single zone problem with continuous heating

[Deenz]

Hello, I have a problem with my evohome wifi system. I have 6 zones, 5 of them works fine. 6th zone has a problem it is used only for continuous heating, no schedule setted up. Power of radiator is not very high so it takes time to heat it up. Problem is when I want evohome to keep higher temperature for long time. More tham 8 hours. In moment I raise temperature central unit sends signal to BDR91 which starts up my boiler and keeps it running for some time. After about 8 hours when temperature is not possible to reach setted temparature because of small radiator, evohome "gives up" heating. HR92 keeps radiator opened, central unit says 100% on boiler but LED on BDR91 does not shine and heating is not working.

Example:
2PM: zone temperature 18°C requested 22°C - boiler starts up
10PM: zone temperature 22°C requested 22°C - boiler turned off and switch to very often on/off which should be ok to keep the temperature.
3AM: zone temperature 18°C requested 22°C - boiler still in often on/off mode but since temperature is more than 1.5°C lower than requested it should switch to continuous which it doesn't.
6AM: zone temperature 17°C requested 22°C - due to not switching to continuous heating temperature keeps lowering.

When I lower the temperature for at least 10 minutes and than turn it back it repeats the scenario.
If I turn on heating in another zone temperature raising because radiator valve is opened as expected so HR92 works fine comunication also looks fine because info about temprature is always on display pretty accurate.

[Richard]

Why not fit a radiator sufficiently sized to heat the room? Correctly sized radiator should heat the room in circa 1 hour. evohome is good, but cannot bend the rules of physics.