlet. You’d have to reverse the connections again if there was a plumbing crossover during the installation.

Then, check out the pipe areas of the hot water side for leakage. Tighten fitting and repair if there are any leaks.

Note: Always hire a professional plumber to install your water heater to avoid such risky issues.

AO Smith Water Heater Not Heating [Gas Heaters]

We’ve already discussed the most common issues that occur in electric water heaters. But if you own a gas heater, many of those issues will not apply to you.

AO smith water heater

Here are the most common reasons why your AO Smith gas water heater is not producing hot water:

  • Gas supply issues.
  • Pilot light issues.
  • Defective igniter.
  • Faulty thermopile.
  • Faulty gas control thermostat.

Check out our detailed guide on AO Smith water heater won’t light to find out all about those issues in detail and learn about how you can fix them.

Apart from them, there are other issues that may give hot water from your gas heater but the water won’t be hot enough. Here are the main reasons behind it:

  • Setting the thermostat too low.
  • Bare piping.
  • Faulty thermostatic mixing valve.
  • Using an undersized heater.
  • Faulty dip tube.
  • Clogged main burner.

Let’s look at these issues in detail and learn how you can fix them:

1. Setting The Thermostat Too Low

It’s kind of obvious that you’ll get little hot water from your heater if someone in your house has the thermostat too low. It may seem like a silly thing to check if you didn’t touch your heater but someone else could have done that.


If the thermostat is too low, rotate the thermostat knob to get hot water again. Make sure you don’t set the temperature too high. Otherwise, the thermal switch can get tripped.

2. Bare Piping

The water travels through the hot water pipes to the faucets. During that traveling, there is some heat loss of the water. But if the hot water pipes you have are exposed, the heat loss will be much more and the water coming out of the faucet won’t be hot enough.


Although bare piping is an issue, you’d be happy to know that there’s a simple fix. Just grab some insulating material and cover the hot water pipes with them. The heat loss will be reduced due to that and you’ll get hot water again.

3. Faulty Thermostatic Mixing Valve

Many people use thermostatic mixing valves. It’s a great safety device that mixes hot water with cold water to prevent scalding. But when the thermostatic mixing valves go bad, you wouldn’t get hot water at your desired temperature.


First, you have to make sure that you’re dealing with a bad thermostatic mixing valve. Turn on other hot water faucets in your house and check the water temperature.

If the temperature of the hot water at the other faucets is fine, that means the thermostatic mixing valve in your faucet has gone bad. All you have to do is to replace it and you’ll get sufficient hot water from your hot water faucet again.

4. Using An Undersized Heater

Overload is also a major problem if the hot water temperature doesn’t reach your desired level. Mostly, this may happen when you push your heater beyond its supply limitation.

You may encounter trouble due to using an undersized water heater. Typically, your water heater has a limit or capacity to heat up the water.

But when you’re using many hot water faucets at a time, your heater can’t cope with the high demand and you won’t get hot water from the heater. Most homeowners do it and face the trouble.


First of all, consider the capacity of your heater and check if it is able to heat the required hot water. If you have a high demand for hot water, ensure the right size of the tank as you need that covers your hot water needs.

5. Faulty Dip Tube

A broken dip tube is another culprit behind the AO Smith water heater not being hot enough.  Mechanically, a dip tube is an internal pipe of a water heater.

ao smith water heater faulty dip tube

The main job of a dip tube is to push  ‘Cold Water’ to the bottom of the hot water heater tank. And then the heated water circulates and rises to the top. One side of the pipe is connected to the inlet from the top area, and the other is submerged in water. 

When the dip tube gets cracked or broken, it fails to circulate the water in the tank sufficiently, and then, you’ll get cold water instead of getting hot water.


After identifying a cracked or broken dip tube, the only solution to the problem is to replace it with an intake one. Because this part isn’t serviceable at all.

6. Clogged Main Burner

If your AO Smith water heater is operated by gas, a clogged main burner could be the main reason why you aren’t getting enough hot water from it.

AO smith water heater defective burner

Typically, a gas-fired water heater burner plays a central role in the heating process. In this case, if the gas valve doesn’t open, the pilot light fails to ignite the burner. As a result, you won’t get hot water.

But if the burner fails to light up, it can’t transfer heat to the water. And then, you’re facing a hot water issue.

The burner itself may also be dirty even if everything else is fine. Carbon deposits build up on the burner over time. If there’s too much carbon build-up, it reduces the ability of the burner to heat the water and that’s why you get little cold water.


First, ensure the ignition source is operating properly. If everything else is fine, you need to check your burner. If the burner is dirty, you can try taking it out and cleaning it first.

Hopefully, that’ll do the trick. But if that doesn’t work, it’s also possible that the burner has gone bad. In that case, you’d have to replace it. Follow these steps to do it:

  • Move the gas control knob to the “Off” position. Unplug the power cord and shut off the gas valve. Remove the burner access panel and unscrew the nuts that are secured on both sides of the burner.
  • Disconnect the electronics from the gas control valve including the piezo switch, the gas lines, and the thermocouple.
  • Now, you can slide the burner assembly out of the water heater. Clean the area where the burner was sitting as it’s most probably dirty by now.
  • Put the new burner assembly in place of the old one and reconnect the lines to the gas control thermostat.
  • Tighten the screws of the burner assembly and put the cover back in. Test for gas leaks by spraying some soapy water and turning on the gas valve. If there are no leaks, your installation is complete. Otherwise, you’d have to do it again.


What temperature should I set for my AO Smith water heater?

You should set 120°F. as it is an ideal temperature setting with your AO Smith water heater. However, if it is needed, you can increase it up to 140° F. just by turning the dial indicator.

What is the difference between an electric and a gas water heater?

Mostly, the difference varies with the source of heating of the heaters. To heat up, an electric heater uses heating elements while a gas-fired heater uses a burner assembly.

Why is my AO Smith water heater tripping the reset button?

AO Smith water heater reset button has been designed to trip as a  safety precaution. It trips when the temperature goes out of control due to a bad heating element, a faulty thermostat, or electric problems.


Hopefully, we have covered all the common reasons behind your AO Smith water heater not heating along with their fixes. So, follow the guideline and deal with the issues whenever you face any trouble getting hot water from your AO Smith water heater.

We strongly suggest periodic maintenance to avoid such frustrating issues further. And, don’t forget to share your valuable thoughts with us in the comments.

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

Leave a Comment