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Had this problem for a while. Delta, single handle, shower. Sometimes showers are hot. Sometimes luke warm to cool. To fix the luke warm shower we have found that turning the hot side of the sink (same bathroom) on causes the shower to get steamy hot (good). It doesn't take a lot of sink flow, just a little does the trick. Turn the sink off and we go back to luke warm shower water.

To make the mystery even more interesting is that the hot water side of the sink comes out cold while the shower is on. Both faucets (sink and shower) are fed from the same hot water line. How is cold water being introduced to the hot side of the sink.

Changed out the shower cartridge, as well as the sink cartridges within the last six months. This did not fix the problem.

faucet

shower

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2 Answers 2

Try pulling the cartridge out and turn on the water valve back on. Look to see if you have good flow coming from both hot and cold water holes. Doing this could blow out any debris in the lines restricting flow. Cartridges act weird when water pressure is not balanced between hot and cold.

Try turning off just cold supply and then just hot supply so 100% of water pressure is directed to clear out any debris.

Some showers have built-in valves for hot and cold. Adjust them so same flow is coming out of both holes.

Did you try adjusting the hot water safety limiter? It basicly just stops the handle from turning all the way to hot.

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I have the exact same issue. I have a condo with a tankless hot water heater that will get all faucets hot except the shower, which it goes back and forth between hot and cold. I have found the only way to get a constant temp is to turn on the faucet on the same line.

I wonder if the hot valve of the nearby faucet when closed is the real issue.

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