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Our hot water runs out during one not-too-long shower. By the end of the shower, I have the cold water completely off and the hot water coming out is only lukewarm. My husband already replaced both heating elements and both thermostats. What else could be causing this?

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Has this always been a problem, or has it just started? When your husband was replacing parts, did he check to make sure there isn't a bunch of sediment at the bottom of the tank? Is the tank set to the proper temperature? – Tester101 Feb 5 '14 at 19:54
At some point it's more economical to replace the tank, rather than continually replacing parts. – Tester101 Feb 6 '14 at 10:54

The most likely reason is a degraded/broken dip tube in the hot water heater. If this were the case, there would likely be little bits of plastic in your water, which perhaps could be seen if you removed a faucet aerator. To replace the dip tube, the cold water inlet must be removed, and the new tube installed. One related cause is if the inlet and outlet of the water heater are swapped. If swapped, the cold water in the bottom of the tank is supplied to fixtures.

Another reason is an unintentional mixing of the hot water and cold water. It's possible that there is a broken faucet somewhere that allows the water to mix, providing lukewarm water.

I suggest also verifying that the hot water is reaching the proper temperature. If you drain a little bit of water from the tank, is it the proper temperature? If not perhaps the wiring is wrong, the thermostat is broken, or the thermostat is set to the wrong temperature.

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The tank being plumbed backwards (cold/hot reversed) will also cause this. – gregmac Feb 5 '14 at 20:00
+1 for dip tube... Another idea: Is it possible your incoming water temperature is colder than normal? If so a tank that might have been large enough for a shower before is no longer large enough. How big a capacity is the tank? Do you have the model number so the gallons per hour and recovery rate can be checked? – mfarver Feb 5 '14 at 20:52
Thank you SO much for the replies, this is so helpful! We are going to replace the dip tube and see what happens. This started happening a few months ago, having not been a problem previously, so we are pretty sure the capacity of the tank is okay. Thanks again! – Sarah Feb 6 '14 at 7:00
While you're at it, you should check the condition of the anode rod and the emergency relief valve, and replace them if needed. – Pigrew Feb 6 '14 at 15:45

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