How Water Moves Through a Building
Supply up, waste down: understanding the two systems makes every plumbing decision easier.
The supply side
Mains pressure brings cold water in and either feeds everything directly (combi and unvented systems) or lifts to a loft tank that gravity-feeds the house. Which system you have decides your shower pressure, your fixes and your upgrade options.
The waste side
Everything drains by gravity to the soil stack, and the stack vents above the roof so falling water does not suck the traps dry. Every fitting keeps a water seal between your rooms and the sewer, which is why dry traps announce themselves by smell.
Why this matters to you
Half of plumbing misdiagnosis is treating a supply symptom with a waste fix or vice versa. Knowing which side of the wall the problem lives on is most of the way to the right trade, the right part and the right price.
Supply: mains, tank and pressure
Water arrives under mains pressure at your stopcock. In a direct system every cold outlet runs off that main. In an indirect system the main fills a loft tank and most outlets run from it under gravity, which is why the pressure upstairs is lower. Knowing which you have explains most pressure complaints without any investigation at all.
Waste: traps, branches and the stack
Every fixture drains into a trap holding a water seal, then into a branch, then into the soil stack, then underground. Falls are shallow and deliberate, too steep and the water outruns the solids, leaving them behind. That counterintuitive fact is behind a surprising number of blockages in retrofitted bathrooms.
Why all this explains your symptoms
A gurgle is air being pulled through a trap. Slow drainage across several fixtures is a shared branch. Weak hot but strong cold is a gravity hot supply and a mains cold. Once you can picture the route, most plumbing symptoms stop being mysterious and start telling you roughly where to look.