How to choose the right Grundfos circulation pump for heating
How to choose the right Grundfos circulation pump for heating
The circulation pump is the heart of every heating system. Even the most expensive boiler and the best quality radiators are almost useless if the water does not circulate properly in the system – either too slowly and the rooms do not warm up, or too fast and the pump unnecessarily draws current, makes noise and wears out quickly. From practice, I know that choosing a pump is a topic where customers make surprisingly frequent mistakes – usually either underestimating the power, or (just as common) buying an unnecessarily oversized pump because they think "more power is always better." In this article, we will go through everything you need to know: from basic concepts through the selection of a specific model to practical scenarios from real customer cases.
What a circulation pump actually does and why its selection matters
A circulation pump (also called a circulator pump) ensures continuous movement of hot water from the boiler to the radiators and back. Without it, heat would spread only by natural convection – slow and uneven. In modern closed-loop heating systems, the pump is an essential component. A properly selected pump ensures that each radiator receives enough hot water at the right time, temperature regulation will be accurate and energy consumption will remain acceptable.
Grundfos is a Danish brand that is globally among the leaders in pump manufacturing. Their product portfolio is extensive – from simple fixed-speed pumps for small apartments to sophisticated frequency-controlled units for large buildings with multiple circuits. If you don't know which model to choose, you are not alone – that is exactly why this guide was created.
Basic concepts you need to know before choosing
Before we get into specific models, it is essential to understand three basic parameters, without which pump selection makes as little sense as buying a car without knowing how many people you will carry in it.
Flow rate (Q) – how much water the pump will move
Flow rate is given in liters per hour (l/h) or cubic meters per hour (m³/h). It determines how much hot water the pump will move through the circuit per unit of time. Too low a flow rate means that radiators far from the boiler will remain cold, while too high a flow rate causes noise and energy waste. For a typical family home with a heating capacity of around 10–15 kW, the usual flow rate is somewhere between 600 and 1,200 l/h.
Head (H) – what resistance the pump can overcome
Head is given in meters of water column (m w.c.) and expresses what hydraulic resistance the pump can overcome. It is not about the physical height of the building, but about the total sum of resistances – friction in the pipes, elbows, valves, thermostatic heads, boiler, heat exchanger. The longer the piping, the more branches and regulating elements, the higher the required head. For a typical family home, the calculation usually results in 2 to 6 m w.c.
Power consumption (P) – how much electrical energy the pump consumes
Older fixed-speed pumps had a power consumption of typically 60–100 W. Modern energy-efficient pumps of class A (e.g., Grundfos Magna or Alpha) consume only 5–25 W at the same performance. The difference in electricity costs over 10 years of operation is in the tens of euros – sometimes even more than the cost of the pump itself.
Overview of Grundfos pump types for heating
Grundfos produces several product lines intended for different applications. It is important to understand what each of them offers so that you can choose the right one – buying a submersible pump instead of a circulation pump is a basic mistake, but it happens.
UPS series – classic circulation pumps for heating
The UPS series is historically the most widespread line of Grundfos circulation pumps. These are fixed-speed three-speed pumps that have manual power switching (1st, 2nd, 3rd speed). They are simple, reliable and affordable. The disadvantage is that they do not have automatic regulation – they always operate at the set speed regardless of the system's current needs.
A typical example from this series is the GRUNDFOS UPS 25-30, a compact circulation pump with an axial distance of 180 mm, suitable for small to medium heating systems. The number "25" indicates the connection diameter in mm (DN25 = 1 inch), the number "30" indicates the maximum head in tenths of a meter – that is 3.0 m w.c. at zero flow. It is a pump that can be found in tens of thousands of Slovak households precisely because of its reliability and simple replacement for an older model.
Solar series – pumps for solar collectors
Solar pumps from Grundfos are specially designed for pumping the heat transfer medium in solar collector systems. The medium in the solar circuit is not pure water – it is a mixture of water and antifreeze (propylene glycol), which has different viscosity and chemical properties. A regular circulation pump would not last much longer in such an environment.
From the selection on atria.sk, for example, the GRUNDFOS Solar 25-120, 180 mm are available for larger solar installations (maximum head up to 12 m w.c.!) and the more compact model GRUNDFOS Solar 15-80, 130 mm for smaller collector areas. Learn more about their use and differences from standard circulation pumps in the articles Grundfos solar pumps – what they are used for and how they work and Grundfos UPS vs. Grundfos solar pumps – which one is suitable for your system.
UNILIFT KP series – submersible sewage pumps
It is important to stop here and explain one thing that causes confusion: UNILIFT KP are not circulation pumps for heating. These are submersible pumps designed for pumping water – from pits, basements, floods, or pools. I mention them here because they are found in the same category on atria.sk and some customers mistakenly consider them to be heating pumps.
Specific models such as GRUNDFOS UNILIFT KP 150 A1 and GRUNDFOS UNILIFT KP 250 A1 with 10 m cable have a power of 150 W and 250 W respectively and are intended for pumping clean or slightly contaminated water during emergencies or construction work. They absolutely do not belong in a heating circuit – they are designed for unidirectional flow, not for a closed pressure circuit.
How to calculate the required pump power
This is where most people make a mistake – either they guess or choose a pump "like the neighbor had". The correct approach is to calculate the required flow and head for a specific system. A detailed methodology with a specific formula can be found in the article What Grundfos pump power do I need for my house, but here we will explain the basic principle.
Calculation of required flow
The required flow is derived from the boiler's output (or the heating system's output) and the temperature difference between the supply and return. The basic formula is:
Q [l/h] = P [W] × 0.86 / ΔT [°C]
where P is the thermal power in W and ΔT is the temperature difference between the supply and return. For a classic heating system with 75/60 °C, ΔT = 15 °C. For a low-temperature floor heating system with 40/30 °C, ΔT = 10 °C.
Practical example: A family house with a 15 kW boiler (= 15,000 W), temperature difference 75/60 °C (ΔT = 15 °C):
Q = 15,000 × 0.86 / 15 = 860 l/h ≈ 0.86 m³/h
For floor heating with the same boiler and a temperature difference of 40/30 °C:
Q = 15,000 × 0.86 / 10 = 1,290 l/h ≈ 1.29 m³/h
Do you see why floor heating requires a higher flow – and thus a more powerful pump?
Calculation of required head
The head depends on the hydraulic resistance of the entire system. For an approximate calculation, the rule is: calculate 100–150 Pa resistance per meter of pipe (in both directions – supply and return). Add the resistance of fittings, valves, and the boiler according to the technical documentation.
Example: A house has the longest circuit of 25 m, so the total pipe length in both directions = 50 m. At 120 Pa/m:
Pipe resistance = 50 × 120 = 6,000 Pa = 0.6 m w.c.
Boiler resistance (according to the catalog) = 2,000 Pa = 0.2 m w.c.
Valve and control resistance ≈ 3,000 Pa = 0.3 m w.c.
Total ≈ 1.1 m w.c.
In this case, a pump with a head of 2 m w.c. at the required flow would be sufficient. For the UPS 25-30 (maximum head 3.0 m w.c.), this task is manageable.
Practical scenarios for pump selection – examples from real customer orders
Theory is fine, but in practice, you have to choose between specific products. Here are four typical situations I encounter most often:
Scenario 1: Replacing an old pump in an apartment
A customer from a panel house, 60 m² apartment, with a private circuit and a 6 kW electric boiler. The old pump has jammed and needs to be replaced with something reliable and affordable. The old pump's shaft distance is 180 mm, connection DN25.
Calculation: Q = 6,000 × 0.86 / 15 = 344 l/h. The circuit is short, about 20 m. The required head will be less than 2 m w.c. Ideal choice: GRUNDFOS UPS 25-30 – at a head of 2 m w.c., the flow is approximately 700 l/h (according to the characteristic curve), which is more than sufficient. Installation is straightforward, an installer can replace it in 30 minutes.
Scenario 2: New construction of a family house with floor heating
A family house 180 m², air-to-water heat pump 12 kW, floor heating in all rooms, temperature difference 35/28 °C (ΔT = 7 °C). Several zones with their own control valves.
Calculation: Q = 12,000 × 0.86 / 7 = 1,474 l/h ≈ 1.5 m³/h. The hydraulic resistance of an extensive distribution system with control valves will be higher – we estimate 4–5 m w.c. Here, the UPS 25-30 is not sufficient. We need something more powerful – for example, Grundfos UPS 25-60 or even better, an energy-efficient pump from the Alpha series with electronic control. In such systems, the investment in a smarter pump pays off in 3–5 years due to lower consumption.
Scenario 3: Solar system with 4 collectors for domestic hot water heating
The customer has 4 flat collectors (total area 8 m²) connected to a 300-liter tank. The heat transfer medium is 40% propylene glycol. The longest circuit (collectors – tank – back) is about 30 m.
Here it is crucial that it is not water but a glycol mixture – the viscosity is higher, and therefore the resistance in the pipes is also higher. At the same time, temperatures in summer can rise to 90–100 °C and even higher during short outages. A standard circulation pump would not survive such conditions for long. The right choice: GRUNDFOS Solar 25-120 – special seals, resistance to high temperatures and glycol-based media, a head of up to 12 m w.c. is sufficient even for larger installations.
Scenario 4: Replacing a pump in a boiler room of an apartment building
An apartment building with 12 apartments, a shared boiler room, a 80 kW gas boiler, and an old heating system with cast iron radiators. The longest circuit is 80 m. The original pump had an axial distance of 280 mm and a DN40 connection.
Flow calculation: Q = 80 000 × 0.86 / 15 = 4 587 l/h ≈ 4.6 m³/h. Here we have reached a level where small UPS pumps are no longer sufficient – you need a larger pump from other Grundfos series (CM, TP or Magna3). Such orders are beyond the scope of this article, but the reason is clear: each system has its own parameter envelope and you need to hit the right spot.
What to pay attention to when choosing – common mistakes from practice
Over the years I have seen many poorly designed heating systems where the pump was the first cause of problems. Here are the most common mistakes:
Mistake No. 1: Selecting based on boiler power without considering the system
Many customers come with the information "I have a 20 kW boiler, what pump do I need?" – but without information about the type of heating system, the length of the piping, the number of circuits, or the type of control. For floor heating with the same boiler, you need 2–3× more flow than for a radiator system. Dimension the pump according to the system, not the boiler.
Mistake No. 2: Ignoring the center distance
The center distance (the distance between the centers of the connections) must fit exactly. Standard dimensions are 130 mm and 180 mm. If you replace a pump with a different dimension, you will end up with extension nipples, leaks, and problems. Always measure the center distance of the old pump before ordering or check the technical data sheet.
Mistake No. 3: Forgetting to bleed the system
The new pump may be running, but the system is not heating. The cause? Air in the pipes. Every pump replacement is a good opportunity to bleed the entire system. Without this, the pump may run "dry" and be flooded with air, which can cause overheating of the bearings. More about this issue can be found in the article Grundfos pump not pumping or making noise – causes and solutions.
Mistake No. 4: Incorrect pump speed setting
With UPS pumps, many people leave the pump on the third (highest) speed and don't care. Result: unnecessary electricity consumption, noise, and sometimes system instability (cavitation, bypassing through thermostatic heads). I recommend setting the speed so that the return temperature is at least 10 °C lower than the supply temperature – this is a sign that the system is operating efficiently.
Mistake No. 5: Confusing a circulation pump with a drainage pump
As mentioned above – submersible pumps UNILIFT KP are intended for drainage, not for circulation in a closed loop. They are unidirectional, open (do not operate under pressure), and do not tolerate continuous operation with hot water. Installing them as a circulation pump would cause them to fail quickly and potentially also damage the entire heating system.
Energy efficiency – why class A matters
Since 2013, the EU has had regulations that gradually remove pumps with low energy efficiency from the market. Since 2015, new circulation pumps must meet a minimum energy efficiency index EEI ≤ 0.23. In practice, this means that classic three-speed pumps from older generations of UPS are increasingly being replaced by frequency-controlled models.
The difference in consumption is really significant. An old model with a power of 80 W operating for 5,000 hours per year (typically from September to April plus transitional periods) consumes 400 kWh/year. At a cost of 0.20 €/kWh, this is 80 €/year just for the pump. A modern energy-saving pump with EEI ≤ 0.20 consumes only 50–80 kWh/year over the same period, saving 60–70 €/year. Over 10 years, you save 600–700 € – a sum greater than the cost of the pump itself.
Installation and Mounting – What Must Be Met
Choosing the right pump is only half the success. Equally important is the correct installation. Here are the key requirements that must be followed without exception:
- Pump orientation: The motor shaft must always be horizontal. Installation with a vertical shaft (motor up or down) shortens the bearing life and can cause noisy operation. The water flow direction can be vertical or horizontal, but the motor axis = horizontally.
- Sealing and threads: Always use new seals (O-rings or flat rubber seals depending on the type of connection). Never seal internal threads with Teflon tape – this can cause the pump casing to crack when tightened.
- Air vent valve: The pump must be vented before the first start. Grundfos UPS has a vent screw on the side of the motor – loosen it, wait until water flows out without bubbles, then tighten it.
- Electrical connection: Always connect the pump to a protected circuit with a 10 A circuit breaker. Earthing is mandatory. Never run the pump "dry" – this will immediately damage the bearings.
- Filtration and dirt traps: Always install a fine filter (minimum 300 μm mesh) before the pump (on the inlet). Dirt in the system is the main cause of premature bearing wear.
The complete installation procedure can be found in the article Mounting a Grundfos Circulation Pump – Step by Step and for UNILIFT models in the article How to Connect a Grundfos UNILIFT Pump – Installation Procedure.
Grundfos Pump Maintenance – What to Do Every Year
Grundfos pumps are generally very reliable and require almost no maintenance under normal conditions. Nevertheless, there are a few things I recommend checking at least once a year – ideally in spring after the heating season ends:
- Check if the pump is making unusual noise (clicking, metallic scraping, strong vibrations) – this may indicate bearing wear or a foreign object in the impeller.
- Inspect the connections and seals – moisture or water droplets around the pump are signs of a developing leak.
- Check the pump body temperature after one hour of operation – an overly hot body (much warmer than the system water) may indicate a cooling or bearing issue.
- If the pump starts up hard or not at all after the summer break – see the article Common Grundfos Pump Faults and How to Fix Them.
- We recommend a preventive pump replacement every 5–7 years – especially for older models with mechanical seals instead of ceramic ones.
More about annual service can be found in the article Grundfos Pump Maintenance and Service – What to Check Every Year.
Decision Table – Which Grundfos Model for Your Case
| Situation | Recommended Series | Typical Model | Note |
|---|---|---|---|
| Small apartment, boiler up to 10 kW, radiators | UPS | UPS 25-30 | Shaft length 180 mm, DN25 |
| Family house, boiler 15–20 kW, radiators | UPS / Alpha | UPS 25-60 or Alpha2 | For long pipe runs choose Alpha |
| Underfloor heating, low temperature | Alpha / Magna | Alpha2 25-60 / Magna3 | Higher flow, automatic regulation |
| Solar collectors, glycol mixture | Solar | Solar 15-80 / Solar 25-120 | Depending on collector area |
| Water removal, basement, flooding | UNILIFT KP | KP 150 / KP 250 | NOT for heating circuits! |
Most Frequently Asked Questions (FAQ)
Can I replace the pump myself, or should I call a heating technician?
It depends on the specific system and your skills. Replacing a pump in a closed circuit requires closing the appropriate shut-off valves (ball valves before and after the pump), draining a small amount of water, unscrewing the old pump, installing the new one with new seals, and re-pressurizing the system. If you have basic plumbing skills and the system is equipped with shut-off valves (which every properly designed system should have), you can do it yourself. For more details, read the article Mounting a Grundfos Circulation Pump – Step by Step. If you are unsure or the system does not have shut-off valves, call a professional.
What is the lifespan of a Grundfos UPS pump?
Under normal conditions (clean water, system pressure 1–2 bar, no dirt), Grundfos UPS pumps typically last 10–15 years without any service. Their lifespan is reduced by running without water, air in the system, aggressive media (hard water with high calcium salt content), or dirt. Some units from the 90s are still working – this is a product of the notoriously reliable German and Danish engineering culture.
The pump is humming – is that normal?
Mild humming at higher speeds is normal. If the pump suddenly starts humming or the noise is metallic, clicking, or crackling, it is a different case – it is usually due to air in the system, worn bearings, or a foreign object in the impeller. Detailed causes and solutions can be found in the article Grundfos Pump Not Pumping or Making Noise – Causes and Solutions. Basic rule: if the pump suddenly changes its sound profile without an obvious reason, the system should be checked.
What speed should I set the UPS pump to?
Most home systems work best at the second speed. The third speed is suitable for extreme cold or if the system has very long pipe runs. The first speed is sufficient for small circuits (e.g., one apartment). You will know the correct setting when the return temperature is 10–20 °C lower than the supply temperature – if the difference is smaller (e.g., only 5 °C), the pump is running too fast and you can reduce the speed.
What is the difference between Solar 15-80 and Solar 25-120?
The numbers in the name say it all: "15" and "25" indicate the connection diameter (DN15 = ½ inch, DN25 = 1 inch), "80" and "120" indicate the maximum head in tenths of a meter (i.e., 8.0 m and 12.0 m). Solar 15-80 is intended for smaller solar systems with 2–4 collectors and shorter pipe runs (up to 20 m). Solar 25-120 can handle larger installations with 6–10 collectors and longer pipe runs or greater vertical differences between the collectors and the storage tank.
Do I need to replace the pump if the new boiler has its own built-in pump?
Modern condensing wall-hung boilers have an integrated circulation module – but that doesn't mean it's always sufficient. The built-in module usually handles a small circuit (one apartment, one-story house). If you have a branched system with multiple circuits (floor heating zone + radiator zone + TÜV tank + solar), each secondary circuit typically requires its own pump. The boiler manufacturer always specifies this in the technical manual – check the hydraulic diagram of recommended connections.
Conclusion: Pump selection is not a lottery
Choosing the right Grundfos circulation pump is not a matter of luck or bravery – it's the result of three simple steps: determining the system parameters (capacity, pipe length, medium type), calculating the required flow rate and head, and then selecting a model whose characteristic covers these parameters with a reasonable reserve (10–20 %). Do not overdimension unnecessarily – bigger is not always better and in most cases it is harmful.
If you are hesitating between models or have an atypical system, check out other articles in this Knowledge Center – especially What Grundfos pump capacity do I need for my home with a complete calculation methodology, or Common questions about Grundfos pumps for quick answers to common dilemmas. The full Grundfos range is available at atria.sk/grundfos/, where you can filter by parameters and compare specific models.
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.