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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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    I’m sure you’ve fixed your problem by now (or moved) but for others that have the same problem, this sounds like a plumbing cross over caused by a faulty mixing valve. This problem allows cold water to get into the hot water system and would explain why cold water came out of the spigot when the hot water valve was open in the sink. This Rheem technical service bulletin describes a test to check for this condition: cdn.globalimageserver.com/…
    – Paul
    Commented Dec 4, 2019 at 4:04

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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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Do you have a recirculation line to keep the hot water in the hot lines? I had this same problem with an on demand hot water heater and the backflow valve before the HWH recirc pump was broken. After I replaced it the problem disappeared. See my post here:

Shower gradually cools, gets hotter when sink is on. Navien tankless whole house HWH

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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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  • Low flow shower heads sometimes don' t work well with tankless water heaters. Our Bosch tankless worked fine with a 2.5 gpm shower head, but became intermittent with a 1.4 gpm head. I removed the flow restrictor on the 1.4 gpm head and the water heater worked again.
    – Jim Stewart
    Commented Apr 16, 2017 at 15:02
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I had the same issue. I swapped the old shower head for a water saving one and it fixed the problem for me.

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