Ok with node-red-contrib-amazon-echo or node-red-contrib-virtual-smarthome, you could simply say "alexa set water temp to 25" screen shot below device is set as a dimmer. In screenshot you can see the percentage. i said "set water temp to 30" as well.
Also in node-red-contrib-alexa-remote2, you can use the event listener set to device activity. Then when you say "alexa blah blah", you can see the spoken word in msg.payload.description.summary
. You would then have to parse out the temp.