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Here’s what every installer, apprentice, and property owner needs to understand about pipe sizing and how to make sure it’s done right.
Why Pipe Size Matters
Water Flow and Pressure
The most obvious effect of pipe diameter is how much water can flow through it. A pipe that’s too narrow will restrict flow, especially if multiple outlets are in use at the same time think showers running while someone’s using the kitchen tap.
On the other hand, an oversized pipe might reduce water velocity, allowing sediment to build up and increasing installation costs unnecessarily.
System Efficiency
In heating systems, undersized pipes force pumps to work harder, which increases wear and wastes energy. Oversized pipes might slow water circulation, leading to uneven heat distribution.
The correct size ensures the system is balanced and efficient something that matters whether you're running underfloor heating or traditional radiators.
Noise and Vibration
Incorrect pipe sizing often leads to “water hammer” or general system noise. Water moving too quickly through a narrow pipe can cause vibration, especially if not clipped properly. Get the sizing right, and you reduce both sound and strain on fittings.
What Affects Pipe Size Choice?
It’s not a one-size-fits-all approach. Here are the key things to consider when deciding on pipe diameter:
Flow Rate Demand
How much water is needed at any given point? For example, a single basin tap needs much less than a bath tap, combi boiler, or unvented cylinder.
Length of Run
Longer pipe runs can experience pressure drops, especially if using narrow pipes. You may need to upsize slightly to maintain pressure over distance.
Type of System
Pressurised systems (like combi boilers or unvented cylinders) can usually tolerate smaller bore pipes than older gravity-fed systems — but each has specific requirements.
Number of Outlets
Installing multiple appliances or outlets on the same run? You’ll need to account for peak simultaneous usage, not just individual demand.
Pipe Material
Different materials offer different internal diameters (even if their external measurements are the same). For example, 15mm copper and 15mm plastic may not behave the same way under pressure. Always account for actual bore size, not just labelled size.
Common Domestic Pipe Sizes (UK)
Here’s a rough guide to the most common sizes used in domestic plumbing:
15mm – The standard for most taps and radiators. Good for short runs or single fixtures.
22mm – Typically used for bath taps, hot water feeds, or key parts of heating systems.
28mm – Often used in older gravity-fed systems or for main feeds to high-demand systems.
10mm – Microbore pipes used in some radiator systems; popular in retrofits but prone to blockage if not properly filtered.
For heating pipework, you’ll also see 16mm and 20mm sizes in modern plastic systems, especially for underfloor heating or where pipe-in-pipe systems are used.
How to Avoid Mistakes
Use sizing calculators for more complex jobs, especially heating or multiple-outlet plumbing.
Stick to manufacturer recommendations where connecting to appliances or systems (like boilers or cylinders).
Don’t guess based on “what looks right” even if you’ve done similar jobs before. Every system behaves differently.
Consult with experienced merchants (like us!) if unsure we’ll help you select the right components for your system layout and usage demands.
Need Help with Pipe Sizing or Materials?
Our team at Collister and Glover is always happy to help whether you’re an experienced installer or just starting out. We stock a full range of copper, plastic, and push-fit pipework in all standard sizes, plus insulation, fittings, and valves to match.
Call or visit us in Deeside to get advice on your next project and make sure your pipework performs exactly as it should.
