How to choose a circulation pump for central heating
Circulation pump for central heating: a comprehensive guide to selection
The circulation pump is the heart of any central heating system. It ensures continuous flow of hot water between the boiler and radiators, without which the whole system would stop working – or would work only very inefficiently due to natural thermosiphon circulation, which is practically unacceptable for modern low-temperature systems. Despite this, the selection of a pump is a topic that most homeowners address only when the old one dies, or when the plumber places three different models on the table and asks what they want to choose. This article will help you understand what questions to ask, what to compare, and where not to make compromises.
Basic terms you need to know
Before we get into specific parameters and recommendations, it is important to understand three key quantities that define each circulation pump. Without understanding them, you will get lost in the catalog sheets and specifications.
Flow rate (Q) – how much water the pump will move
Flow rate indicates the volume of water the pump will transport per unit of time. It is usually given in m³/h (cubic meters per hour) or in l/min (liters per minute). For a typical family house with a central heating system, we usually operate in the range of 0.5 to 3 m³/h, with most houses with 5–10 radiators being sufficient with a pump with a flow rate of around 1–1.5 m³/h.
Flow rate depends mainly on the thermal output of the system and the designed temperature drop (difference between supply and return water temperatures). For a standard system with a temperature drop of 20 °C (e.g. 70/50 °C), the simplified formula is:
Q [m³/h] = P [kW] / (1.163 × ΔT [°C])
For example: boiler 15 kW, temperature drop 20 °C → Q = 15 / (1.163 × 20) = 15 / 23.26 ≈ 0.65 m³/h. Modern condensing boilers and heat pumps operate at lower temperatures and higher flow rates – at ΔT = 5 °C it would be 2.58 m³/h for the same power.
Head (H) – the pressure the pump can overcome
Head, also called manometric height or "head", indicates the resistance of the entire heating system that the pump must overcome. It is given in meters of water column [mH₂O] or in kPa (kilopascals). Note: 1 mH₂O ≈ 9.81 kPa ≈ 0.1 bar.
The resistance of the system depends on the length of the pipes, their diameter, the number of fittings, valves, thermostatic valves and other components. For a typical family house, a typical value is 2–6 mH₂O. Multi-storey houses, long distribution loops or systems with floor heating may require 6–10 mH₂O.
Electric power (P) – what you will pay for operation
The power of the pump is given in watts [W] and directly affects operating costs. Old pumps with simple technology had a power of 60–120 W and operated at constant speeds without the possibility of regulation. Modern pumps of class A with electronically commutated motors (EC motors) have a power of 5–45 W and automatically adjust the power to the current needs of the system. The difference in electricity consumption per year can be 300–500 kWh – which at today's energy prices represents 100–200 € annual savings.
The intersection of the pump curve and the system curve is the so-called operating point – this is exactly where the pump is actually working. A properly selected pump has the operating point in the middle of its characteristic, not at the edges.
Wet-rotor vs. dry-rotor pump: a crucial choice
This is the first and most important decision to make. There are two basic types of circulation pumps in terms of motor construction, and each has its place of use. A detailed analysis can be found in a separate article Difference between wet-rotor and dry-rotor circulation pump, here we summarize the essentials.
Wet-rotor pump
In a wet-rotor pump, the motor rotor and impeller are immersed directly in the medium being pumped – that is, in hot water. The water also serves as a coolant and lubricant for the bearings. The result is quiet operation, maintenance-free, long life and compact dimensions. The disadvantage is slightly lower efficiency compared to dry-rotor pumps at high outputs.
For family houses and apartment units, a wet-rotor pump is almost always the right choice. 99 % of circulation pumps in central heating in the residential sector are exactly wet-rotor pumps.
Dry-rotor pump
The motor of a dry-rotor pump is separated from the pumped medium. The motor is air-cooled and the bearings are externally lubricated. This design allows for higher output and better efficiency at large flows, but requires regular maintenance, is noisier and takes up more space. Dry-rotor pumps are found in industrial buildings, apartment buildings with dozens of apartments, schools and similar.
Energy efficiency classes – why it is important today
Since 2013, Regulation (EC) No 641/2009 has been in force in the EU, which banned the sale of pumps with an energy efficiency index (EEI) higher than 0.27. Since 2015, a stricter limit of EEI ≤ 0.23 has been in force. In practice, this means that new pumps on the market are energy efficient and mostly belong to class A or A+.
If you have a pump in your home that is older than 10–15 years, replacing it with a modern class A pump will likely pay for itself in 2–3 years just from electricity savings. Without any subsidies or discounts. This is an investment with a better return than most other energy measures in the house.
Regulation and control algorithms – where the real difference lies
Modern circulation pumps are equipped with electronics that allow them to adjust their performance to current conditions in the system. There are three basic regulation modes:
Constant speed (fixed stage)
The simplest mode – the pump operates at a set stage (1, 2, or 3), regardless of the current state of the system. Suitable for old, simple systems without thermostatic valves. When some thermostats are closed, the pressure in the system increases and the pump becomes noisy. This mode is used today only in old installations or when commissioning a new system.
Proportional pressure regulation (Δp-v)
The pump maintains a differential pressure that increases linearly with flow. When thermostatic valves close (lower flow), the pump reduces its speed and pressure. The result is quiet operation and energy savings. Suitable for most modern systems with thermostatic valves on radiators.
Constant pressure regulation (Δp-c)
The pump maintains a constant differential pressure regardless of the flow. Suitable for long distribution loops and systems where there are no thermostatic valves, or where constant pressure is required due to the nature of the system (e.g., floor heating with balancing valves).
The best modern pumps (e.g., from the Grundfos Magna3, Wilo Stratos or DAB Evoplus series) also offer an automatic mode, where the pump itself analyzes the system and selects the optimal control algorithm. When choosing a pump, therefore, pay attention not only to hydraulic parameters, but also to available control modes – in a modern house with thermostatic valves, proportional regulation is a necessity, not a luxury.
How to determine the correct pump size for your home
This is where the most mistakes are made. Either a pump that is too powerful is purchased (which means noise, vibrations, unnecessary consumption and rapid wear), or one that is too weak (the system does not heat evenly, distant radiators are cold). A detailed calculation method can be found in the article What circulation pump power do I need for my home, here we will go through the basic procedure.
Step 1: Find out the boiler's thermal output
The boiler output (in kW) is stated on the nameplate or in the documentation. Note – we take the nominal output, not the maximum. For a standard family house with an area of 150 m², it is usually 12–20 kW for gas boilers, 8–15 kW for heat pumps.
Step 2: Determine the system temperature drop
- Classic steel radiators: temperature drop 70/50 °C → ΔT = 20 °C
- Modern condensing boilers with radiators: 60/40 °C → ΔT = 20 °C
- Floor heating: 35/28 °C → ΔT = 7 °C (requires significantly higher flow!)
- Heat pumps: 45/40 °C → ΔT = 5 °C (highest flow)
Step 3: Calculate the required flow
Use the formula: Q = P / (1.163 × ΔT). For a 15 kW boiler and floor heating with ΔT = 7 °C: Q = 15 / (1.163 × 7) = 15 / 8.14 ≈ 1.84 m³/h.
Step 4: Estimate the required head
If you do not have access to design documentation and pressure loss calculations, you can use approximate values:
- Single-story house, short pipes (up to 40 bm): 2–4 mH₂O
- Two-story house, longer pipes (40–80 bm): 4–6 mH₂O
- Larger house, more complex system (over 80 bm): 6–8 mH₂O
- Floor heating with a large number of loops: 3–7 mH₂O (depends on the manifold)
Step 5: Choose a pump with a working point in the middle of the curve
Look in the catalog at the characteristic curves and find a model whose curve passes through your calculated working point (Q, H) in the middle – not at the end or the beginning. If you are working with a variable speed pump, set the flows and pressures after installation according to the actual conditions.
Mounting position, dimensions and connection
Even if you have correctly calculated the hydraulic parameters, when purchasing you must pay attention to several practical dimensions and mounting conditions.
Construction length and connection dimension
Circulation pumps for domestic installations are manufactured in standard construction lengths. The most common are 130 mm (for small domestic systems) and 180 mm (standard for most family homes). Less common, but also available, are 110 mm, 150 mm and 200 mm. Before purchasing, always measure the distance between the center of the flange and the center of the flange on the existing installation.
The connecting threads are typically G 1" (most small pumps), G 1¼" or G 1½" (medium and larger pumps). Note – manufacturers of pumps usually include adapters (reductions) in the packaging, so a pump with G 1½" can be connected to a G 1" pipe, but consider whether it makes hydraulic sense.
Mounting position
Wet rotor pumps can be installed with the rotor axis in a horizontal position (most common) or vertically upwards. Installation with a downward-facing rotor axis is not permitted for most models – air would accumulate in the bearings and cause failure. Always check the allowed positions in the manual for the specific model.
The pump should be placed on the return pipe (cold side) – not only because the lower temperature is more favorable for electronics and seals, but also to ensure that suction pressures are always positive and cavitation does not occur. Details on correct installation are described in the article Installing a Circulation Pump Step by Step.
Special applications: floor heating, heat pumps and solar systems
Selecting a pump for special types of systems deserves special attention. Mistakes here are expensive – an unsuitable pump can cause uneven heating, overheating of collectors or reduced efficiency of the entire system.
Floor heating
Floor heating operates at low temperatures (35–45 °C) and a low temperature difference (5–10 °C). Therefore, the flow is significantly higher than in radiator heating of the same power. In addition, a distributor with many circuits generates considerable pressure losses. For a house with 15 kW of floor heating and ΔT = 7 °C, you need a flow of about 1.8 m³/h and a head of 4–6 mH₂O just for the distribution after the distributor. The total resistance including the distributor can be as high as 8–10 mH₂O.
Heat pumps
Heat pumps are very sensitive to the flow of the medium in the primary and secondary circuit. Too low a flow causes poor heat exchange, a drop in COP and in extreme cases freezing of the evaporator (in air-to-water heat pumps) or compressor failure. Heat pump manufacturers usually specify the required flow precisely – this must be respected. Read more in the article Circulation pump for a heat pump or solar system.
Solar systems
Solar collectors require special pumps capable of operating with a heat transfer medium (usually propylene glycol/water 1:1) at temperatures up to 130 °C and pressures of 3–6 bar. Standard pumps for central heating are not sufficient here – you must use models specifically designed for solar applications, with more durable seals and higher thermal resistance.
What to pay attention to when comparing brands and models
Three European brands dominate the market, covering most domestic and commercial applications: Grundfos (Danish), Wilo (German) and DAB (Italian). Each offers several lines of pumps for different segments – from basic economic models to premium "smart" pumps with Wi-Fi connectivity and cloud monitoring. A detailed comparison can be found in the article Comparison of Grundfos, Wilo and DAB circulation pumps.
When comparing, focus on these parameters:
- EEI (Energy Efficiency Index): the lower, the better. Values below 0.20 are excellent.
- Maximum medium temperature: for standard heating systems 110 °C is sufficient, for solar systems you need 130 °C+
- Maximum operating pressure: 10 bar is standard for domestic systems
- Operating temperature range: most pumps are certified for ambient temperatures of 2–40 °C
- Warranty: standard warranty is 2 years, premium models offer 5 years
- Availability of spare parts: with established brands this is less of a problem, but with unknown imports, repairs may be impossible after 5 years
Typical scenarios from practice: what we most often solve
From practice, we know that most customers come with one of the following situations. We describe how to proceed in each case.
Scenario 1: Replacing an old pump with an identical one
The most common case. After 15–20 years the pump fails and the customer wants a quick replacement. The temptation is to buy "the same" – the same construction length, the same thread, done. But it is a missed opportunity. For the same or even lower price today you can get a pump that consumes 5× less electricity. Therefore, we always recommend switching to a modern EC motor when replacing – the additional investment (if any at all) pays off within a year.
Scenario 2: New installation – condensing boiler + radiators
For a modern condensing boiler up to 25 kW with radiators in a family home (area 120–200 m²), you typically need a pump with a flow of 1.0–2.0 m³/h and a head of 4–6 mH₂O. It is important to set the correct regulation mode – proportional pressure (Δp-v). Boilers such as Vaillant ecoTEC, Viessmann Vitodens or Bosch Condens have an integrated pump – check this when buying the boiler to avoid unnecessarily buying it again.
Scenario 3: A house with floor heating + radiators in combination
Combined systems require separate circuits with their own pumps – one for the low-temperature floor heating circuit, the other for the radiators. A mixing valve (three-way) divides the temperatures. Each circuit has its own hydraulic parameters and its own pump. A fundamental mistake is to install one pump for both circuits – it will never work properly.
Scenario 4: Noisy Pump and the Need for Quiet Operation
Noise can have multiple causes: air in the system, cavitation, pipe resonance, worn bearings, or simply an improperly set speed. If a new pump is noisy – first bleed the system and reduce the speed. If an old pump is noisy – consider replacement, since the bearings of wet rotor pumps wear over time and replacement is often more expensive than a new pump. Modern EC pumps are almost silent at low flow rates (under 35 dB).
Pump Setup After Installation
Installing a pump is just the beginning. Proper setup significantly affects comfort, noise level, and energy consumption. Basic setup includes selecting the control mode and setting the reference pressure or speed. A detailed procedure is described in the article Setting the Speed and Performance of a Circulation Pump.
Practical recommendation: at first startup, set the pump to a medium level or to proportional control with a set Δp value in the middle of the scale. Then, during full heating (all thermostatic valves open, boiler at full power), check whether all radiators are evenly warm. If distant radiators are cold, increase the set pressure value. If you hear noise or vibrations, reduce the value. The whole balancing process is iterative and requires some patience, but the result is worth it.
Maintenance and Lifespan of Circulation Pumps
Wet rotor pumps are practically maintenance-free throughout their entire lifespan, which with quality models can reach 15–25 years. The only thing you can do preventively:
- Check at least once a year whether the pump is making unusual noise or showing vibrations
- Monitor for water leakage from the pump housing (a sign of seal wear)
- Maintain water quality in the system – corrosion inhibitors and correct pH (7.0–8.5) prolong the lifespan of the entire system, including the pump
- After a long summer shutdown (e.g., 3–4 months), briefly start the pump to prevent rotor seizing due to corrosion
If the pump is seized (the rotor is blocked by deposits or corrosion), an experienced plumber can usually unblock it manually using a screwdriver (most pumps have a bleed/relief screw on the body). More about faults and their solutions can be found in the article Common Circulation Pump Faults and Their Solutions and in the article Maintenance and Service of Circulation Pumps.
Price and Return on Investment
The prices of circulation pumps vary widely. Basic models suitable for small home systems start at around 40–80 €. The mid-range with an EC motor and proportional control ranges from 100 to 200 €. Premium models with advanced electronics, Wi-Fi, and Bluetooth communication reach 250–400 €.
From the perspective of return on investment, the key difference is in power consumption. If you replace an old pump with a power consumption of 80 W with a new one with an average consumption of 20 W (a typical value for an EC pump under real load), you will save 60 W × 8,000 hours of operation/year = 480 kWh/year. At an electricity price of 0.30 €/kWh, this is a saving of 144 € per year. A mid-range pump (130 €) will therefore pay for itself in less than one year.
Most Frequently Asked Questions About Circulation Pumps (FAQ)
How do I find out what pump I currently have installed at home?
Every pump has a nameplate on its body with the model designation, serial number, power consumption, and hydraulic parameters. Usually, it is enough to enter the model designation into a search engine – manufacturers such as Grundfos, Wilo, or DAB have detailed product databases available online, where you can find all technical parameters and data sheets. Alternatively, you can measure the construction length and connection dimensions from the designation, which is sufficient for selecting a mechanically compatible replacement.
Can I install a pump myself without a professional?
Technically, replacing a pump with an identical one (same construction length, same thread) is a relatively simple task – close the ball valves, replace the pump, tighten the flange connections, bleed the system, and start it up. The law in the Slovak Republic does not require professional qualification for this task, unlike work on gas appliances. However, in case of any doubts, when working on a more complex system, or during the first installation, we recommend calling a professional. One improperly sealed flange can cause water damage that far exceeds the cost of the pump.
The pump is humming and is audible throughout the house – what should I do?
The most common cause is air in the system. Bleed the radiators and the pump itself (if it has a bleed screw). The second cause is an improperly set speed – reduce the level or the set pressure value. If the noise persists, check whether the pipe is tightly attached to the wall (it can be a source of resonance) and whether the flexible couplings are correctly installed. Worn bearings in an old pump are indicated by high-frequency whining or rubbing – in such cases, the solution is to replace the entire pump, not to service it.
Do I need one or multiple pumps for my house?
A single-family home with one heating circuit (only radiators or only floor heating) is sufficient with one pump. If you have a combined system (radiators + floor heating), two separate pumps are recommended – one for each circuit with its own temperature and pressure parameters. Similarly, in a system with domestic hot water (DHW) preparation via a storage heater with a circulation pump, it is a separate circuit with its own pump. The total number of pumps in a house can be 2–4 with standard equipment.
Is it worth buying a "smart" pump with Wi-Fi and an app?
Yes, in new builds and renovations with a smart home – the ability to remotely monitor consumption, error notifications, and settings is practical and can help in early detection of faults. For a simple replacement of an old pump in a standard family home, smart features are a nice bonus, but not a necessity. More important is having the correct hydraulic parameters and a good control algorithm – these are available even in models without Wi-Fi.
How long will a new circulation pump last?
A quality wet rotor pump from a reputable manufacturer (Grundfos, Wilo, DAB) has a lifespan of 15–25 years with proper installation and maintained water quality. The key is the water quality in the system – pH 7–8.5, low oxygen content (closed system with an expansion tank), and no excessive solid impurities. Regular bleeding and corrosion inhibitor checks multiply extend not only the pump's lifespan but also the entire system, including the boiler and radiators.
Conclusion: How to Make the Right Decision
Selecting a circulation pump for central heating is not about buying the most expensive model or looking for the cheapest replacement. It is about understanding what your system actually needs – what flow rate, what head, what type of control. Once you know these values (or can derive them from boiler and system parameters), selecting the right pump is relatively straightforward.
The most important recommendations in conclusion:
- Always choose a pump with an EC motor of class A – the return on investment compared to old models is excellent
- Do not buy an unnecessarily large pump – an oversized pump is noisy, vibrates, and operates at a suboptimal point on its curve
- For a modern system with thermostatic valves, always set up proportional pressure control
- Install the pump on the return pipe, correctly oriented according to the instructions
- For combined systems (radiators + floor heating), plan separate circuits with their own pumps
- Pay attention to water quality in the system – this is an investment that pays off in the long lifespan of the entire system
If you are unsure about the selection or have a specific system (heat pump, solar, combined system), see other articles in the Knowledge Center or contact us directly in the circulation pump category on atria.sk, where you will find the current range of models for all common applications in homes and small commercial buildings.
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Having trouble deciding or dealing with a specific situation in your home? Write to us - we'll be happy to help.