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1960s house with 2 showers. Just replaced the 55gal electric hot water tank elements, and there is hot water in all of the faucets, showers.

However one of the showers only runs lukewarm. Obviously there is an issue with that shower’s mixing valve. But, this lukewarm shower takes/uses all of the hot water. So, if someone has a lukewarm shower, the hot shower has no more hot water.

It is a small hot water tank, so back when both showers were not it was not unusual for this to happen. But now that barely any hot water is coming out of the lukewarm shower, it’s a mystery where all that hot water has gone.

Checked for leaks and can’t see any in the hot water supply. Checked the hot water tank and it’s working. So where is all the hot water going if the shower is lukewarm? Next step, after looking at the mixing valve is to put a low-flow head on that shower and see if it’s just the sheer volume of cold water overwhelming the hot water and making it feel lukewarm while simultaneously putting all the hot water down the drain.

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    Unless the "lukewarm" shower has tremendous flow, your assumption is faulty. Something else is restricting flow to the cold shower.
    – isherwood
    Commented Nov 11, 2020 at 20:21
  • What kind of mixing valve is in the shower that gives only lukewarm water: old style dual valve (separate valves for hot and cold) or new single valve? Do you have a recirculation system that sends unheated water from the hot supply back to the inlet of the water heater?
    – Jim Stewart
    Commented Nov 12, 2020 at 17:14

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There is a good chance that the lines are oversized for that bathroom. I dealt with this in a shower in France that I needed to redo. It was basically close to our equivalent of 3/4" for the cold/hot pipes and I reduced them to a little under 1/2" and it resolved the issue.
If someone turned on both sides high it was out of hot water in 12-15 minutes before we reduced the pipe size, hot water gone from a good size tank. After it lasted about 35 minutes - which is good for 3 people to shower separately.

The other issue I have ran across is the rain shower heads. These things seem like water is coming out normal but they are usually so big there is upwards of 3 times the amount of water used. It boggles my mind how much these things use.

As for your issue, if you remove the shower head and take a picture of the insides it is highly likely you can block off a lot of the flow with a simple rubber gasket/ring. Without getting into the wall this is your first and easiest option. A valve from the 60s or even more than 20 years old... I honestly wouldn't touch it as my number 1 piece of advice with intricate hardware - especially old - if it works leave it alone.

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