Meldrew No, as I said an RO unit will not remove all hardness and alkalinity. My point is you need to take measures in a service boiler to prevent it OR descale. Your options are:
Use water that can’t scale
Use water that can form scale, but not at your service boiler temps.
Use water that will scale.
The people using bottled water (Ashbeck, Volvic, Waitrose essential etc) and those using RO and zero filter jugs, fall under options 2 and 3, whether they add bicarbonate or not. Obviously, option 3 requires descaling at some point. With option 2 you can still avoid having to descale by managing the water in the boiler, which you can do by regularly flushing it out to refresh the water in there and prevent the ionic concentration from increasing as you use the steam (as ionic concentration increases the temp that scale will form decreases, so while it starts off non-scale forming it will not stay that way as you use steam). The other option is to accept the need to perform a light descale of the service boiler at some point….I completely flush my boiler every 5 weeks, fill it with distilled, and supply it with the brew water of about 30 GH and 40 KH via the auto fill. Others simply draw 200ml or so off via the hot water tap daily, and eventually will drain the entire thing and refresh.
Some people go for option 1) using a distiller or maybe RODI with added bicarbonate.
I can’t say what is best. I used to go for option 1 but the coffee has improved with some hardness and I wouldn’t go back to it just to avoid having to manage the service boiler. It is an individual preference, you may well prefer the coffee with no hardness.