Short answer: room heat load sets the required output and therefore the spacing; flow, internal diameter and allowable pressure loss then set circuit length; and only the available differential pressure decides whether that length can be accepted. Copying spacing, diameter and loop length as three separate values is how a system can look compliant yet heat unevenly.
- Heat load is calculated from the envelope, room use and the design code of the destination country — not from a catalogue table.
- Spacing follows the required output of each zone; external walls and full-height glazing may need denser loops.
- Flow, internal diameter and specific pressure loss are checked together.
- A circuit length is accepted only after the available pump pressure covers the loop, manifold, valves and local losses.
- The design professional owns the calculation; a supplier can match the pipe to the design input.
One chain, five outputs
Start with the heat-load calculation under the rules that apply at the destination. Confirm the floor build-up and finish, fix the design supply and return temperatures, and derive the output each zone must emit. Spacing is the result, with denser loops only where the loss is higher. Next calculate flow per circuit; combine it with internal diameter to obtain velocity and specific pressure loss. Accept a circuit length only after the available differential pressure — pump head minus mains, other circuits and control-valve demand — covers straight-run plus local losses. Finish by comparing circuit resistance and regrouping or balancing where the spread is too wide.
| Stage | Input | Output | Record |
|---|---|---|---|
| 1. Heat load | Envelope, room use, destination code | Required output per room | Calculation and code edition |
| 2. Spacing | Required output, floor build-up, water temperatures | Spacing layout with cold-edge densification | Layout and assumptions |
| 3. Circuit flow | Output and design ΔT | Flow per circuit | Flow schedule by loop |
| 4. Diameter and length | Flow, velocity and friction limits | Internal diameter and circuit length | Pipe schedule and lengths |
| 5. Δp check | Pump curve, manifold and valve losses | Accepted length or regrouping | Available Δp against calculated loss |
Why the “100 m” figure is not a material limit
A routine limit around 100 m appears in many projects because, at conventional water temperatures and common pipe sizes, the control valve and pump margin are roughly used up at that length. Change the internal diameter, the design temperature difference, the finish conductivity or the pump margin and the acceptable length moves. Presenting the figure as a property of the pipe is a misreading in one direction; forcing a much longer loop into one circuit is a misreading in the other, and the far end simply stays cold. A calculation should show the assumptions behind the accepted length, not a fixed guarantee.
What changes from 16 to 20 mm
At the same flow, internal diameter sets velocity and friction loss. It also sets coil stiffness, minimum bend radius and the depth needed in the floor build-up. A smaller pipe bends more easily and sits lower but resists more; a larger pipe reduces resistance and carries more water but needs more turning space and a thicker construction. The 16 versus 20 decision is therefore a system trade-off, not a ranking. The pipe schedule should follow the hydraulic result and the floor section, not a preference stated before either is known.
Three items a copied spacing misses
- Cold edges. External walls and full-height glazing concentrate loss; zoning and local spacing should reflect it instead of one uniform grid.
- Boundaries and joints. Edge insulation and movement joints in large areas affect both heat transfer and long-term cracking risk.
- Finish conductivity. Stone and timber respond differently to the same water temperature; the finish must be fixed at design stage, not corrected by raising the temperature after handover.
Where the supplier fits
A verifiable supply record can give the pipe structure, outside diameter × wall thickness, coil length, batch and accompanying documents. SANLI PIPE reports a scope that includes PEX-AL-PEX, PE-RT / PE-Xa heating pipe, aluminium-plastic PP-R and PE-RT alloy-aluminium oxygen-barrier pipe, with 20 extrusion and composite lines and 26 factory inspection steps. The supplier can configure diameter, wall thickness, coil length, packaging and selected accessories to the design input; it does not replace the calculation. For loop quantities and zoning, see the quantity guide; for climate inputs, see the cold-winter design note.
Why a copied "100 m" misbehaves — and the check order that works
Total pressure loss is the sum of straight-run friction (pipe length × friction per metre) plus local losses (bends, manifold, valves). Friction per metre is set mainly by flow and internal diameter: more flow or a smaller bore means higher friction; water temperature shifts viscosity and density, but to a limited extent. Because straight-run friction is roughly proportional to length, the circuit length is the easiest factor to lose control of — it multiplies the friction directly.
The ceiling is not the pipe: it is the available differential pressure — pump head at that flow minus the mains, the other circuits and the minimum pressure difference the control valve needs to regulate. Thermostatic valves, especially self-acting ones, work within a pressure-difference band: too much and the valve becomes noisy and unstable; too little and it does not open, so the end of the loop stays cold. The length rule is therefore "make the worst circuit's loss fall inside the valve and pump working band", not "as long as possible". A 100 m order of magnitude is what typical temperature differences, common small diameters and ordinary valve bands produce together — change any one input and the number should be recalculated.
Two misuses are classic. One: a long loop accepted by habit, the far end starved of flow, half the floor cool — on site this shows as "the boiler is running but the room is not warm" and is often blamed on the pipe. Two: many short loops, low resistance each, but more manifold circuits and valves, harder balancing, higher cost, and large length differences between loops that are almost impossible to balance.
The executable check order is: (1) calculate each circuit's required flow from room load and the design supply–return difference; (2) choose the diameter and read the friction per metre at that flow from a design handbook or software, not from hearsay; (3) use the available differential pressure to back-calculate the maximum acceptable length, then divide circuits by floor geometry; (4) keep the loss spread between circuits within a balanceable range, otherwise adjust lengths or add per-circuit balancing; (5) write the measured length of every circuit into the concealed-works acceptance record, in step with the installation acceptance flow.
What the supplier can verify is geometric consistency: outside diameter and wall thickness (which fix the internal diameter), coil length and batch marking, permanent coil identification, and wall-thickness uniformity inside the 26 factory checks. This page gives no friction coefficients and no recommended loop length: those values must come from the design calculation and the hydraulic data of the chosen fittings — a supplier must not stand in for the designer's conclusion.
Scope and limits
This page explains a design method and its constraints; it is not a design calculation or a code-compliance conclusion. It deliberately gives no spacing, flow, pressure-loss or circuit-length values: those depend on the project and must be produced by the responsible designer under the current local rules. The “100 m” figure is cited only as an example of a convention, not as a standard limit.
Sources
- EN 1264 — embedded heating and cooling systems: design and installation framework, current official edition (CEN).
- СП РК 4.02-101-2012, СП РК 2.04-01-2017, СН РК 2.04-21-2004 — Kazakh design framework (adilet.zan.kz, retrieved 1 October 2026).
- Uzbekistan heating reform — closed-loop networks and in-unit heating, hot water and metering from 1 June 2025 (Gazeta.uz, retrieved 30 September 2026).
Frequently asked questions
Can a spacing chart be used before the heat load is known?
Only as a preliminary discussion aid. Final spacing follows the project heat load, floor construction, water temperatures and the destination-country design rules.
Does a larger pipe always solve excessive pressure drop?
No. It changes velocity, friction, water volume, stiffness, bending space and the floor build-up. The full hydraulic and construction check still governs.
Is circuit length limited by pipe strength?
Normally the decision is governed by flow, pressure loss, available pump pressure, controls and balancing. A routine length convention is not a universal material limit.
Who confirms compliance?
The responsible design professional, applying the current local rules and project data. A manufacturer supplies pipes and test documents but does not issue a project design conclusion.
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