Length of solar pipe: how to correctly measure the route from the collector to the tank

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Solar pipe length: why is it important at all?

At first glance, it seems like a trivial question: measure the route from the collector to the storage tank, buy a pipe slightly longer and that's it. In practice, however, it doesn't work this way. Incorrect measurement of the route is one of the most common reasons why customers return the pipe, or why the installer has to improvise on site with an unsuitably short or unnecessarily long material. And in the case of solar systems, it has a direct impact on hydraulics, heat losses, pressure conditions in the circuit and the long-term durability of the entire installation.

A solar circuit is not an ordinary water supply route. It works with a heat transfer liquid at temperatures that can reach 180 to 200 °C at the collector (even more in a stagnation state), pressures in the system typically range between 2 and 6 bar, and the entire route must be designed so that the liquid can circulate without problems, deaerate and thermal expansion does not cause mechanical stress. All of this starts with the correct measurement of the pipe length.

This article will guide you through the entire process: from measuring the route on the roof, through calculating the length for a specific product, to common mistakes from practice and their consequences. If you are looking for a comparison of pipe types or deciding between a flexible hose and a rigid corrugated pipe, also see the article How to choose solar pipe: flexible hose vs. rigid corrugated pipe in this Knowledge Center.

What affects the actual length of the pipe (not just the straight-line distance)

This is a key point that many customers underestimate. The actual length of the pipe is always significantly greater than the direct distance between the collector and the storage tank. There are several reasons for this:

Height change of the route – collectors are on the roof, the storage tank is in a technical room in the basement or ground floor. The vertical part of the route is an additional real length.
Obstacles around structural elements – beams, columns, wall edges, chimneys, ventilation pipes – each obstacle adds length.
Passages through the structure – passages through ceilings, walls and roofs are always made with a certain reserve; the pipe cannot be fixed "exactly on the spot" at the passage site.
Minimum bend radii – stainless steel corrugated pipe is not a hose that you can bend at any angle. Each bend requires a certain radius, which increases the real route length compared to an ideal straight line.
Reserve for connection – at both the collector and the storage tank, you need a free end for connecting fittings; usually 20–40 cm extra on each side.
Expansion loops – for long routes (over 10 m), it is advisable to design at least one expansion loop, which will absorb the thermal expansion of the pipe.

In practice, this means that if the straight-line distance between the collector and the storage tank is 6 meters, the actual pipe length will most often be 10 to 14 meters. This is a rule, not an exception.

Step by step: how to measure the route correctly

The following procedure is based on dozens of completed customer projects – single-family homes, cottages, semi-detached houses and small apartment buildings. The situation is not always the same, but the measurement principle is always the same.

1. Identifying the outlet and inlet points

Start by precisely determining two points: the outlet of the solar medium from the collector (usually the lower collector in a series, or a distributor) and the inlet to the storage tank (heat exchanger in the tank or the solar inlet on the tank). These points are fixed – the entire route is based on them. Also note the elevation of both points, as this information will be important later.

2. Dividing the route into sections

Divide the entire route into measurable sections according to the nature of the route:

Vertical section on the roof – from the collector down to the edge of the roof or to the point of passage through the roof structure
Roof passage / external section – the external part, possibly within the roof cladding
Vertical section inside – from the ceiling/roof passage down to the storage tank
Horizontal section inside – if the pipe runs horizontally along the wall of the technical room

Measure each section separately. The best tool is a tape measure (not a laser distance meter – it measures the air distance, not the actual pipe route length). For vertical sections, measure vertically from the floor to the ceiling/edge, for horizontal sections along the wall or ceiling.

3. Accounting for bends and direction changes

Each 90° turn in stainless steel corrugated pipe with a minimum bend radius (typically 5–8× DN) adds approximately 15–25 cm to the route compared to a sharp geometric corner. If you have four elbows on the route, add 60–100 cm extra. This is an estimate that applies to standard routes – for more complicated routing, the difference will be greater.

4. Reserve for connection

Leave at least 30 cm of free pipe on each side of the route (collector and storage tank). This reserve is used for easy connection of fittings, air vents and possible reconnection after service. In practice, I recommend 40 cm rather than 30 cm, especially for the first installation, when the exact location of the storage tank may not yet be final.

5. Expansion loop

For routes longer than 10 meters, I recommend designing at least one expansion loop – usually a U-shape with a height of 30–40 cm and a width of 20–30 cm. This loop will absorb the thermal expansion of the pipe (stainless steel expands by about 1.8 mm/m at 150 °C compared to 20 °C). This loop is also an extra length – add 80–120 cm for each loop.

A concrete example of calculation from practice

Let's imagine a common situation: a single-family house, collectors on the southwest roof (slope 40°), a technical room in the ground floor under the stairs. The customer estimates the route to be "about 5 meters". After a real measurement, it looks like this:

Section	Description	Length
A	Vertical section from the collector to the roof passage (along the slope)	1.8 m
B	Roof and attic passage (including beam bypass)	2.2 m
C	Vertical drop along the wall in the corridor to the ground floor	2.8 m
D	Horizontal section along the wall to the tank	1.5 m
E	Expansion loop (1 pc, U-shape)	0.9 m
F	Reserve for connection (collector + tank)	0.8 m
Total pipe length		10.0 m

From "about 5 meters" we ended up with exactly 10 meters. For such a route, the ideal choice is Solárna – Flexihadica 2 v 1, 10 m – a twin pipe that runs both branches (supply and return) in one protective sheath, which simplifies the entire installation and reduces the installation time.

In the case where the route comes out to, for example, 12 or 14 meters, there is no other option but to choose Solárna – Flexihadica 2 v 1, 20 m – the excess can always be adjusted during installation, and the remaining pipe can be insulated and brought under the tank or combined with a rigid corrugated pipe.

Height difference between the collector and the tank: why it is important not only for the length

You need to know the vertical position of the collector and the tank for two reasons. The first is purely metric – the vertical distance directly affects the length of the pipe. The second is hydraulic: the pressure difference of the liquid column between the collector and the tank affects the setting of the circulation pump, and in cases of forced flow, it also affects the choice of the pump.

Typical values for a single-family house:

Height of the collector above the tank: 4–8 m (typical single-family house, collector on the roof, tank in the ground floor or basement)
Static pressure difference: at a height of 6 m and a density of 20% propylene glycol solution (~1.02 kg/l), it is about 0.6 bar – not negligible, but standard pumps can handle it without problems

What is important, however: if the tank is placed higher than the collector (for example, on the attic and the tank on the first floor), this must be taken into account when setting up the hydro-pneumatic device and the pressure in the expansion tank. We could write a lot about this – if you want to know more, you will find relevant information in the article Solárna flexihadica 2 v 1: what does the twin pipe design mean and when is it worth it.

Outdoor route vs. indoor route: different measurement rules

Many installations combine an outdoor and an indoor route – for example, the pipe runs along the facade of the house and enters the interior through the wall. In such a case, measuring the route is a bit more complicated, as you need to take into account:

Outdoor part of the route

The pipe is usually run along the facade, consoles, or in a protective sleeve on the outdoor part. Measurement is straightforward – you follow the actual route of the pipe along the facade, including bypassing corners, window frames, etc. Do not forget that the outdoor part of the pipe must be sufficiently resistant to UV radiation and temperature shocks, which is not a problem in the case of stainless steel corrugated pipe, but the insulation must be special (outdoor). More about this in the article Insulation of solar pipe: what not to forget when installing outdoors and indoors.

Wall or ceiling passage

For each passage through the ceiling or roof, add at least 20–30 cm extra (construction thickness plus protective sleeve). In older houses with thick walls (45–60 cm brick), a wall passage can add up to 60–70 cm. It is important to measure it in reality, not to estimate.

Indoor part of the route

Inside the house, you have more flexibility in routing the route, but the same rule applies: measure the actual route along the wall or ceiling, not the air distance. In a technical room with a low ceiling and dense distribution equipment, the route from the ceiling passage to the tank can easily reach 3–4 meters, even if the tank is only 1.5 meters from the wall.

How to measure the route on a sloped roof: specifics

A sloped roof is a special case because the pipe does not run vertically, but at an angle corresponding to the roof slope. If the slope is 35° and the vertical height from the collector to the edge of the gutter section is 2 meters, the actual length of the pipe running along the roof surface is:

Actual length = vertical height / sin(angle of slope) = 2 m / sin(35°) = 2 / 0.574 ≈ 3.48 m

In other words: 2 meters of vertical drop on a 35° roof means almost 3.5 meters of actual pipe length. This is a common mistake – customers measure only the vertical height and then find the pipe is too short. Always measure the actual route along the surface, not the projection.

In practice, you do this by placing the tape directly on the roof surface (or estimating along the line where the pipe will run) and measuring the actual distance from the collector to the roof edge. This issue is irrelevant for flat roofs.

Pipe diameter and its influence on route measurement

The diameter itself (DN12, DN16, DN20) does not directly affect how you measure the route, but it does affect what bend radius you must follow. A larger diameter = a larger minimum bend radius = a longer route in places where the direction changes.

DN12: minimum bend radius approx. 60–80 mm, suitable for shorter routes and smaller systems
DN16: minimum bend radius approx. 80–100 mm – Stainless steel pipe DN16 is the most commonly used diameter for family homes
DN20: minimum bend radius approx. 100–130 mm – Stainless steel pipe DN20 is suitable for larger collector areas (6 or more collectors) or long routes over 15 m

If you are unsure which diameter to choose, visit the article What solar pipe diameter do I need: DN12, DN16 or DN20 in this Knowledge Center – you will find a detailed table according to the number of collectors and route length there.

Common mistakes when measuring the route – and how to avoid them

From my own experience, I know that 80% of problems with incorrect pipe length stem from a few recurring mistakes:

Mistake No. 1: Measuring the air distance

A customer comes to the store and says: "The collector is 4 meters from the tank in a straight line." They buy a 5-meter pipe and on the site find out that they are missing 4 meters. A straight line is never the pipe route.

Mistake No. 2: Forgetting about construction penetrations

Each penetration through the ceiling, wall or roof adds length. They may forget about 20 cm thickness of the structure, but if there are three penetrations, it is 60 cm – which with DN16 is the difference between a tight connection and the impossibility to connect the fittings.

Mistake No. 3: Neglecting expansion loops

The installer installs the pipe without an expansion loop, and within half a year, micro-cracks appear at the point of pipe fixation. An expansion loop requires about 0.8–1.2 m of extra pipe.

Mistake No. 4: No reserve for connection

The pipe is cut "to the last inch" – exactly to the required length. At the first service (unscrewing the fitting, cleaning the filter), it turns out that there is no reserve for reconnection. Always leave 30–40 cm on each side.

Mistake No. 5: Ignoring future changes

The tank is moved, the manifold is added. Pipe cut "to the millimeter" gives no flexibility. Better 50 cm extra than 10 cm short.

How to proceed without direct access: measuring via rope

Not always do you have the possibility to walk the entire route with a tape. For example, on a steep roof where you do not have safe access, or for wall routing in an inaccessible place. In such cases, a proven technique works: rope or ropes.

You take a piece of rope (rope, garden hose, electrical cable – anything that is long enough and flexible), and fix one end at the collector location. Then the rope is led along the same route that the pipe will take – along the roof, through the penetration, down the roof, along the facade, through the wall, along the wall inside the building to the tank. Finally, the rope is cut at the tank location and its length is measured. To the measured result, you add 15–20% as a safety reserve and the result is a reliable estimate of the pipe length.

This method is sometimes more accurate than measuring with a tape, because the rope naturally adapts to all bends and does not depend on the mathematical precision of the slope, angles and so on.

Choosing the right product length after measurement

When you have the final measurement result (including reserves), it is time to decide on a specific product. For most family home installations, the following applies:

Route up to 10 m: Solar – Flexihose 2 in 1, 10 m – covers most single-story houses and cottages
Route 10–20 m: Solar – Flexihose 2 in 1, 20 m – suitable for multi-story houses and houses with basements
Route over 20 m or non-standard layout: usually solved via Stainless steel pipe – corrugated per meter with individual cutting, or a combination

Important notice: do not unnecessarily shorten the 2 in 1 flexi-tube. Any remaining length is always better insulated and safely stored than discarded somewhere. If you have 2–3 meters left, you can use them for an expansion loop or as a service reserve in the technical room.

Thermal expansion: why you must consider pipe length changes

Stainless steel corrugated pipe expands when heated. The linear thermal expansion coefficient of stainless steel is approximately 16–17 × 10⁻⁶ K⁻¹. What does this mean in practice? For a 10-meter pipe that is heated from a winter temperature of –15 °C to an operating temperature of 140 °C (a difference of 155 K), the elongation is:

ΔL = L × α × ΔT = 10 m × 17 × 10⁻⁶ × 155 = 0,026 m = 2,6 cm

This may seem small, but if the pipe is rigidly fixed at both ends without an expansion loop, those 2.6 cm will create stress in the material, which over time leads to metal fatigue and the formation of microcracks – especially at the locations of nuts and connections. That is why an expansion loop is not made "when you feel like it," but always for routes longer than 8–10 meters. For more information on problems caused by poor installation, see the article Common problems with solar piping: overheating, leaks and stainless steel corrosion.

Summary: a quick checklist before purchasing

Before ordering solar piping, go through this quick checklist:

☐ Have I divided the route into measurable sections?
☐ Have I measured the actual route (not the straight-line distance)?
☐ Have I taken into account the roof slope when measuring on a sloped roof?
☐ Have I accounted for every ceiling, wall or roof penetration?
☐ Have I considered the minimum bending radii at each elbow?
☐ Have I included a reserve of 30–40 cm on each side (collector + tank)?
☐ Have I planned for an expansion loop (for routes longer than 8 m)?
☐ Do I know which DN diameter I need?
☐ Is the total length ≥ real route + 15% reserve?

Most frequently asked questions (FAQ)

How do I determine the length of the route if I do not have access to the roof?

Use the rope technique: lower a long rope from the collector location along the same route the pipe will follow (along the façade, through a window, through a wall penetration, etc.) all the way to the tank. Then measure the rope on the ground. Alternatively, you can measure the route in segments from accessible locations (from a window, from a ladder, from the ground) and add the partial values. Always add at least 15% safety reserve.

What if my pipe comes out to 11 meters – should I buy 10 m or 20 m?

Unambiguously 20 m. Short pipe cannot be extended on site without an additional joint, and each joint is a potential weak point. In addition, you will need to buy additional fittings. The leftover longer pipe can be used for an expansion loop or insulated and stored in the technical room as a service reserve. From a cost perspective, having 10 extra meters is cheaper than returning to the store and dealing with complications on site.

Does the length of the pipe change so much when heated that it affects installation?

During installation at cold (ambient) temperature, the pipe is not yet stretched to its operating temperature. This means you are installing the pipe in its "cold" state – it is therefore shorter than it will be in full operation. That is why it is important to leave enough reserve and design an expansion loop: it will absorb the elongation when heated. For a 10 m route and a temperature difference of 130 K, expect an elongation of approximately 2.2–2.6 cm – exactly what a small U-loop can absorb.

Is there a difference in measuring the route for a 2 in 1 flexi-tube compared to separate corrugated pipes?

The principle of measurement is the same – you measure one route, because both tubes (supply and return) run side by side in one braid. The difference is that the 2 in 1 flexi-tube has a slightly larger overall diameter (two tubes + insulation + braid), which can be relevant for tight wall penetrations. Before installation, check the diameter of the penetration – for a standard DN16 2 in 1 flexi-tube, you usually need a minimum 60–70 mm hole. For rigid corrugated pipes (for example, Stainless steel pipe – corrugated), you measure and plan both penetrations separately.

How many extra meters should I always buy as a safety reserve?

Practitioners agree on the rule: add at least 15% to the measured result (including bends and turns), but no less than 1 meter absolutely. For a typical single-family house with a 10 m route, this means buying 12 m or directly 20 m product. Better to have extra pipe than to have it too short – and in the case of a 2 in 1 double pipe, the leftover is always useful.

What happens if the pipe is too long and I do not use it all?

Leftover pipe does not affect the system's functionality in any way. A longer route slightly increases hydraulic resistance and heat loss, but if the pipe is properly insulated, these losses are minimal (in the order of tenths of kW·h per year). A problem would be a pipe that is unnecessarily long by dozens of meters – in that case, the hydraulics would go out of the optimal range. With a normal excess of 3–4 meters, there is no reason for concern. Simply neatly form the leftover into an expansion loop or store it in the technical room.

Conclusion: Measuring the route is the foundation of the entire solar installation

Correctly measuring the length of the solar pipe is the first and most important step of any installation. It is not just a matter of logistics (so you don't have to go to the store twice), but primarily the foundation for the correct hydraulics of the system, safe expansion and long-term reliability of the solar circuit. Invest 30 minutes in thorough measurement and dividing the route into sections – you will save hours of work on site and avoid potential costs for returns or emergency repairs.

If you have doubts about the pipe diameter, type of fittings or insulation method during measurement, you will find answers in other articles in this Knowledge Center. And if you are still unsure about choosing a specific product, take a look at the entire category of solar piping and stainless steel corrugated pipes – products are sorted by diameter and length, so you can easily orient yourself after measuring.

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Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.