ok i figure with my reading of 75 i should be ok with just a di vessel. My only real query would be for you James is does the di vessel get water pure enough and is it cost effective or would u recommend getting a ro.
Yes, i use an 11ltr vessel and it works perfect.
I use around 1000 ltrs a week, 200 a day as we only use WFP for top windows. The resin will need changing after about 3 months.
A bag of resin costs about £70 which has 25ltrs worth in there, so it will last about 7.5 months.
If you want to add a 2nd DI to save resin then you can, or if you have money for an RO then again you can.
Hope this helps.