Just my two bits recently joined this site after getting myself a used BM3 I3 Rex (2015 UK model) Purchase only a few days ago with 12k miles on clock. I also am/was ? having problems re no heating. Somewhere I then read that if its in Eco mode, either takes a long time to heat or perhaps, stays off. I fiddled with settings this morning and turned off eco drive mode, it then appears to have started heating . I had set the climate control "departure times" to come on at a set time. Prior to it coming on, I fiddled with a lot of controls however, no heating ? Anyway, long and short, predeparture heating did start and warm up the cab. I am still uncertain what or why or where I may be going wrong. I spoke to another person who has a Merc Van (electric) and he also told me that his van does not heat up fast. He sets off normal mode and then after a few miles switched it to eco. Hope this assists ? Appreciate, you may already have discounted this
PS - Unsure whether my I3 has a heat pump ?