When choosing a pipe, it is easy to focus on one number: a neighbour used sixteen-millimetre pipe, the installer suggests seventeen-millimetre pipe and twenty-millimetre pipe is praised online. But the diameter printed on the coil does not tell you how water will flow in a particular circuit. You need the wall thickness, material, route length and required flow. A good choice is not the largest pipe but a coordinated system that does not need several extra components to compensate for one mistake.
This article specifically compares diameters. Heating capacity for the whole home, pipe spacing and temperature need to be established separately. Replacing the pipe with a larger one will not automatically make an uninsulated room suitable for low-temperature heating.
What do the two numbers on a pipe mean?
A marking such as 16 × 2,0 mm normally gives the outside diameter and wall thickness. In simplified terms, subtracting two wall thicknesses from the outside diameter gives the internal diameter. This geometric example leaves 12 mm. For 17 × 2,0 mm, the result is 13 mm; for 20 × 2,0 mm, 16 mm. These are examples of the stated dimensions only: an actual product's wall thickness may differ.
| Example dimension | Calculated internal diameter | Approximate water volume per metre |
|---|---|---|
| 16 × 2,0 mm | 12 mm | 0,113 l |
| 17 × 2,0 mm | 13 mm | 0,133 l |
| 20 × 2,0 mm | 16 mm | 0,201 l |
Volumes are calculated from circular cross-sectional area, not taken as declarations for any brand. One extra millimetre externally may clearly be more than a minor difference for the water. Additional volume does not itself mean greater heat output into the room, however.
Why does space for water matter?
For the same flow in a narrower pipe, water travels faster and pressure losses generally rise. The precise difference must be read from the specific pipe's hydraulic chart, since water properties and temperature matter too. In the Danfoss design guide, circuit assessment relates to flow, length and pressure losses, not just the diameter name.
Imagine two equal-length circuits. One heats a small bedroom, the other a zone needing more output. The second may require a higher flow, so even identical pipes will not experience equal resistance. The statement that a pipe suits a hundred metres therefore needs a further question: at what flow and in which system?
A larger pipe also affects installation cost
A larger outside diameter needs appropriate space in the floor construction. Bending, turn layouts, clips and manifold connections differ. If a thin renovation build-up is specified, you cannot arbitrarily substitute another pipe and assume the manufacturer's tables remain valid. The Uponor systems overview shows that underfloor heating includes different constructions, not just one universal coil.
Pipes cluster near windows, narrow passages and the manifold. A larger-diameter option may require a different layout there. The minimum permissible bending radius for the selected product matters. An excessively sharp bend is not an acceptable way to save floor height. The installer must assess damage according to the system's repair procedure.
Material and fittings are not minor details
Identical outside dimensions do not guarantee identical fittings. Pipe type, wall thickness and manifold connection kit must match. A similar-looking shop component is not technical proof of compatibility. Keep the product name, a photo of its marking and the invoice so you do not later have to guess what is hidden in the floor.
Suitability for heating, oxygen barrier properties and permitted operating conditions are checked too. Do not apply one brand's advertising claim to every pipe of the same diameter. System warranty terms may depend on the component combination, installation procedure and documents. Read these before purchasing, not after screeding.
How can three quotations be compared in practice?
Ask for the same room plan and proposed floor covering in each quotation. Then compare not the coil price but the number of circuits, their lengths including lead-in runs, design flows and overall installation scope. Otherwise, a cheaper pipe may mean an extra manifold branch and a more expensive one a completely different floor build-up.
For example, a small floor may have two proposals: more short circuits using smaller pipe or fewer longer circuits using larger pipe. Both may be justified in theory. The decision depends not on a general claim of better, but on hydraulic checks, zoning, installation options and future controls. Fewer branches are no advantage if rooms requiring different temperatures are combined.
What should be recorded before ordering?
If you buy materials yourself
Give the selected product data sheets to the installer before ordering. Agree who checks compatibility and handles leftovers. Equal coil lengths do not mean equal work: one circuit can be laid continuously, while a leftover may be too short for the planned loop. A total pipe length calculated from floor area is not a coil allocation plan. Each loop must fit a suitable continuous length according to the design.
If the quotation merely says underfloor pipe, request clarification before purchasing. An unspecified wall thickness later prevents not only flow comparisons but checking that the agreed material was actually delivered. Inspect markings and packaging when accepting a coil, and record any damage. A pipe accidentally crushed in transport cannot be excused because it will soon be concealed in screed.
Keep one short summary: pipe dimensions including wall thickness, exact model, circuit lengths, manifold and connection fittings. Record floor construction and covering separately. If any element changes, the designer must assess whether the earlier calculation remains valid. A verbal promise that this is how we always do it is not enough.
For an initial length estimate, use the pipe length calculator, but it does not determine permissible pressure loss. For the broader system principles, see the underfloor heating selection guide. If you want to compare actual quotations, use the contact form to send their technical sections and a floor plan. A question about three diameters then becomes a specific, verifiable solution for your home.
Information sources
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