Design of Softening System | Reisser Aqua Tenera

Hello,
our plumber offered us two systems for water softening, namely the Aqua Tenera 1 (1-column) and the Aqua Tenera 2 (2-columns) from Reisser.
These are Judo systems produced and labeled for Reisser.
I have attached the data sheets below. Our annual consumption last year was 120 cbm of water. We are building a single-family house with currently 2 adults + 2 children. The garden water runs through separate pipes past the system.

We have 21 °dH and want to soften it to about 6 to 8 °dH.

What would you recommend based on the data? Many thanks for your assessment. If more input is needed, please let me know. Thanks!

Normally, I would have said a single system is completely sufficient. But with the small amounts of resin installed here, it looks a bit different. If I’ve calculated correctly on the fly, the 2.5 liters of resin are enough for about 170 liters of water with a hardener reduction of 15 degrees dH. Depending on how long the regeneration takes, you won’t have soft water during this time. I would rather go for the double system.

Thanks in advance. That would mean that regeneration occurs about twice a day, correct?
Which formula did you use to convert from the amount of resin to the amount of soft water?

You want to reduce 15 degrees dH, one mmol/l Ca is 5.6 degrees dH, i.e. 2.67 mmol/l. The system has a capacity of 0.45 mol, i.e. 450 mmol divided by 2.67 = 168 liters.