ᐅ Sizing of Water Softening System | Reisser Aqua Tenera

Hello,
our plumber has offered us two water softening systems: the Aqua Tenera 1 (single-tank) and the Aqua Tenera 2 (twin-tank) from Reisser. These are Judo systems manufactured and labeled for Reisser.
I have attached the datasheets below. Our annual water consumption last year was 120m³ (130 cubic yards). We are building a single-family house with currently 2 adults and 2 children. The garden water runs through separate pipes bypassing the system.

Our water hardness is 21 °dH and we want to soften it to about 6 to 8 °dH.

Based on the data, what would you recommend? Many thanks for your assessment. If you need more information, please let me know. Thanks!

Normally, I would have said a single system is completely sufficient. But with the small amounts of resin used here, it looks a bit different. If I quickly calculated correctly, the 2.5 liters (2.6 quarts) of resin, considering a hardness reduction of 15 degrees dH, are enough for about 170 liters (45 gallons) of water. Depending on how long the regeneration takes, you won’t have soft water during that time. I would prefer to go with a double system in that case.

Thanks in advance. That would mean that regeneration occurs about twice a day, correct?

Which formula did you use to convert the amount of resin to the volume of soft water?

You want to reduce the hardness by 15 degrees dH; one mmol/l of calcium equals 5.6 degrees dH, which means 2.67 mmol/l. The system has a capacity of 0.45 mol, which means 450 mmol divided by 2.67 equals 168 liters.