I'm not a certified repair person. But, before you call one, consider this. Inside water heaters are 2 metal rods. They are designed to "corrode" to protect your water heater. Some take several years to carrode, some only a few years. They are usually accessed from the top of the water heater. They are inexpensive to replace. If you have a "operations manual" that came with the heater. Instructions on how to replace them may be in it. But ALWAYS make sure your heater is UNPLUGGED! On my water heater, these carroding rods are accessible by removing the 2 plastic plugs on TOP, near water supply lines. If your handy you may try to at least INSPECT those rods. If your not sure of how they should look, take them into a Home Depot, Lowes or other plumber supply. If your not handy, may be time to call for servicing by a pro. By they way, in my manual, those rods are to be inspected EVERY YEAR..
I would replace the Tstats on the water heater ahd the heating elements. Depending on your gallonage the water heater has 2 heat strips, cut the power off at the breaker cut the water supply off drain the tank, once drained the elements take a specific wrench or Socket, take them both out or using the ohm meter ohm them out if bad then take them out. remove the tstat the lower is connected to the top so take off the bottom take it to HD or Lowes get 2 just like the one you have now. Install the new one on bottom first then the top be sure you either mark the wiring sequence or number them and put them back on each screw exactly the way you removed them install the tstat refill with water opening the furtherest faucet from the water heater, this helps remove the air that is in the lines, once the water is coming out of the faucet flip the breaker back on. Water should be hot in a couple of hours and life should be good
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