I have a 40-gal A.O. Smith Promax electric water heater (see pics below). It was a few years old when we bought the house, and we've been living here about 10 years. So I realize the heater may be reaching the end of its life, but I'm trying to see if I can replace a specific part and squeeze a few more years out of it.

It's always ran perfectly fine. A few days ago, we went to do dishes and noticed there was no hot water, only lukewarm water (again, not hot, but not freezing cold either). I went down to the basement and can confirm that there was no water leaking out from underneath it. I started to monkey around with it, and did the following:

  1. Cut the power to it
  2. Used a multimeter on both the elements (there are two) to confirm conductivity across them, and there was
  3. Used the multimeter to also confirm conductivity going into and out of the thermostat (there was)
  4. Turned the power back on to it
  5. Pressed the reset button (red button in the photo below), and immediately heard it turn on and start working
  6. Waited a few hours and confirmed we had hot water (although it bounces back and forth between frigid and scalding hot...)

Hoping we were in the clear, I went back to business as usual.

up close whole unit

Last night, same thing: no hot water, only lukewarm. This time I just hit the red reset button and -- voila -- 2 hrs later, hot water is restored. I went to take a shower and again, the water bounced back and forth between freezing and scalding hot, took a lot of fine tuning with the faucet to get the water just right.

I'm wondering what the possible problems could be? Is it just a bad thermostat perhaps, or a similar/related part? I'd rather try and swap that out with a new one for ~$100 than replace the entire tank!

Is there any way to test the thermostat or related parts to see if they are in fact functioning improperly? What diagnostic steps can be recommended here to rule out before "going nuclear" and replacing the whole unit?

2 Answers 2


Water heaters have replaceable parts in them:

  • The anti-corrosion anode rods (what a boater calls a "zinc")
  • The heating elements proper
  • The dip tube which is an internal baffle which brings the cold water down to the bottom of the tank

The dip tube needs further explaining. In stagnant water, hot water will rise to the top. However if it were actively stirred, the temperature of the water quickly becomes the average of all the water in the tank. As you use hot water, cold water comes in. If stirring were happening, the heater's water would continuously decline in temperature as you used it. That would be no good!

So the hot water tank is optimized to work more like a toothpaste tube - the entering cold water pushes the hot water upward -- so it doesn't stir or mix, but the hot water moves up like a piston until it is used up.

The dip tube's job is to carry cold water to the bottom of the tank, so it can do the piston thing. Otherwise, cold water would just "take the shortcut" from the cold inlet to the hot outlet just 9" away.

Your dip tube is broken, hence the flashes of hot and cold. Cold water is shortcutting. Fair chance it broke because it corroded off. As such, it's worth looking at the anode, whose one job is to corrode first. If it's used up, corrosion spreads to other components; good heater design calls for it to be easily replaced components.

The heating elements can also be swapped, and that is likely to take care of the tripping. You know the tripping is a safety device. It's like a circuit breaker; when a breaker trips, the breaker isn't bad, the circuit is being overloaded.

  • OK thanks @Harper (+1) - let me regurgitate this back to you to confirm I'm actually comprehending what you're saying, and if I am, I have a few followup questions for you, if you don't mind! It sounds like you're pretty sure that the dip tube is broken/corroded and needs to be replaced. It also sounds like you're expecting the anode to be corroded since that should fail first. It also sounds like you're suggesting I take the elements out and have a look at them, they might need to be replaced too. So question (1): is everything that I just summarized inline with you're thinking? Commented Dec 3, 2019 at 18:15
  • If it's not, then please provide a wee bit of clarification! And if I'm understanding you, then my followup question is: (2) are the elements the only thing that could be causing the tripping? Why would they be causing the tripping? Thanks again! Commented Dec 3, 2019 at 18:16
  • @hotmeatballsoup yeah, you got it. And if you're starting to glance in the direction of a new unit, that's reasonable. Why would elements cause tripping? Because the thermostat is obviously a safety protective circuit (because it has a reset button), and why would it only protect itself? The thermostat controls the elements, and that is what's most likely to have a serious problem, so putting a safety trip on a thermostat just makes sense. Commented Dec 3, 2019 at 18:20
  • OK thanks again. So it looks like I can replace both elements, the thermostat, the anode and the dip tube for a few hundred bucks. Knee-jerk, non-binding answer: would you recommend trying these first or just say screw it and replace the whole thing. I'm leaning towards replacing it because its already toward the end of its life and who knows what else is wrong with it besides these 3 components... Commented Dec 3, 2019 at 18:27
  • @hotmeatballsoup I would keep that option in my back pocket, and do some diagnostics to see if I could narrow it down. If you're hiring all this work done I would say that troubleshooting won't be worth the labor and I'd just go for replacement. Commented Dec 3, 2019 at 18:42

Buy a tune up kit. Both heating elements both thermostate's under $50. Replaced two heaters with the kits this last month. They work like new. When draining the heater clean out the calcium build up in the bottom of the heater also while your at it.
To clean out calcium it's some time easier to unscrew the valve so the calcium comes out easier.

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