For up to 4 bedrooms you will need a 1/2" 15mm Limetron® to be installed on your incoming main
For more than 4 bedrooms you will need a 3/4" (22mm) Limetron® installed on your incoming main.
For small commercial applications choose Scaletron®
For large Commercial applications choose MagCAT®
For Industrial applications choose Colloid-A-Tron®