Grundfos solar pumps – what they are used for and how they work
Grundfos solar pumps – what they do and how they work
Solar thermal systems have become a common feature in family homes, recreational buildings and commercial buildings in recent years. Whether it is the preparation of hot water, heating support or pool supply, behind every functional solar circuit stands a discreet but key device – the solar circulation pump. And it is precisely here that Grundfos enters with its Solar range, which was designed directly for these demanding conditions. In this article, we will look at what solar pumps actually do, why you cannot simply connect a standard circulation pump to a solar collector, and how to choose the right model for your specific system.
What is a solar circuit and why it needs a special pump
A solar circuit is a closed hydraulic loop that connects solar collectors on the roof with an accumulator or heat exchanger. A heat transfer medium circulates in this circuit – usually a non-freezing mixture based on propylene glycol and water in a ratio of 40 – 60 %. This mixture has significantly different physical properties than pure water: it is denser, has a higher viscosity (especially at low temperatures), a higher boiling point and is chemically more aggressive to common elastomers and brass components.
In addition, solar collectors reach fluid temperatures of 120 – 180 °C on hot summer days, and during stagnation (when the pump is not working or the tank is full), the temperature in the collector can even exceed 200 °C. Such conditions would simply destroy a standard circulation pump designed for a heating system with temperatures up to 95 – 110 °C.
This means that a solar pump must meet several strict requirements:
- Temperature resistance of at least 130 °C (up to 140 °C for short peaks)
- Material compatibility with glycol mixtures and other solar media
- Seals made of EPDM or PTFE, not NBR rubber
- Bearing surfaces capable of operating without oil lubrication (the medium is the only lubricant)
- Resistance to steam-air cavitation conditions during stagnation
- Ability to restore circulation after the circuit is refilled with steam
That is why Grundfos developed the Solar range – pumps that are constructed from the ground up for solar applications and not just "modified" versions of standard circulation pumps.
Grundfos Solar 15-80 and Solar 25-120 – parameters and differences
Grundfos offers two main models in the basic solar range that cover the vast majority of family and smaller commercial installations. Let's look at them in detail:
Grundfos Solar 15-80, 130 mm
Grundfos Solar 15-80, 130 mm is a compact model intended for smaller solar systems. The numbers in the name are not random – "15" indicates the diameter of the flange connection (DN 15, i.e. ½"), "80" expresses the maximum head in tenths of a meter (i.e. 8 meters of water column) and "130 mm" is the axial distance of the connections. This model is ideal for solar systems with 2 – 4 flat collectors or 1 – 2 tubular collectors, which corresponds to a typical family household with 3 – 5 people.
The maximum flow of this pump is approximately 3 m³/h, which at recommended solar collector flows of 15 – 25 l/(m²·h) is sufficient for a collector area of up to about 8 – 12 m². The motor power ranges from 45 – 65 W depending on the speed setting. The pump has three manually switchable speeds, so it is possible to adjust the flow according to the specific system.
Grundfos Solar 25-120, 180 mm
Grundfos Solar 25-120, 180 mm is a larger model suitable for more extensive solar installations. The DN 25 (1") flange allows a higher flow without excessive pressure losses, and the maximum head of 12 m of water column covers longer and more complex pipe runs. The axial distance of 180 mm corresponds to the European standard for larger solar stations.
This model can handle a collector area of up to 20 – 30 m², which corresponds to systems with combined hot water preparation and heating support, or solar systems for a larger family home or apartment building with several units. The motor power ranges from 60 to 100 W, again with three manually selectable speeds.
Both models share typical Grundfos solar pump design features: a rotor made of stainless steel, a pump body made of cast iron with anti-corrosion surface treatment, graphite and ceramic bearing surfaces, EPDM seals and a supply voltage of 230 V / 50 Hz.
How a solar pump actually works – the physics behind it
To understand how a solar pump works, we first need to understand what happens in the solar circuit. Solar collectors absorb solar radiation and transfer it to a heat transfer medium. The pump circulates this medium through the collectors, where it is heated, and through the heat exchanger in the storage tank, where it transfers its energy to the water supply. The cooled medium then returns to the collector – and the cycle repeats.
Grundfos Solar uses wet rotor pump technology – the motor rotor is in direct contact with the heat transfer medium, which also cools and lubricates the sliding bearings. This eliminates the need for a mechanical seal and significantly extends the pump's lifespan. Another advantage of the wet rotor design is quiet operation – the noise level is significantly lower compared to dry rotor pumps, which is important when installed in residential buildings.
The pump operation is controlled by an external solar controller (e.g., Grundfos DMS/DMH or controllers from Resol, Deltasol, etc.). The controller compares the temperature at the collector (sensor T1) with the temperature in the storage tank (sensor T2). If the temperature difference is greater than the set hysteresis (typically 5 – 8 °C), the controller turns the pump on. When the difference drops below the minimum value (typically 2 – 3 °C), the pump is turned off. This simple but effective algorithm prevents the pump from unnecessarily circulating the medium and cooling the tank.
In practice, we also encounter the situation of stagnation – this occurs when the tank is fully charged and the sun is still shining. The temperature in the collector rises above the boiling point of the glycol mixture (at 50% propylene glycol this is about 165 °C at atmospheric pressure), the medium evaporates and the vapors push the liquid back into the expansion tank. The pump must be able to withstand hot vapors without damage – and that is exactly what Grundfos solar pumps are designed for.
Material and structural details that matter in practice
When I have seen systems in practice where customers installed a standard Grundfos UPS circulation pump or a similar standard model instead of a solar pump, the result was always the same: after 1 – 3 seasons, problems began. Torn NBR rubber seals, rusted rotors, seized shaft seals or melted plastic parts in the pump housing. This is not about the poor quality of standard pumps – they are simply not built for these conditions.
Grundfos Solar solar pumps differ in these key structural features:
- Seals made of EPDM (ethylene-propylene-diene rubber): EPDM is resistant to temperatures up to 150 °C and compatible with glycol mixtures. Standard NBR seals decompose at 120 °C when in contact with glycol.
- Stainless steel AISI 316 rotor: Resistance to corrosion caused by degraded solar fluid (as it ages, the pH changes and the fluid becomes more aggressive).
- Graphite-ceramic sliding bearings: They operate reliably even at high temperatures and require no lubrication. They do not use ball bearings with plastic cages, which fail at 130+ °C.
- Cast iron pump housing with epoxy coating: Cast iron withstands thermal shocks better than plastic materials, and the epoxy coating protects against corrosion.
- Maximum operating temperature 130 °C (short peaks 140 °C): This is a manufacturer-guaranteed value for continuous operation, not just a laboratory test.
For comparison: the standard Grundfos UPS 25-30 is an excellent circulation pump for heating, but its maximum operating temperature is 110 °C and the seals are not compatible with glycol. Therefore, it is not suitable for installation in a solar circuit. More about the differences between pump types can be found in the article Grundfos UPS vs. Grundfos solar pumps – which one is suitable for your system in this Knowledge Center.
Sizing – how to choose the right size of solar pump
Selecting the correct size of pump is one of the most frequently underestimated phases of designing a solar system. An undersized pump will not allow sufficient flow, which leads to overheating of the collectors and rapid aging of the heat transfer fluid. An oversized pump, although it circulates the medium quickly, increases electrical energy consumption and shortens the time it takes for the fluid in the collector to heat up – the system efficiency decreases.
Basic rule for sizing: flow rate of 15 – 25 liters per hour per square meter of collector area. At higher flow rates, the fluid does not heat up sufficiently (small delta-T), while at lower flow rates, stagnation conditions occur too early.
Example calculation: A family house has 8 m² of flat solar collectors. The recommended flow rate is 15 × 8 = 120 liters per hour, which is 0.12 m³/h. This number is far below the maximum flow rate of Grundfos Solar 15-80 (3 m³/h), so from the flow rate perspective, even the smallest model is sufficient. The decisive factor is then the head – pressure losses in the system.
Pressure losses are calculated as the sum of:
- Losses in the piping (depends on diameter, length and flow rate)
- Losses in the solar collectors (usually 0.1 – 0.5 bar depending on the type)
- Losses in the storage tank heat exchanger (0.2 – 0.8 bar)
- Losses in fittings, valves and solar station (0.1 – 0.3 bar)
In typical single-family homes with 6 – 10 m of collector pipe (copper DN 18 × 1 mm) and 1 – 2 flat plate collectors, the total pressure loss is 0.3 – 0.7 bar, which corresponds to 3 – 7 m of water column. Solar 15-80 with a maximum pumping head of 8 m WC covers most of these applications.
For larger systems – for example, a combination of TÚV + heating support for a house with 5+ people, collector area 12 – 25 m², longer piping or tubular (vacuum) collectors with higher pressure losses – Solar 25-120 is more suitable. For more information on the correct dimensioning procedure, see the article What Grundfos pump power do I need for my home?.
Installation of solar pump in practice
The solar pump is always installed on the return line of the solar circuit (cold branch), i.e., on the pipe that leads the cooled fluid back from the storage tank to the collectors. There are two reasons for this: first, the temperature of the fluid is lower on the return line, which prolongs the pump's lifespan. Second, the pressure on the return line is higher due to the expansion tank, which reduces the risk of cavitation.
The installation position is horizontal – the pump shaft must be horizontal. Vertical installation with the motor at the bottom is not permitted (the medium would drain the bearings), while vertical installation with the motor at the top is acceptable for most Solar models, but Grundfos recommends a horizontal position for maximum lifespan.
In practice, so-called solar stations (Grundfos Solar Station, Roth, Resol, etc.) are used during installation. These compact units integrate the pump, ball valves, check valve, safety valve, thermometer, flow meter, and pressure gauge in one block. This solution is easier to install, hydraulically balanced, and aesthetically cleaner.
The electrical connection of the pump is simple: 230 V / 50 Hz power supply via a solar controller. The controller switches the pump on and off based on temperature differences (T1 collector − T2 storage tank). The recommended cable cross-section is at least 1.5 mm², and the cable length should not exceed 20 m without increasing the cross-section. The pump does not have its own thermostat or control module – it is a purely mechanical device controlled externally.
If you are interested in a detailed installation procedure including hydraulic connection, air venting, and controller setup, read the article Installation of Grundfos circulation pump step by step in this Knowledge Center.
Maintenance of Grundfos Solar pumps
Solar pumps are designed as low-maintenance devices – in the ideal case, they operate for years without the need for intervention. Nevertheless, there are several steps of preventive maintenance that significantly extend the system's lifespan:
Checking and replacing the heat transfer fluid
This is, according to my experience, the most commonly neglected aspect of solar system maintenance. Propylene glycol ages over time – it oxidizes, changes pH, and loses corrosion inhibitors. The recommended interval for checking the fluid's pH is every 2 years. If the pH drops below 7.0 (the fluid becomes acidic), fluid replacement is necessary, otherwise corrosion of the pump and collectors occurs. A good heat transfer fluid based on propylene glycol should have a pH between 7.5 and 9.0. Fluid replacement is recommended every 5 – 10 years regardless of measurement results.
Checking operating pressure
The solar circuit should have a pressure of 1.5 – 2.5 bar in the cold state (at 20 °C) according to the design. A pressure drop below 1 bar indicates a leak or insufficient filling. Regular visual inspection of the pressure gauge on the solar station will detect problems before they cause pump cavitation. If the pump operates at a pressure below 0.5 bar, immediate cavitation damage is likely.
Visual inspection of seals and connections
Once a year (ideally before the season, i.e., in spring), inspect the visible seals of the solar station, the connecting bolts of the pump, and flexible hoses if used. The heat transfer fluid is quite visibly colored (typically orange or pink), and any leak is noticeable. At the first sign of a leak, the seal should be replaced – aged fluid with low pH is aggressive and a small leak quickly worsens.
Flow and Delta-T test
If the system has a flow meter (which it should), check the flow during a clear sunny day. It should match the design value (typically 15 – 25 l/m²·h). Reduced flow may indicate a partially clogged heat exchanger (deposits from degraded fluid), an air bubble in the system, or an emerging pump issue. Delta-T between the collector and the storage tank should be 5 – 15 °C under normal load.
The complete annual plan for preventive maintenance can be found in the article Maintenance and service of Grundfos pumps – what to check every year.
Solar pumps vs. other Grundfos pumps – what to use where
Customers often ask us whether they can use something cheaper and more readily available instead of a special solar pump. Let’s break it down:
Grundfos UPS 25-30 (view product page) is an excellent circulation pump for heating circuits with temperatures up to 95 °C and clean water or inhibited water with a maximum of 10 % glycol. It is not suitable for solar circuits with glycol and temperatures up to 130 °C – NBR seals can withstand contact with concentrated glycol at 120 °C for only a few months.
Grundfos UNILIFT KP 150 A1 (GRUNDFOS UNILIFT KP 150 A1) and UNILIFT KP 250 A1 (GRUNDFOS UNILIFT KP 250 A1) are submersible water pumps – they belong to a completely different category and are not intended for solar or heating applications. Their place is in pumping rainwater, pumping during floods, draining basements, etc.
The conclusion is clear: use Grundfos Solar for solar thermal circuits, Grundfos UPS or newer Alpha/Magna for heating, and Grundfos UNILIFT for water pumping. Each range was designed for specific conditions and trying to substitute them will eventually cost you more.
Lifespan and economics of solar pumps
A high-quality Grundfos Solar pump has a lifespan of 15 – 25 years with proper installation and maintenance. Power consumption ranges from 45 to 100 W depending on the model and set speed. In a typical operating time of a solar system in Central Europe (1,400 – 1,800 hours per year) and an average power consumption of 60 W, this means an annual consumption of about 84 – 108 kWh, which at an electricity price of 0.20 €/kWh amounts to 17 – 22 € per year. In terms of return on investment, this is not a significant item compared to what a solar system saves on water heating.
For comparison: modernization to a solar pump with an EC motor (electronically commutated motor) – if the system size allows it – can reduce consumption by 40 – 60 %. Grundfos offers various variants in this area, and for new installations we recommend discussing this option with your sales representative.
Frequently asked questions (FAQ)
Can I use a standard circulation pump (e.g. Grundfos UPS) instead of a solar pump?
No, we do not recommend it. Standard circulation pumps are not designed for glycol-based heat transfer media, operating temperatures above 110 °C, or stagnation conditions at 140 – 180 °C. NBR rubber seals degrade, rotors corrode, and overall the lifespan of such a pump in a solar circuit is reduced to 1 – 3 years instead of the standard 15 – 25 years. In addition, warranty conditions from the collector and tank manufacturer may be void when using an unsuitable pump.
What glycol solution should I use for a solar circuit with Grundfos Solar?
Grundfos recommends a heat transfer fluid based on propylene glycol (not ethylene glycol – which is toxic) in a concentration of 40 – 50 % aqueous solution. This concentration protects against frost down to –25 to –30 °C and has a boiling point above 160 °C at atmospheric pressure. The fluid must contain corrosion inhibitors approved for contact with copper, aluminum, and cast iron. Never use automotive antifreeze (ethylene glycol) or unbranded frost-resistant solutions without solar system labeling.
How noisy are Grundfos Solar pumps?
Grundfos Solar pumps belong to the category of quiet wet-rotor pumps – noise levels range around 35 – 42 dB(A) under normal load, which is comparable to a quiet computer fan. In a technical room, this is not a problem. If the pump starts making louder noises (buzzing, knocking), it is a sign of air bubbles in the system or beginning bearing damage – see the article Grundfos pump not pumping or making noise – causes and solutions.
What happens to the solar pump during a long-term power outage?
The pump will stop and stagnation will occur in the collector. The medium will evaporate, pressure in the system will temporarily rise (which is why a safety valve is mandatory), and then the vapor will condense in the expansion tank. After power is restored and the pump is turned on, the system usually resumes circulation on its own – the vapor is pushed back into the collector, where it condenses. Grundfos Solar pumps are designed for such cycles. If jamming occurs repeatedly after restart, the cause is likely an insufficient expansion tank volume or excessively high pre-charge pressure.
Do I need to replace the heat transfer fluid when replacing the pump?
Not necessarily, but it is a good opportunity. When replacing the pump, you must drain and partially empty the circuit – the ideal time to take a sample of the fluid and measure pH. If the fluid is older than 8 – 10 years or has a pH below 7.5, it is reasonable to replace it at this opportunity. This will save you the cost of another intervention in a few years.
What is the difference between Grundfos Solar 15-80 and Solar 25-120 – how to decide?
Basic rule: Solar 15-80 for systems with a collector area up to 10 – 12 m² and total pressure loss up to 6 – 7 m WC. Solar 25-120 for areas of 12 – 30 m², longer pipe runs (over 15 m), vacuum tube collectors (which have higher pressure losses than flat ones), and systems with combined TÚV + heating support. If in doubt, always consult with a designer or sales representative – underdimensioning is worse than overdimensioning, but even a stronger pump is not a cure for a poorly designed system. More selection criteria can be found in the article How to choose the right Grundfos circulation pump for heating.
Conclusion – why investing in the right pump pays off
A solar pump is a relatively small item compared to the overall cost of a solar system – typically 80 – 180 € depending on the model. Despite this, it determines whether your solar system will operate reliably for 20 years, or whether you will be dealing with failures, leaks, and expensive repairs every 2 – 3 years. Grundfos Solar 15-80 and Solar 25-120 are proven solutions with a long history in thousands of installations across Europe. They are not the cheapest on the market, but they are reliable, spare parts are readily available, and the Grundfos service network operates in Slovakia as well.
If you are unsure which model is right for your specific solar system – what is the collector area, what type (flat vs. vacuum), how many meters of piping – do not hesitate to look at other articles in this Knowledge Center. Questions about selection, dimensioning, and common problems are covered in the article Frequently asked questions about Grundfos pumps, which you can find in the same section.
Do you have a question about this topic?
Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.