Corsa c heater help

[nova speed]

Hi having trouble with the heater on my 1.2 Corsa c it gets hot when the engine is above 2.5k rpm but when it idling it just blows out cold air. I have flushed the whole system and made sure there are no airlocks in the system has anyone else had this problem thanks

[mowgli]

is the heater controlled electrically? maybe there is a sensor out