Plumbing

A house holds two plumbing systems that never touch. Supply brings clean water in under pressure, the city’s pressure, so its pipes can be small and go anywhere: up, down, round corners. Drainage takes used water out by gravity alone, so its pipes are large and every one of them must fall, all the way to the street. A third set of pipes carries nothing but air: the vents, without which the falling water would pull the water out of every trap and let the sewer’s gas into the rooms.

The city keeps its main at 50 to 80 psi; the figure’s runs at 70 psi (483 kPa), 6 ft (1830) under the street. A corporation stop tapped into the main’s top starts the house’s line. The water service runs 6 ft deep all the way to the house, because the frost here reaches 42 in (1065) and a frozen service bursts. At the lot line a curb stop, worked from the surface through a curb box, divides the city’s pipe from the owner’s. The service passes the foundation wall in a sleeve, rises inside it to the meter between two valves, and the downstream valve is the house’s main shutoff.

Every foot the water climbs costs pressure, whatever the pipe:

P = P_main − 0.433 h − lossesP pressure at a fixture (psi) · h height above the main (ft) · losses: friction in pipe, meter, heater and fittings

The lavatory upstairs is 24 ft (7.3 m) above the main, so 10 psi goes in lifting the water and 60 psi remains when nothing flows. With the whole house at its peak about 52 psi still reaches the faucet. The code asks for 8 psi flowing at a lavatory and 20 psi at a flush valve, and for no more than 80 psi standing anywhere: above that a pressure-reducing valve goes in after the meter, or washers and valves wear fast.

Pipes are sized by water supply fixture units, a weight for how much and how often each fixture draws. Because not everything runs at once, the units convert to a peak flow along a curve, not a sum:

7.2 gpm (27 L/min) through the 1 in service is 3.8 ft/s, under the 8 ft/s at which pipes start to hum and erode. The water heater takes cold water at its top and a dip tube carries it to the bottom; the hot leaves from the top. Its relief valve opens at 150 psi or 210 °F (99 °C), and its pipe runs down to 6 in above the floor so a discharge scalds no one.

From the heater the water can be distributed two ways. A trunk and branch system runs a ¾ in trunk under the joists and tees ½ in branches off it: the least pipe, but a joint at every tee and elbow. A manifold puts a valved port for every fixture in one place and runs one unbroken ½ in line to each, a home run: more pipe, no joints in the walls, and a shutoff for every fixture at the manifold. Switch the supply pipe in the figure to see both.

The drain, waste and vent system (DWV) is sized by drainage fixture units, and its pipes get bigger as they go, never smaller. Every horizontal drain falls: ¼ in per ft (2 %) for pipes up to 2½ in, and at least ⅛ in per ft for 3 in and larger, though most plumbers keep ¼ in throughout. That fall is what makes water run fast enough, about 2 ft/s (0.6 m/s), to carry solids with it. Too little and they settle; the figure’s 3 in building drain runs at 3.8 ft/s half full.

Trap

A U under every fixture that stays full: 2 to 4 in (50 to 100) of water between the room and the sewer.

Trap arm

The pipe from the trap to its vent, limited in length so the trap cannot siphon: 5 ft for 1¼ in, 6 ft for 1½ in, 8 ft for 2 in, 12 ft for 3 in.

Stack

The vertical drain the branches join. Here 3 in (75), rated for 48 fixture units; the house puts 9 on it.

Building drain

The lowest horizontal drain inside the house, here under the basement slab, where the laundry can drain to it by gravity.

Building sewer

From 3 ft outside the wall to the main: 4 in PVC on a bed of stone, kept 5 ft (1525) from the water service.

Cleanouts

A capped opening wherever a line could clog: at the stack’s foot, just outside the wall, at the lot line.

Water falling down a pipe pushes air ahead of it and leaves a partial vacuum behind it. Without a way for air to follow, that vacuum pulls on the water in the traps upstream, and a trap that loses its water is an open door to the sewer. A sink can even siphon its own trap: a full basin running out drags the last of the water after it. Vents stop both. Each trap connects to a pipe that rises to the open air, so the pressure in the drains stays at the pressure in the room.

In the figure the stack itself vents the toilet, and continues up through the roof as the stack vent. The lavatory has its own vent up the side wall, and its drain vents the tub on the way, a wet vent. The kitchen sink sits under a window, so its vent rises beside the window, through two floors and the attic, and ties into the stack vent above the highest fixture: no waste can ever reach a vent that joins that high. The laundry has an air admittance valve, a one-way valve that opens to let air in and closes against sewer gas; allowed for branches, never in place of the open stack. Through the roof the vent rises 12 in (305), and stays 3 in all the way through the cold attic, so frost from its own vapour cannot close it.

Pipes cross the structure everywhere, and every hole weakens what it passes. The code limits them closely:

The limits decide the layout. A 2×10 joist allows a 3 in hole; a 3 in drain needs 3¾ in. So where the toilet’s branch must cross a joist, the joist is cut and headered off: hung in steel hangers from two short headers, framed like a little stairwell. The wall behind the toilet is framed in 2×6, a wet wall, because a 3½ in pipe cannot fit a 3½ in stud. And a long drain cannot run through bored holes at all: over 31 ft the kitchen’s 2 in drain falls 7¾ in, more than the band a 2×10 leaves between its edge limits, so it hangs below the joists on straps every 4 ft. Supply pipes, which need no fall, pass through holes at mid-depth. The same holes and the order they are bored in are drawn in platform framing.

Where the building drain leaves, it passes the footing in a sleeve two pipe sizes larger, cast in, so the house can settle without shearing it. The footing drain beside it carries only groundwater, to a sump or to daylight; joining it to the sanitary sewer is forbidden.

Everything above is inspected at rough-in, while the walls are open. The drains are capped and filled with water to 10 ft (3 m) above the highest fitting, or pumped with air to 5 psi, and must hold for 15 minutes without dropping. The supply is held at its working pressure, or with air where the pipe’s maker allows it. Only then do insulation and drywall go on.

Three decisions shape the plumbing of a house like this one: where the waste goes, what the water runs in, and what heats it. None of them is the same everywhere.