What's your tap Tds reading .because in my opinion your trying to save money by not using a lot of resin (not having a go ) why don't you use a 4040 ro a mate of mine has had one for nearly three years and over the last year found it's cheaper for him to change membranes once a year filters twice a year and only go through one bag of resin ,He told me he was using a minimum off six bags off resin a year and at £75 a bag that's £450 a year where as doing it the other way saves him £100 a year minimum