I have installed the Rinnai integration, for my new tankless water heater. I want to create a scene to set the water temperature (the DW likes to have her baths at 108 degrees Fahrenheit). But, in SmartThings it seems to appear as a Thermostat, which reasonably limits the maximum temperature to 40 degrees Celcius (104 degrees Fahrenheit).
Is there any workaround you might suggest?
Is there offset setting ?
No, there is no offset in rinnai app, or physically on the device. Could something be implemented in SmartThings?
Perhaps a simple app wouldn’t be limited like a Scene is.
Please excuse me for my ignorance, but I don’t know the Rinnai integration at all. I am guessing that is one of the official integrations through the app. If so, then you need to contact Rinnai to update it. Most likely, as you have explained, it uses the thermostat capability which has a temperature range defined in the representation from 0 to 40 Celsius. Rinnai has to build a custom capability based on the thermostat one, and adjust that range to those values what the heater actually supports. I guess up to 80 Celsius.
I would recommend reporting it to support@smartthings.com
There are two types of limits, one from within the SmartThings mobile app and another from the device handler (or the associates VID properties of the device handler reported to SmartThings which can place limits on the maximum and minimum thresholds). Are you using a custom device handler?
I wonder if you did an incorrect C to F conversion or something. 104°F is barely lukewarm.
US CPSC recommended hot water setting is 120°F.
We run our tankless water heater at 135°F
UK recommends tank water heaters be at 60°C (140°F) in order to kill legionella bacteria.
I am not using a custom device handler yet. With the Rinnai official device handler I can set the temperature up to 140 degrees Farehnheit. It’s just if I try to create a “scene” it won’t go above 40. Maybe I can do it within some other thing, that isn’t a scene, like an app?
104F is a perfect shower temperature. 108 is my wife’s perfect bath temperature, I prefer 110F for my baths. With this temperature, we run hot only into the shower/bath, and it’s perfect.
When someone isn’t bathing, I like to leave it set at 125F.
The Rinnai App and the SmartThings integration allow us to set the temperature up to 140F. It’s just in a Scene that it’s restricted to 40C.
In the Rinnai official app, and in the Rinnai Smartthings integration, it goes up to 140F. It’s just in a Scene it’s limited to 40C. Is this consistent with what you are telling me about “custom capability”?
ActionTiles can set the temperature to 108F. I wonder why ActionTiles works when Scenes are limited to 40C?
If it works with the device then it should work with ActionTiles as well as it sets the device.
Have you tried Automations?
The capabilities have the tile settings and the automation setting part. The range might be wrong there. You can use the CLI to query the DH for the capability used and then check the content of the capability with the CLI, as I can remember.
But @orangebucket has a better knowledge about the CLI if you want to go down the rabbit’s hole.
I don’t have the CLI working.
But, I decided to try Webcore, sending the wife’s preferred bath temperature when the switch “Wife’s Bath” was activated. Webcore sent the temperature (43C, 109F) and the Rinnai responded, but it was limited to 98 degrees Fahrenheit 
which is much too cold for a bath.
I just found this article Supply House Times Rinnai Water Heater that says “With SmartThings’ new Water Heater category, featuring only Rinnai Tankless Water Heaters”. Does this mean that there is a Water Heater “capability”? My Rinnai is definitely showing up as a Thermostat, not a Water Heater.
I emailed Rinnai support the today, and sent them the link to this discussion thread.
If there’s a Water Heater category already, they should know about it already, right? Would that be the same as a “capability”?
That article also suggests that perhaps I can achieve my temperature setting objectives using Google Home, or Alexa, but not through Smartthings.
There is no Water Heater capability.
Could you grab the CLI and get the details of your device please? Then we can have a look on the device representation and see where the limits are set.
Just download it (I guess you have a Windows machine, just run it through command prompt), run it once, log in yo your Samsung account and then you can easily browse the details. The help provides all the useful commands.