Sizing solar piping and pump unit – how to do it

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Sizing solar pipe and pump unit – how to do it

When I talk to customers who are considering the installation of a solar system for domestic hot water heating, I quickly notice one thing: most of them carefully select collectors, compare storage tanks, and think about control – but they treat the pipe and pump unit as a given, something they "just buy". Yet it is precisely here that most of the problems arise, which service technicians encounter during operation. Underdimensioned pipe, too weak a pump, poor hydraulics – all of this leads to the system working, but not with the efficiency for which you paid. This article will explain in detail how to size the pipe and pump unit, what physical laws are behind it, and what it specifically means for a typical family home.

Why pipe sizing is key to the overall efficiency of the system

A solar system is essentially a hydraulic circuit through which a heat transfer fluid – usually a mixture of water and glycol – flows from the collector to the storage tank and back. The power generated by the collector must be removed quickly enough; otherwise, the collector stagnates, the fluid overheats, and the system is shut down by a thermal fuse or a pressure relief valve. On the other hand, if the fluid flows too quickly (overpressurized circuit), the pump works inefficiently, consumes unnecessary electrical energy, and hydraulic resistance disrupts the balancing of the entire system.

Experience shows that the most common problem is not underdimensioning of collectors (where people are cautious), but underdimensioning of the pipe. I have seen installations where the customer saved money and used a thinner pipe with a diameter of 12 mm, where it should have been 16 mm – and the result? The pump struggled with resistance, the delta T on the collector was 40 °C all morning instead of the standard 8–12 °C, and the tank heated up to 60 °C just before sunset. The energy loss was estimated at 25–30 % of the potential performance over the season.

Basic quantities we work with

Before we move on to the actual calculations, let's establish the terminology and key physical quantities we will be working with:

Flow rate (q) – the volume of heat transfer fluid flowing through the pipe per unit of time, given in liters per minute (l/min) or liters per hour (l/h). In solar systems, we most often work with values of 0.5 – 3 l/min per one collector.
Flow velocity (v) – the speed of the fluid in the pipe, given in m/s. The recommended value for solar pipes is 0.3 – 0.8 m/s. Below 0.3 m/s, sedimentation of impurities and air pockets is a risk; above 1 m/s, noise and pipe wear occur.
Pressure loss (Δp) – the resistance of the pipe to fluid flow, given in Pa or kPa. It consists of friction against the pipe walls and local resistances (bends, fittings, valves, collector, heat exchanger in the tank).
Pump head (H) – the height of the fluid column the pump can overcome, given in meters of water column (m.w.c.) or in kPa (1 m.w.c. = 9.81 kPa ≈ 10 kPa).
Viscosity – depends on the composition and temperature of the fluid. A glycol mixture has significantly higher viscosity than pure water at low temperatures – this increases pressure losses and must be considered in sizing.

Sizing the pipe diameter – step by step

The correct procedure for sizing the pipe starts with determining the flow rate and the required flow velocity. From these, we calculate the minimum internal pipe diameter. We then choose the next larger standard size.

Step 1: Determining the flow rate

The flow rate of the solar circuit depends on the number and area of the collectors. Professional practice works with a "specific flow rate", i.e., the volume of fluid flowing per square meter of collector area per hour. Recommended values range from 30 – 50 l/(m²·h), and for most common flat collectors, we choose 40 l/(m²·h) as a good compromise between efficiency and pressure losses.

Example: A family house, 2 pieces of flat frame solar collector IVAR.SOLAR 210 M5, each with an area of 2.09 m². Total collector area: 2 × 2.09 = 4.18 m².

Flow rate: q = 40 l/(m²·h) × 4.18 m² = 167 l/h = 2.78 l/min

Thus, the pump must ensure a flow rate of approximately 165–170 l/h.

Step 2: Calculating the internal pipe diameter

From the flow rate and the desired flow velocity, we calculate the required internal diameter (d) using the formula:

d = √(4 × q / (π × v))

where q is the flow rate in m³/s and v is the velocity in m/s.

For our example: q = 167 l/h = 0.0000464 m³/s, target velocity v = 0.5 m/s

d = √(4 × 0.0000464 / (3.14 × 0.5)) = √(0.0001856 / 1.57) = √(0.0001182) = 0.01088 m ≈ 10.9 mm

This means that the internal diameter must be at least 10.9 mm, which corresponds to a pipe with an outer diameter of 15 mm (typical copper pipe 15×1 mm with an internal diameter of 13 mm) or 16 mm stainless steel corrugated pipe (internal diameter approx. 12.5 mm). Using a pipe with a diameter of 12 mm (internal ≈ 10 mm) would be on the edge, and flow velocities would exceed 0.6 m/s even with 2 collectors.

For family solar systems with 2–4 collectors, an outer diameter of 16 mm is standard – which exactly corresponds to the product stainless steel pipe with rubber insulation – double, 2× pr. 16, 10 m. This pipe is pre-installed – it contains both circuits (supply and return) in one insulated sheath, which significantly simplifies installation and minimizes heat losses.

Step 3: Calculation of Pressure Losses

Pressure loss is the sum of linear losses (friction in pipes) and local losses (fittings, valves, collector, heat exchanger). For pump sizing, we need the total pressure loss of the circuit.

Linear losses are calculated according to the Darcy-Weisbach equation. In practice, for a quick orientation, we use so-called specific pressure losses R (Pa/m), which are tabulated for different diameters and flows. For copper or stainless steel corrugated pipe DN 16 at a flow rate of 170 l/h, a typical value of R is around 60–100 Pa/m.

Example calculation: Pipe length from the collector to the storage tank (both ways) = 2 × 12 m = 24 m. Specific loss 80 Pa/m → linear loss = 24 × 80 = 1 920 Pa = 1.92 kPa.

Local losses are most easily estimated as a multiple of linear losses. For systems with 2 collectors and simple hydraulics: local losses ≈ 50–80 % of linear losses. In practice, therefore: 1.92 × 0.65 = 1.25 kPa.

Collector resistance: From the technical sheet of the IVAR.SOLAR 210 M5 collector, we read the pressure loss at nominal flow – typically 1–3 kPa per one collector. For 2 collectors connected in parallel, it is the same value as for one collector (parallel connection does not increase resistance at the same flow to each collector). Let's assume 2.5 kPa.

Storage tank heat exchanger resistance: From the tank catalog – typically 2–5 kPa. Let's assume 3.5 kPa.

Pump unit resistance (valves, ball valves, check valve, flow meter, safety valve): we estimate overall 2–4 kPa. Let's assume 3 kPa.

Total pressure loss: 1.92 + 1.25 + 2.5 + 3.5 + 3.0 = 12.17 kPa ≈ 12.2 kPa = 1.24 m.w.s.

This is the minimum head that the pump must overcome. For safety, we choose a pump with a reserve of 20–30 %, so we need a pump with a head of at least 1.5–1.6 m.w.s. at a flow rate of 170 l/h.

Selection of the pump unit – what it includes and what to watch for

A solar pump unit is not just a pump. It is a complex that integrates all the functions needed for safe and efficient operation of the solar circuit:

Circulation pump with speed control (usually 3-speed or EC motor)
Pressure relief valve (protection against overpressure – typically set at 6 bar)
Thermometer on the supply and/or return pipe
Flow meter – allows setting the nominal flow and visually checking the circulation
Check valve – prevents gravitational circulation at night
Ball valves – for service disconnection
Drain valve – to remove air from the circuit
Connection for pressure gauge and filling valve
In a better version also a controller with a temperature sensor

A typical example of such a complex unit is the IVAR.SOLAR K pump unit with regulation IVAR.SOLAR IMTDC. This unit integrates a differential controller that controls the pump start based on the temperature difference between the collector and the storage tank – precisely the function you will read more about in the article Sensors and control of the solar system – how they work and when to replace them.

How to choose the right size of the pump unit

The selection of the pump unit is guided by two parameters that you must find in its characteristic (Q-H curve):

Flow Q – must correspond to the calculated flow of the circuit. In our example, 170 l/h.
Head H – must overcome the total pressure loss. In our example, at least 1.5 m.w.s. at the corresponding flow.

The operating point of the pump must lie on the Q-H curve of the pump for the given speed level. Ideally, so that the pump is set to the middle speed at nominal flow and has a reserve upwards (for the case of clogged filter, higher viscosity at cold start, etc.).

In practice, for family homes with 2–4 collectors (4–8 m² area), most standard solar units with a pump of the Grundfos UP 25-40 class or equivalent, i.e., with a maximum flow of about 2–3 m³/h and a maximum head of 4–6 m.w.s., are sufficient. These parameters are significantly excessive – the operating point lies comfortably in the middle range of the curve, which is desirable.

Effect of glycol on sizing – what changes with a water/glycol mixture

The heat transfer fluid in the solar circuit is not pure water, but a mixture of water and propylene glycol, usually in a ratio of 1:1 (50 % glycol = protection up to about −28 °C). This mixture has different physical properties than pure water:

Density: ~1060 kg/m³ (about 6 % higher than water)
Specific heat capacity: ~3600 J/(kg·K) (about 14 % lower than water)
Viscosity at 20 °C: about 4–5× higher than water, up to 10× higher at 0 °C

The lower specific heat capacity means that for the same heat transfer, you need a higher flow rate (about 14–20 % higher). The higher viscosity increases pressure losses significantly – especially with cold fluid (spring, autumn, morning). Therefore, always use correction factors for glycol mixtures in calculations, not the values for pure water.

Specifically: if you calculated the flow rate for pure water at 160 l/h, increase it to 180–190 l/h for a glycol mixture. Pressure losses in the piping can be 1.5–2× higher at cold start – so the pump must have sufficient performance reserve to start the system even after a cold night.

Collector connection – series vs. parallel and its impact on hydraulics

The way collectors are connected to each other significantly affects the hydraulics of the entire system:

Parallel connection

Each collector is connected separately between the common supply and return. The flow is evenly distributed among the collectors (with symmetric connection). The pressure loss in the collector section equals the pressure loss of one collector (not the sum). The advantage is low resistance, the disadvantage is the need for hydraulic balancing – each collector must receive the same amount of fluid, which may not be achieved with asymmetric piping.

Series connection

The fluid passes through the collectors one after another. The pressure loss is the sum of the resistances of all collectors. The fluid is heated in each collector, so it enters the second collector warmer – this reduces its efficiency. For standard installations, a series connection is recommended only for 2 collectors, with 3 or more collectors the resistance is too high and the temperature differences are unacceptable.

In practice, for most single-family homes (2–4 collectors), we choose a parallel connection with hydraulic balancing – the common "reverse return" (Tichelman). More about this can also be found in the article Installation of a solar system step by step – from the collector to the storage tank.

Thermal insulation of piping – why it matters less than you think

Thermal insulation of solar piping is a topic that is often underestimated in practice. The heat transfer fluid travels from the collector to the storage tank at temperatures of 60–90 °C, with a path that can be 6–15 meters long. Without insulation, you would lose part of the heat to the environment before it reaches the storage tank – and in winter, the piping would draw heat away so quickly that the pump would not be able to maintain a reasonable delta T.

The recommended insulation thickness for solar piping is at least 20 mm of rubber or mineral insulation with a temperature resistance of at least 150 °C (during stagnation, the temperature in the circuit can briefly exceed 120 °C). Rubber insulation (e.g., Armaflex HT) is standard for these applications – it is temperature-resistant, UV-resistant (for external routing), and mechanically flexible.

The product stainless steel pipe in double rubber insulation, 2× DN 16, 10 m solves insulation right at the installation stage – both circuits are insulated and twisted into one compact hose. It also includes an electrical cable for the sensor or pump. This significantly shortens the installation time and eliminates the risk of unsuitable insulation from other suppliers.

Sensors as part of the hydraulic system – where to place them and why

Control of the solar system is based on temperature – specifically, on the temperature difference between the collector (T1) and the bottom of the storage tank (T2). The pump starts when T1 is a set Δt higher than T2 (usually 6–10 °C), and stops when the difference drops below 2–4 °C. Therefore, the correct placement of sensors is just as important as the correct sizing of the piping.

The sensor on the collector must be placed in a well on the absorber plate or in the collector pipe at the outlet. It must withstand temperatures up to 200 °C (stagnation). For this purpose, for example, temperature sensor for well – 180 °C, 2 m cable is used – specifically designed for collector applications with extremely high temperatures during stagnation.

The sensor in the storage tank is placed in a well in the lower part of the tank (1/3 of the height from the bottom), where the coldest water is. This ensures that the pump runs even when the lower layer of the tank is not yet heated, which maximizes the total energy extracted from the collector. For storage tank applications with lower temperature requirements, sensor for well – PVC cable 4 m, 95 °C is used – its 4-meter cable is practical for larger tanks and greater distances from the controller.

More about the correct placement and function of sensors can be found in the article Sensors and regulation of a solar system – how they work and when to replace them.

Typical mistakes in dimensioning – from practical installation experience

Over the years of practical experience, I have seen these recurring mistakes:

Undersized pipe – the most common mistake. The installer chooses a 12 mm pipe "because it's cheaper and easier to lay" and the result is high pressure loss, the pump running at full capacity, and a high temperature gradient on the collector. The system works, but on hot days it stagnates sooner than it should.
Too long a series connection of collectors – three collectors in series = huge resistance, the pump is drowning in work, the third collector operates at such a low efficiency that it could just as well be omitted.
Insufficient insulation – thin mineral wool instead of 20 mm rubber, or no insulation at all through unheated areas (attic, external wall). Measured heat losses on a long uninsulated pipe in winter can swallow up 10–15 % of the daily production of the collectors.
Incorrect placement of the tank sensor – the sensor in the middle or in the upper part of the tank. Result: the pump turns off too early (the top layer is warm, but the bottom is not yet) and the tank is heated to 60 °C only partially.
Forgotten hydraulic balancing – in a parallel connection of 3–4 collectors without balancing, most of the fluid flows through the first collector, the others are undersized. This is manifested by uneven temperatures at the outputs of individual collectors.
Overpumped circuit – flow set to maximum, pump on 3rd speed level. Result: delta T is 3–4 °C, the fluid in the tank heat exchanger does not have time to transfer heat and efficiency drops. The optimal delta T is 6–12 °C – that's why the flow meter is in the pump unit.

Practical dimensioning process from the beginning – summary for the installer

For those who want to have the whole process in one place:

Determine the number and area of collectors → calculate the total area Ac [m²]
Calculate the flow: q = 40 × Ac [l/h], increase by 15 % for glycol mix
Choose the target flow velocity 0.4–0.6 m/s
Calculate the minimum internal diameter and choose the nearest standard pipe size
Measure the total length of the pipe (supply + return)
Calculate linear pressure losses (R × L), estimate local losses (50–80 % of linear)
From the collector and tank catalogs, add their resistances at nominal flow
Add the resistance of the pump unit (~2–4 kPa)
Total pressure loss = operating point, choose the pump according to it (add 20–30 % reserve)
Check that the operating point lies on the Q-H curve of the pump in the middle third (not at the end or beginning of the curve)
Verify the placement and type of sensors according to temperature requirements

Frequently asked questions (FAQ)

Can I use regular copper pipe instead of corrugated stainless steel?

Yes, copper pipe (e.g. Cu 15×1 or Cu 18×1) is fully compatible with glycol-based heat transfer fluids and is commonly used in solar systems. It has a smooth inner surface, which means less friction than corrugated stainless steel pipe of the same diameter. On the other hand, corrugated stainless steel pipe is flexible – easier to run through ceilings and walls, and the pre-insulated double version with cable significantly shortens the installation. The choice depends on the layout solution – copper is cheaper for straight runs, while corrugated stainless steel saves time and money for complicated routes (attics, wall penetrations).

How important is the accuracy of the calculation? Isn't an estimate enough?

For small systems (2–3 collectors, short runs up to 10 m), the deviation from the optimal point is usually tolerable, because solar pump units are designed with a performance reserve. For long runs (15 m and more), a large number of collectors (4+) or series-parallel assemblies, an accurate calculation is worthwhile – it would be a pity to discover after installation that the pump is insufficient, or conversely, that it is running unnecessarily at full capacity for 8 hours a day and costing you tens of euros extra on electricity annually.

On which speed levels should a standard pump operate?

Most solar systems for family homes operate optimally on the 2nd speed level. The first level usually does not overcome the resistance at full flow sufficiently, the third level overpumps the circuit and reduces delta T. If the pump is equipped with an EC motor with smooth regulation, the regulator automatically adjusts the speed according to delta T. This is the preferred option for modern systems – it consumes less electricity and the system is smoother.

What happens if the pipe is undersized (small diameter)?

Higher flow velocity → significantly higher hydraulic resistance (resistance increases with the square of velocity) → the pump operates in an unsuitable operating point, or does not reach the required flow at all. Result: the collector overheats, stagnation occurs, the safety valve vents steam, the system loses glycol mixture. Repeated stagnation leads to glycol degradation, carbonization and damage to seals. The system must be refilled, cleaned and in the worst case the pump must be replaced.

Is hydraulic balancing necessary for 2 collectors in a parallel connection?

With two collectors with a symmetrical connection (equally long supplies) the balancing is automatic – provided the collectors are of the same type and area. If one pipe is visibly longer than the other, or if the collectors are not identical, we recommend setting the regulating valves according to the flow meter in the pump unit. With three or more collectors, hydraulic balancing is always necessary – the most reliable solution is the "Tichelman" connection (reverse return).

What is the correct operating pressure in the solar circuit?

At a cold system (20 °C) the pressure should be 1.5–2.5 bar depending on the height of the collectors above the pump unit (add +0.1 bar for each meter of height plus 0.5–1 bar static reserve). At a hot circuit (80 °C) the pressure increases by 0.5–1.5 bar due to thermal expansion – therefore the expansion tank must be correctly calculated and the safety valve is typically set to 6 bar. Underpressure (pressure below 1 bar) even at a hot circuit indicates a lack of fluid or a problem with the expansion tank.

Conclusion – dimensioning is worth doing properly

Dimensioning of the pipe and pump unit of a solar system is not some mysterious science. It is a concrete calculation with clear inputs and outputs – and the result is directly reflected in how much hot water your system produces and at what cost. A properly dimensioned system runs quietly, efficiently and with minimal heat losses; the pump is at a reasonable point on the performance curve, delta T on the collector is in the optimal range of 8–12 °C and the tank is heated evenly throughout the usable part of the day.

If you are working on a project from the beginning, start with the selection of collectors (the topic is thoroughly discussed in the article What solar collector power do I need – calculation according to the number of people and consumption), and then proceed to dimensioning the pipe and pump according to the procedure described in this article. The resulting system will serve you for 20 or more years – and every extra percent of its performance represents real money saved every summer season.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we'd be happy to help.