Hot water in sink luke warm water in shower

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Justinwayne

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Joined
Sep 14, 2021
Messages
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Location
Seattle
Hi everyone. I have a question dealing with my garage bathroom. I built this bathroom using 1/2-inch pex piping. The pipe comes into the bathroom, the first drop down is to the sink, the second drop down is to the toilet, and the third drop down is to the shower. When i constructed the bathroom, I connected the pipes from the old washing machine connections for the hot and cold water, since I did not want to cut the water lines in the house. I get really good water pressure in this bathroom. In order to get hot water in the shower I had to turn up the gas hot water heater all the way to very hot. The sink had nice hot water, but the shower needed the higher setting to have hot water. Recently I installed a Rheem 13 Kw tankless water heater just for this bathroom. Now the problem: I have the Rheem set to 140 degrees. The sink is 140 degrees when I run it. When I run the shower i just get lukewarm water with the sink not running. When I flush the toilet, I get hot water until the toilet finishes filling the tank then back to lukewarm. I even turned off the cold water and just ran the hot the same thing goes. I thought it might be the shower valve, so I changed that out. I built this bathroom about 7 months ago that is actually when i change out the valve. Maybe I should change out the valve again? I hope someone could help me with this. Any theory's would be welcomed.
 
Try adjusting the hot water limit stop on the shower faucet.

Post a pic of the shower faucet and if you know the name and model number that would be helpful.
 
Sounds like the valve is poorly adjusted as per twowaxhack above, or the tankless isn’t getting enough water flow (see above) to fully heat the water. Try removing the showerhead (is it low-flow?) and see if you get hot water out of the pipe at full flow
 
Sounds like the showerhead is flowing too much hot water for the water heater to supply.

Get a very low flow shower head.
 
Twowaxhack said:
Sounds like the showerhead is flowing too much hot water for the water heater to supply.

Get a very low flow shower head.
Click to expand...
Thanks for all the suggestions. The shower head I have is for low pressure is that the same thing as a low flow shower head? I did remove the valve and turned on only the hot water and it was still lukewarm. So, what I did is to slow the flow of cold water into the water heater now the shower has warm water. I also changed the valve since i had another one. I just wonder when I put the plumbing into the bathroom should I have started at the shower and then ran to the toilet and then the sink. Because the sink has really hot water.
 
Justinwayne said:
Thanks for all the suggestions. The shower head I have is for low pressure is that the same thing as a low flow shower head? I did remove the valve and turned on only the hot water and it was still lukewarm. So, what I did is to slow the flow of cold water into the water heater now the shower has warm water. I also changed the valve since i had another one. I just wonder when I put the plumbing into the bathroom should I have started at the shower and then ran to the toilet and then the sink. Because the sink has really hot water.
Click to expand...
I believe your shower head uses more water than your water heater can supply.
 
Twowaxhack said:
I believe your shower head uses more water than your water heater can supply.
Click to expand...
you are probably right i will look for a low flow shower head... I just came back from taking a shower now I do have hot water, but the pressure is less now since I reduced the cold water entering the heater.
 
Food for thought, I jad a similar situation on a job once. Customer installed new water heater to his first floor apartment. When he ran the shower it was lukewarm best. He turned the cold water on his kitchen sink on and the shower got hot. Basement was finished. Luckily I found a spot where I noticed someone had the hot and cold tied in together. You might have a cross connection somewhere. Maybe somehow you connected the wrong pipes somewhere by mistake. Or maybe your sink faucet is bad inside where your pulling from the hot to cold. Try shutting off all supply valves to each fixture then try the shower. I can't see it being 140 degrees on your tankless and only getting lukewarm. It's either your diverter is not mixing right/not set properly, a cross connection or you have bad flow through your pipe itself. Maybe a kinked or crushed pex line could cause not enough water flow.

You can always shut the stops off on the diverter. Take the cartridge out. Put a pipe over your opening. With the cartridge still out turn on the cold. Notice the pressure of flow. Then turn it off and check the hot. See if it matches the same pressure flow then notice if it gets up to the 140 degrees. Make sure no one is using any other fixture to avoid a false positive.

I know I spelled out a lot here and I hope it makes sense. But even if a lower flow rated shower head would only allow the hot water to last longer not increase the temperature. It's still mixing too much cold water somehow. And this why when you flush the toilet you get hot water.
 
Justinwayne said:
you are probably right i will look for a low flow shower head... I just came back from taking a shower now I do have hot water, but the pressure is less now since I reduced the cold water entering the heater.
Click to expand...
Correct, that’s exactly what would happen.

Rheem actually makes an outlet regulator/restrictor for the hot side, the same can be accomplished by choking down the cold inlet, but it’s not recommended.

The problem is difference in volume you’re using at the shower head vs at the lavatory.

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Last edited:
What’s your incoming cold water temp ?

What’s your water heater setting ?

What’s your mixed shower temp ?

How many gallons per minute does your shower run at the mixed temp ?
 
dmmsr said:
Food for thought, I jad a similar situation on a job once. Customer installed new water heater to his first floor apartment. When he ran the shower it was lukewarm best. He turned the cold water on his kitchen sink on and the shower got hot. Basement was finished. Luckily I found a spot where I noticed someone had the hot and cold tied in together. You might have a cross connection somewhere. Maybe somehow you connected the wrong pipes somewhere by mistake. Or maybe your sink faucet is bad inside where your pulling from the hot to cold. Try shutting off all supply valves to each fixture then try the shower. I can't see it being 140 degrees on your tankless and only getting lukewarm. It's either your diverter is not mixing right/not set properly, a cross connection or you have bad flow through your pipe itself. Maybe a kinked or crushed pex line could cause not enough water flow.

You can always shut the stops off on the diverter. Take the cartridge out. Put a pipe over your opening. With the cartridge still out turn on the cold. Notice the pressure of flow. Then turn it off and check the hot. See if it matches the same pressure flow then notice if it gets up to the 140 degrees. Make sure no one is using any other fixture to avoid a false positive.

I know I spelled out a lot here and I hope it makes sense. But even if a lower flow rated shower head would only allow the hot water to last longer not increase the temperature. It's still mixing too much cold water somehow. And this why when you flush the toilet you get hot water.
Click to expand...
All of my lines are good. When I had my bathroom connected on my gas tank hot water heater it was hot water. It only went to lukewarm water when I installed my tankless water heater. But it always went cold when someone else turned on a hot water faucet in another part of the house this is the reason, I wanted the bathroom on its own hot water heater.
 
Twowaxhack said:
What’s your incoming cold water temp ?

What’s your water heater setting ?

What’s your mixed shower temp ?

How many gallons per minute does your shower run at the mixed temp ?
Click to expand...
incoming temperature for this area is around 47 to 50..
water heater setting is 140 degrees
mixed shower temp is lukewarm not sure the exact temp.
gallons per minute is 1.5......

So do you think I got the wrong size water heater. I thought I figured it out. I wanted it for one shower.
 
Justinwayne said:
incoming temperature for this area is around 47 to 50..
water heater setting is 140 degrees
mixed shower temp is lukewarm not sure the exact temp.
gallons per minute is 1.5......

So do you think I got the wrong size water heater. I thought I figured it out. I wanted it for one shower.
Click to expand...
The water heater is operating correctly.

The max temp rise is 59 degrees ( at the water heater outlet) flowing 1.5gpm.

47 degrees incoming water temp
+
59 degree temp rise
=
106 degree shower at 1.5gpm.

So it’s leaving the water heater at around 106 degrees. By the time it hits your skin…….

You get the idea.

Water heater is too small at your incoming water temps to flow a normal shower volume.
 
Justinwayne said:
All of my lines are good. When I had my bathroom connected on my gas tank hot water heater it was hot water. It only went to lukewarm water when I installed my tankless water heater. But it always went cold when someone else turned on a hot water faucet in another part of the house this is the reason, I wanted the bathroom on its own hot water heater.
Click to expand...
Okay, you had in your first description that when someone flushed the toilet the shower would get hot until it finished stopped filling then would go lukewarm.
Now you're saying when you had the gas water heater hooked up it would go cold when someone would run water in separate part of the house. These shower diverted are made to be pressure/temperature balancing. This way you won't get a cold shower or scalding when someone would flush a toilet or run water. This sounds a lot like a cross connection. If it is not cross connection it would stay the same no matter what was used. It would stay tempered. Did you take any pics after you roughed it in?
 
dmmsr said:
Okay, you had in your first description that when someone flushed the toilet the shower would get hot until it finished stopped filling then would go lukewarm.
Now you're saying when you had the gas water heater hooked up it would go cold when someone would run water in separate part of the house. These shower diverted are made to be pressure/temperature balancing. This way you won't get a cold shower or scalding when someone would flush a toilet or run water. This sounds a lot like a cross connection. If it is not cross connection it would stay the same no matter what was used. It would stay tempered. Did you take any pics after you roughed it in?
Click to expand...
The water heater is maxed out. Look at the math above.

But if he wants to check for a cross connection just turn the cold water off to the water heater and go to the lavatory faucet and make sure once the initial hot water pressure is relieved that the flow stops on the hot water side.
 
Twowaxhack said:
The water heater is maxed out. Look at the math above.

But if he wants to check for a cross connection just turn the cold water off to the water heater and go to the lavatory faucet and make sure once the initial hot water pressure is relieved that the flow stops on the hot water side.
Click to expand...
Cross connection meaning that he could've somehow piped the cold to the hot directly. I had went to a job where the landlord replace the water heater himself and right after had the same issue. I found a pipe that did not look right and found it was the cold water line piped to the hot. Only the shower was affected. They had to run the cold water in the kitchen in order to take a hot shower. I wouldn't think it could be possible but have seen it myself. I thought he piped the water heater wrong. Checked the dip tube. Shut off all fixtures. It took me several hours to find it.
 
dmmsr said:
Cross connection meaning that he could've somehow piped the cold to the hot directly. I had went to a job where the landlord replace the water heater himself and right after had the same issue. I found a pipe that did not look right and found it was the cold water line piped to the hot. Only the shower was affected. They had to run the cold water in the kitchen in order to take a hot shower. I wouldn't think it could be possible but have seen it myself. I thought he piped the water heater wrong. Checked the dip tube. Shut off all fixtures. It took me several hours to find it.
Click to expand...
I understand what a cross connection is.

The math shows his water heater is operating and the shower temp being Luke warm is normal.

But sure, check for a cross connection, it only takes 30 seconds to check for one.
 
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