Originally Posted by
DBMandrake
What actually happens is the raw measured temperature is biased up to half a degree towards the setpoint and the result is then rounded (up or down) to the nearest 0.5C for final display.
So for example if the set point is 20C and the true measured temperature is 20.1C, the measured temperature is biased down to 20C (biased towards the set point up to a maximum of 0.5C) and then rounded to the nearest 0.5C - so reports 20C.
If you then change the set point to 20.5C it will now bias the measured temperature up to a maximum of 0.5C above the true temperature. True temperature is 20.1C and set point is 20.5C, this is then rounded and you get 20.5C.
If you turned the set point up to 22C while the measured temperature was 20.1C it would bias it to 20.6C then round it up to 21C...
This "fake temperature display" is independently simulated by all devices - the over the air protocol (and the web API) always reports the true temperature to two decimal places without any rounding or biasing, and the individual devices like DTS92 and HR92 apply this algorithm themselves before displaying the temperature on their screens.
So adjusting the set point half a degree is not "useless", as it does indeed increase or decrease the ultimate room temperature by half a degree (it will take a while though) however I agree that the measured temperature display on the Evohome system is misleading in the way that it rounds the temperature towards the set point to disguise fluctuations around the set point. And one side effect of this is that the display temperature sometimes shifts half a degree as soon as you adjust the set point, and before the room temperature actually changes.
I wish it didn't, unfortunately it is baked into the firmware in all the non-firmware upgradeable devices like HR92's (and others like DTS92 that pre-date Evohome) so they couldn't remove it from the controller and Phone app without introducing discrepancies with the readings on other devices...