Andy Hall wrote:
I looked at doing this at one point, but couldn't find a source of parts
significantly cheaper than ready made units.

There are a few products sold with hefty margins, but if you shop
around, prices are pretty good, so I decided not to bother. I may
look again, though.

The main things, if you want to look into this, are the choice of the
resin units to suit water usage - use the specs to determine size and
type - and then a suitable valve to go with it/them.

Thanks for that Andy. It's interesting, analogous to wanting to assemble
a PC only to find that PC World will do it just as cheaply!

I already have an 8-year old Kinetico which has now expired. It was
while poking inside it that I realized how simply it could be assembled
if only I had the right information.