What gets sized in a plumbing system?
On supply: flows, pressures and pipe sizes. On drainage: fixture units, slopes and sizes of branches, stacks and building drains. Plus equipment like water heaters, expansion tanks and sump pumps.
The calculators give preliminary sizing; a licensed plumber installs to local code.
What pressure and flow does a home need?
Fixture pressure should be roughly 40–80 psi; above 80 add a PRV, below about 30 consider a booster. Minimum flows depend on the fixture.
Each floor of height takes away about 4–5 psi of static pressure.
How do you avoid drain problems?
Adequate pipe sizes, ¼ in per foot slope, vented stacks and accessible cleanouts. Flat runs, bellies between hangers and tight 90° bends on horizontal drains are the usual causes of clogs, gurgling and sewer odor.
Toilets need at least a 3 in drain, and every trap needs a vent within the distance your code allows.