The basics:
how a system works
Four systems run through almost every building: cold water in, hot water made, heat moved around, and waste taken away. Learn how those four fit together and most plumbing jobs stop being a mystery. This unit walks a domestic system end to end, then puts you in front of some faults.
The four systems
Everything else is detailGetting it in
Mains water arrives under pressure from the street, through a stop tap, and is distributed to every cold outlet and to whatever makes your hot water. It is drinking water, so keeping it clean is a legal duty.
Making it
Either on demand as it flows through a combi boiler, or stored in advance in a cylinder. Which one a property has changes almost every decision you make about the rest of the job.
Moving it
A sealed loop of hot water pushed by a pump from the boiler, out through the radiators and back again. Flow out, return back, and a valve on each radiator to control the room.
Taking it away
Gravity, not pressure. Waste falls down a pipe laid to a fall, through a trap that holds a plug of water, into the soil stack and out to the sewer.
Water supply is under pressure and goes where you send it. Waste is under gravity and only goes downhill. Almost every drainage fault you will ever attend comes back to something being flat, blocked, or missing its water seal.
Walk the system
Tap a numbered pointA schematic of an ordinary house with a combi boiler. It is not to scale and the pipe runs are straightened out, which is exactly how you will see it drawn on a job. Tap each numbered point.
Stored or instant?
The first question on any jobBefore you touch anything in a property, find out how it makes hot water. It determines the pressure at the taps, whether there is a tank in the loft, what you are allowed to work on, and how you price the job.
| Type | How it works | Tell-tale signs | Watch for |
|---|---|---|---|
| Combi boiler | Heats water instantly as it flows through. No cylinder, no tank. | No airing cupboard cylinder, no loft tank, mains pressure at every tap. | Flow rate limits. One shower is fine; two at once is not. |
| Unvented cylinder | Stored hot water held at mains pressure in a sealed steel cylinder. | A cylinder with a pressure relief pipe (tundish) and an expansion vessel. | G3 registered work only. These are pressure vessels and legally restricted. |
| Vented (gravity) | Cold tank in the loft feeds a copper cylinder; pressure comes from the height of the tank. | Loft tank, copper cylinder, weak hot flow upstairs. | Low pressure upstairs is normal, not a fault. Check the head before condemning anything. |
On a gravity system, every metre of height between the bottom of the cold tank and the outlet gives you roughly 0.1 bar of pressure. A tank 3 m above a shower gives about 0.3 bar — which is why that shower is disappointing, and why the answer is a pump, not a bigger pipe.
Drainage, traps and falls
Gravity does the workEvery appliance connects to the drain through a trap: a bend that stays full of water and seals the room off from the sewer. Lose that water seal and you get the smell. Most trap failures are one of three things: the trap has dried out from lack of use, it has been siphoned empty by a long unvented run, or it has been blocked with grease and hair until it overflows.
Above ground
32 mm — basin
40 mm — sink, bath, shower
50 mm — long runs and appliances combined
110 mm — WC and the soil stack
Enough, but not too much
Waste pipe is laid to a fall of roughly 18–45 mm per metre. Too flat and solids sit in the pipe; too steep and the water runs away and leaves the solids behind. A common working figure for 40 mm waste is about 1 in 40.
Trap seal depths. A trap on a pipe connected to a soil stack needs a 75 mm water seal. A trap on a gully or a ground-floor appliance discharging outside can be 50 mm. If you fit a shallow trap where a deep one belongs, the seal gets pulled out by the stack and the customer rings you back about a smell.
The rules you work inside
Know these names- Water Supply (Water Fittings) Regulations 1999
- The rules that protect the public drinking water supply. They are why you cannot let a hose end sit in a bucket of chemicals, why outside taps need a check valve, and why fittings must be approved. Notifiable work must be told to the water undertaker before you start.
- Backflow and contamination
- Water must never be able to run backwards into the main. Fluid categories 1 to 5 grade how dangerous a contamination risk is, and each category demands a specific level of protection — an air gap, a check valve, or a full break tank.
- Building Regulations Part G
- Sanitation, hot water safety and water efficiency. Part G3 is the one that makes unvented hot water storage restricted work.
- Building Regulations Part H
- Drainage and waste disposal — pipe sizes, falls, traps, ventilation of stacks and connection to the sewer.
- Building Regulations Part L
- Conservation of fuel and power. Controls, insulation and boiler efficiency requirements live here.
- Gas Safety (Installation and Use) Regulations 1998
- Gas work may only be carried out by someone competent and on the Gas Safe Register. Working on gas without it is a criminal offence, not a technicality.
Fault finding
Three calls, three decisionsDiagnosis is the part of the job that pays. Read each call-out and choose what you would do first — first, not eventually.
Call 1. A customer has a cloakroom that smells of drains. It is only used a few times a month. Everything drains away normally and there is no visible leak.
Call 2. Upstairs shower is weak. There is a cold water tank in the loft and a copper cylinder in the airing cupboard. Downstairs taps are fine.
Call 3. One radiator is cold at the top and warm at the bottom. The rest of the house is fine.
Check what you have taken in
4 marksWhich of these may only be worked on by someone holding the specific qualification for it?
A cold water storage cistern sits with its base 4 metres above a bathroom tap. Estimate the static pressure at that tap, in bar.
Produced for EAL Learn+.
Introduction to Plumbing — a 30-hour pre-apprenticeship short course.