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:
- Cut the power to it
- Used a multimeter on both the elements (there are two) to confirm conductivity across them, and there was
- Used the multimeter to also confirm conductivity going into and out of the thermostat (there was)
- Turned the power back on to it
- Pressed the reset button (red button in the photo below), and immediately heard it turn on and start working
- 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.
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?