Almost every guide to underfloor heating quotes the same formula: floor area divided by spacing, times 1.1 for good measure. It is quick and easy to remember. The trouble is that it leaves out two things that decide whether the loop fits the limit at all - and it is wrong in two opposite directions at the same time.
Where the length limit comes from
The limit is not a convention, it is a consequence of hydraulic resistance. Water flowing through a pipe loses pressure to friction, and the loss grows with length. A circuit that is too long means three things: the pump has to overcome more resistance, the temperature difference between flow and return grows, and balancing that loop against the others on the same manifold becomes difficult or impossible.
That is why every system publishes a maximum circuit length, and why it differs by pipe diameter. Check it in your own system's datasheet - the figures circulating online differ from one another by tens of metres for the same diameter. HeatAlgo uses 80 m (warning) and 90 m (hard limit) for 16x2 pipe, and 90 m and 100 m for 17x2 - always counted together with the run to the manifold, because the water flows through it exactly as it flows through the loop field.
What the formula leaves out
The setback from the walls
A loop is not run right up to the wall. HeatAlgo sets it back by 10 cm, which is what the perimeter expansion joint takes and what the pipe needs to bend without kinking. The field the pipe actually covers is therefore smaller than the room's floor area - and the formula counts the whole area. This error overstates the answer.
The run to the manifold
A pair of pipes runs from the manifold to the room: flow and return. If the manifold stands 15 m away, that is 30 m of pipe that never appears in the loop field at all - and that the formula contains in no form whatsoever. This error understates the answer, and it is usually the larger of the two.
Bend radii
Pipe does not turn a square corner, it turns an arc. The sum of those arcs is a few per cent of the length. It is the smallest of the three corrections, and it is the only one the formula attempts - that is what the 1.1 multiplier is.
How far off it is, measured
A room 3.2 m wide and 4.0 m long, so 12.8 m². The manifold is 15 m away and the run leaves the middle of the shorter wall. The order of the dimensions matters - entered the other way round they give a different loop and different numbers. The "formula" column is (12.8 ÷ spacing) × 1.1; the "pipe in total" column is the result from real loop geometry including the run to the manifold.
| Spacing | Formula | In the room | Run to manifold | Pipe in total |
|---|---|---|---|---|
| 10 cm | 140.8 m | 115.0 m | 29.7 m | 144.8 m |
| 15 cm | 93.9 m | 77.0 m | 29.6 m | 106.6 m |
| 20 cm | 70.4 m | 59.5 m | 29.6 m | 89.1 m |
| 25 cm | 56.3 m | 46.8 m | 29.7 m | 76.5 m |
| 30 cm | 46.9 m | 39.2 m | 29.7 m | 68.9 m |
There is a pattern here that no guide mentions. The formula always overstates the pipe inside the room - by 25.8 m at 10 cm, by 7.7 m at 30 cm. And it always understates the total, because the run to the manifold is missing from it. The two errors point in opposite directions, so sometimes they partly cancel and sometimes they do not: at tight spacing the wall-setback error wins and the formula looks about right, at wide spacing the run wins and the formula is clearly too low.
The row that matters most is 15 cm spacing. The formula gives 93.9 m; the real circuit is 106.6 m - the formula lands 12.7 m short. That is the whole point: 12.7 m is more than the gap between the limits used in practice. Against a 100 m limit the formula says "one loop is enough" and the correct answer is two. Against any other limit the same error can put you on the wrong side of it just as easily - because what is missing is the entire run to the manifold, and here that run is 29.6 m.
The difference is not academic: it is a decision about the number of circuits, taken on site from a number that omits the distance to the manifold.
Work out your own roomHow to choose the spacing
Spacing is not a matter of taste, nor of floor area alone. It follows from how much heat the floor has to deliver into the room. Pipe laid more tightly gives more watts per square metre at the same flow temperature - and conversely, wider spacing needs a higher temperature to deliver the same output.
Hence the practical order:
- Start from the room's heat demand. Without it, choosing a spacing is guesswork. That means a heat load calculation room by room, not a per-square-metre rule of thumb.
- Check the floor can carry that output at the intended flow temperature and with the chosen covering. Tile conducts heat far better than wood or carpet.
- Tighten the spacing in the edge zone, along external walls and under windows where losses are highest. That is the same loop at a varying pitch, not a separate circuit glued around the perimeter.
- Check the length - only now, because only now do you know the spacing.
The lower the flow temperature, the more a tighter spacing pays for itself - and that is the direct link to a heat pump, whose seasonal efficiency rises when it can run cooler. The same output delivered at 30 °C rather than 40 °C is a different electricity bill across a whole winter.
When to split into two loops
Two reasons, and only two.
Length. One circuit including the run to the manifold exceeds the permitted figure. Split the field into two sections of similar area - similar, because two loops of very different length are hard to balance on one manifold.
Shape. L and T shapes, narrow necks, alcoves and obstacles can make a single loop impossible to route without crossing pipes or leaving a patch uncovered. That is not a question of metres, it is geometry. The honest answer there is "split", not "draw something that cannot be laid".
The check at the end
Loop length is not a result in itself. The result is the answer to whether this floor covers the room's demand. So the last step is comparing the loop's output against the room's heat loss from the heat load calculation - and if it falls short, you go back to the spacing or the flow temperature before anything reaches the site.
That is also why the calculator in this article handles one rectangular room and nothing more. A real project is several rooms on one manifold, each with its own heat loss and its own spacing, plus the balance of the whole. L and T shapes, slanted walls, obstacles and multiple floors are handled in the underfloor heating module.
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