We recommend for majority of the time to always leave the red dial on full heat, then control your water temperature by changing the blue dial.
If you want really hot water, leave the blue dial on lowest flow. If you want to lower the water temp, move the blue dial to medium or high water flow.
Once you move the blue dial the water temp should change after a few second, then stabilise after another few seconds. Change the blue dial again, the same thing happens, the water temp starts changing after a few seconds and stabilises after another few seconds.
As a test you can try the following:
Adjust the red dial to HI and blue dial to HI - you'll probably get water about 35-40C on your typical day where the temperature is somewhere around 20-25C.
Start moving the water dial and you'll see the water temperature starts increasing. If you moved the dial and the temp is getting close to 50, increase the water flow and the temp should start dropping again.