I understand what you are saying, gwatpe. But would you not agree that this is all not related to the ECO button and temperature? (As greendwarf kinda confirmed).
As I said, I use ECO all the time. And still I often see the left yellow power scale go to 3/4 when I turn on the heater. Because of this and because of what I have read I am more or less convinced that ECO does not impact power consumption by the heater, as suggested by Wiredgoose.
So, all ECO does is reduce power consumption by the air conditioning and change throttle response. But Wiredgoose was talking about temperature and cranking up the heater, which is not impacted, so the noticable effect is reduced to throttle response. And if ECO mode is able to prevent the engine from starting by changing the throttle response, than so could the driver. If there are hills that are steep enough to make the engine start and you as a driver cannot prevent that, than the ECO mode cannot prevent it either. That was my line of thinking.
And even when I am mistaken and ECO does impact power consumption by the heater, or if Wiredgoose actually meant that he turns on the air conditioning on colder days, instead of just the heater, or both, than the effect of reducing the A/C related power consumption would still be rather limited. To limited to have the effect of an on/off switch as suggested by Wiredgoose when he wrote "the engine cuts in very often and putting the car in ECO prevents this".
Too bad, we don't know yet what temperatures Wiredgoose is talking about.