If the home has a 1-inch main line, use a whole-house system with 1-inch or larger connections whenever practical rather than necking the line down to 3/4 inch. For a smaller 3/4-inch home main, a 1-inch filter can still be installed with reducers, but the existing plumbing remains the limiting section.
Why 1-inch is common on higher-flow systems
SpringWell CF1 and CF4 use 1-inch connections; RKIN CS8, CS9 and CS10 also use 1-inch connections. Larger systems step up further.
Do not confuse port size with service flow
Two filters can both have 1-inch ports but very different media-bed and valve flow ratings. Always compare the published service GPM.
Avoid unnecessary neck-downs
Reducing a 1-inch main to 3/4 inch through the filter adds another restriction and can worsen pressure behavior during simultaneous demand.
Installation fit
Pipe material, bypass valves, elbows, unions and local plumbing code all affect the finished installation.