Hot water from shower but not tub

[mweber2525]

Hello all - I have an issue with a single handle tub/shower combo on a tankless water heater. I get hot water from the shower head, but only lukewarm water from the tub even when handle is cranked all the way over to hot. The unit is a Grohe single handle setup that was installed when the addition was built roughly 1 year ago. Some other notes:

• When shower is running hot water, I can switch the diverter to the tub and will get 2-3 seconds of actual hot water before switching to lukewarm.
  
  
• Measured GPM on tub faucet at ~2.0.
  
  
• I can confirm the anti-scald limiter on Grohe handle unit is maxed out.
  
  
• I do not have access to the pipes feeding the tub/shower.
  
  
• This has been an issue since install. We don't use the bathtub often so didn't identify it as an issue until several months after install.
  

Any thoughts? I suspected the mixing valve, but think that can be ruled out since the same mixing valve feeds the shower (which works as intended). Also suspected water pressure, but not sure how that impacts a tankless heaters, and also not sure why shower would work and tub wouldn't given that tub is only flowing at ~2.0 GPM.

Tankless heater is a Rheem RTEX-13. Not sure of Grohe model number, but was purchased new at Home Depot at time of install one year ago.

 

[DIYorBust]

The RTEX-13 literature says it can provide a temperature rise of 44 degrees for 2.0 gpm. So if you expected 110 degree water at the tap, you'd need the incoming water to be at 66 degrees. However my guess is that this spec is a bit of stretch and on top of that there is some additional cooling between the water heater and the tub spout. Insulating the hot piping might help.

 

[Sponsor]

 

[Toolaholic]

The RTEX-13 literature says it can provide a temperature rise of 44 degrees for 2.0 gpm. So if you expected 110 degree water at the tap, you'd need the incoming water to be at 66 degrees. However my guess is that this spec is a bit of stretch and on top of that there is some additional cooling between the water heater and the tub spout. Insulating the hot piping might help.

A good thing to do in any case ,is to put on an electric

Hello all - I have an issue with a single handle tub/shower combo on a tankless water heater. I get hot water from the shower head, but only lukewarm water from the tub even when handle is cranked all the way over to hot. The unit is a Grohe single handle setup that was installed when the addition was built roughly 1 year ago. Some other notes:

• When shower is running hot water, I can switch the diverter to the tub and will get 2-3 seconds of actual hot water before switching to lukewarm.
  
  
• Measured GPM on tub faucet at ~2.0.
  
  
• I can confirm the anti-scald limiter on Grohe handle unit is maxed out.
  
  
• I do not have access to the pipes feeding the tub/shower.
  
  
• This has been an issue since install. We don't use the bathtub often so didn't identify it as an issue until several months after install.

Any thoughts? I suspected the mixing valve, but think that can be ruled out since the same mixing valve feeds the shower (which works as intended). Also suspected water pressure, but not sure how that impacts a tankless heaters, and also not sure why shower would work and tub wouldn't given that tub is only flowing at ~2.0 GPM.

Tankless heater is a Rheem RTEX-13. Not sure of Grohe model number, but was purchased new at Home Depot at time of install one year ago.

A good thing to do in any case is install a Recirc. pump on Your W.H. Good luck Sully