What pump capacity do I need for my heating system
What pump performance do I need for my heating system?
This is one of the questions that literally everyone deals with – whether you are replacing an old, long-served pump with a new one or building a heating system from scratch. And it's precisely here that the most mistakes are made. People either reach for the cheapest option they can find or, on the contrary, buy the most powerful model in the range, thinking "bigger must be better." Neither approach works well. An underdimensioned pump is unable to push water through the entire system, some radiators remain cold, the boiler operates inefficiently, and you end up paying for heat you don't even feel in your home. An overdimensioned pump, on the other hand, unnecessarily consumes electricity, is noisy, and shortens the lifespan of valves, seals, and the entire piping system.
In this article, we will step by step go through what the performance of a circulation pump actually means, how it is calculated, what typical values are for standard family homes, and how to choose the right model from the Everline range so that your heating system operates reliably and efficiently for many years.
What actually determines the "performance" of a circulation pump?
When we talk about the performance of a circulation pump in a heating system, we mean two independent but interrelated quantities:
- Flow rate (Q) – the amount of water the pump can push through per unit of time. It is given in cubic meters per hour (m³/h) or liters per minute (l/min).
- Head (H) – the pressure the pump must overcome to allow water to flow through the entire system despite the hydraulic resistance of pipes, fittings, valves, radiators, and the boiler. It is given in meters of water column (m w.c.) or in pascals (Pa).
These two values form the so-called pump performance curve – the Q-H curve. Each pump has its own curve, where the available pressure decreases with higher flow rate and vice versa. A properly selected pump operates at the "optimal point" on this curve – neither at the extreme left edge (zero flow, maximum pressure) nor at the right edge (maximum flow, zero pressure).
The electrical power of the pump (given in watts) is the third value that interests you from the perspective of operating costs, but is not a direct measure of hydraulic performance. A 60 W pump may be far less efficient in an unsuitable system than a 45 W pump operating in an optimal system.
Step 1: Calculate the required water flow (Q)
The flow that the pump must provide depends on the thermal output of the heating system and the temperature difference between the supply and return. The basic formula is as follows:
Q [m³/h] = P [kW] ÷ (1.163 × ΔT [°C])
where:
- P is the thermal output of the boiler or the total thermal demand of the system in kilowatts
- 1.163 is a constant for water (specific heat capacity × density, simplified)
- ΔT is the temperature difference between the supply and return in °C
The temperature difference ΔT depends on the type of system. For classic radiator heating (70/50 °C), ΔT = 20 °C. For low-temperature systems – underfloor heating, heat pumps – we work with ΔT = 5–10 °C.
Practical examples of flow calculation
Example 1 – family house 150 m², boiler 20 kW, radiators, ΔT = 20 °C:
Q = 20 ÷ (1.163 × 20) = 20 ÷ 23.26 = 0.86 m³/h
Example 2 – family house 120 m², heat pump 10 kW, underfloor heating, ΔT = 8 °C:
Q = 10 ÷ (1.163 × 8) = 10 ÷ 9.3 = 1.07 m³/h
Example 3 – apartment 65 m², gas boiler 12 kW, radiators, ΔT = 15 °C:
Q = 12 ÷ (1.163 × 15) = 12 ÷ 17.45 = 0.69 m³/h
Interesting, isn't it? You can see that a house with a heat pump and underfloor heating may require a higher flow than a house with a gas boiler and radiators, even though it has a lower thermal output. That's why it's not possible to say generally "a 20 kW boiler needs XY pump."
Step 2: Calculate the required head (H)
This is the more complex part, as the hydraulic resistance of the system depends on dozens of factors: the length and diameter of the pipes, the number of elbows, fittings, thermostatic heads, boiler heat exchanger, the number of circuits and their complexity. There are two approaches:
Precise calculation of hydraulic resistance
A heating system designer works with a hydraulic calculation, where for each pipe section, pressure losses due to friction (R × L) and local losses (Z) are calculated. The result is the sum of losses on the longest circulation loop (so-called calculation loop). This is the correct and only accurate method for new installations or larger projects.
Approximate calculation based on specific resistance
For everyday practice when renovating heating in a family house, you can use an approximate formula:
H [m w.c.] = R × L × 1.3 ÷ 10 000
where R is the specific pressure drop in Pa/m (typically 100–200 Pa/m for well-dimensioned piping, 200–400 Pa/m for older, narrow pipes), L is the length of the calculation loop in meters (total section of supply + return of the furthest radiator), and 1.3 is the coefficient for local losses.
Typical values from practice
Based on long-term experience, the following approximate head ranges apply for family houses:
- Smaller apartment, short piping (up to 50 m loop): 2–3 m w.c.
- Family house with copper or plastic pipes DN 15–20, medium-length piping: 3–5 m w.c.
- Larger family house, long piping, thermostatic heads, multiple circuits: 5–7 m w.c.
- Underfloor heating (high resistance of distributors): 4–8 m w.c.
- Systems with buffer tanks, zone valves: 5–8 m w.c. and more
Step 3: Understand the numbers in the Everline pump designation
Now that you know the required flow and head, let's look at what the numbers in the Everline pump names specifically mean. This is also covered in detail in the separate article What do the numbers in the Everline pump name mean – diameter, head, spacing, but for a quick overview:
The designation, for example, 25/6/180 is read as follows:
- 25 – nominal pipe diameter in millimeters (G 1 1/2" thread, DN 25)
- 6 – maximum head in meters of water column
- 180 – flange spacing (center distance) in millimeters
Thus, the middle number directly indicates the maximum head. For the number 4, it is a pump with a maximum head of 4 m w.c., for the number 6, it is 6 m w.c., and for the number 8, it is 8 m w.c.
Overview and comparison of Everline models
In the current range at atria.sk, you will find several models of the Everline series with different parameters. Let's see which systems they are suitable for:
Everline 25/4 – for smaller and medium systems with short pipe runs
Circulation pump Everline 25/4/130 and Circulation pump Everline 25/4/180 are intended for systems where the head does not exceed 4 m w.c. The difference between them is only in the flange spacing (130 mm vs. 180 mm) – you choose based on the flange spacing of your existing pump or piping in the system.
Typical use of model 25/4: smaller apartments up to 80 m², houses with short pipe runs and modern copper or plastic installation, systems with continuous regulation without thermostatic heads. The flow rate ranges up to about 2.5–3 m³/h (depending on the set speed and actual hydraulic resistance), which covers most standard apartment installations.
Everline 25/6 – a universal model for most family homes
Circulation pump Everline 25/6/130 and Circulation pump Everline 25/6/180 are the most commonly sold models in practice. The maximum head of 6 m w.c. covers the vast majority of family homes with radiator heating – older installations with threaded steel piping, systems with thermostatic heads, houses with multiple floors.
If you are unsure whether model 25/4 is sufficient for you or whether you need 25/6, choose 25/6. The difference in power consumption is minimal, but the hydraulic reserve will allow you to easily regulate the system even with partially closed valves. From practical experience, it is true: during a renovation, when replacing an old pump and you do not know the exact parameters of the original system, model 25/6 is almost always a safe choice for a family home up to 200 m².
Everline 25/8 – for demanding systems with high resistance
Circulation pump Everline 25/8/180 with a maximum head of 8 m w.c. is intended for extensive systems – larger family homes over 200–250 m², systems with floor heating and zone valves, a combination of radiators and floor heating in one system, or older piping made of thin steel pipe with significant pressure losses.
Important note: model 25/8 has the same nominal pipe diameter DN 25 as other models, which means it is not suitable for pipe runs with higher flows (over about 3.5 m³/h) – in such cases, you should consider switching to a larger diameter DN 32. This topic is covered in detail in the article Difference between Everline 25 and 32 pumps – which one is suitable for my system.
Three typical scenarios from practice – how we solved them
Scenario 1: Replacing an old pump in a 30-year-old panel apartment
A customer from Bratislava had a 68 m² apartment in a panel building with a 14 kW gas boiler. Steel threaded pipe ½", loop length about 40 meters, four radiators with manual valves. The old pump was a Grundfos UP 15-14, completely clogged with sludge and the impeller locked.
Calculation: Q = 14 ÷ (1.163 × 20) = 0.60 m³/h. Hydraulic resistance of steel pipe ½" at flow 0.6 m³/h – specific resistance approx. 250 Pa/m, length 40 m: H = 250 × 40 × 1.3 ÷ 10 000 ≈ 1.3 m w.c. System without thermostatic heads, simple, short loop.
Solution: Model Everline 25/4/130 at speed 1 covers the needs with a significant reserve. The customer was satisfied, the price was appropriate, and the problem was solved in half an hour of replacement.
Scenario 2: Reconstruction of a 180 m² family house with a mixed system
A house from 1995, gas boiler 24 kW. Ground floor – floor heating (60 m², one loop through a distributor with 8 pipes), upper floor – radiators with thermostatic heads (120 m²). A combined system via a hydraulic balancer (collector). The total required power was 22 kW, ΔT on radiators 20 °C, ΔT on floor 8 °C.
This was two separate loops – primary (boiler → balancer) and secondary (balancer → radiators, balancer → floor). Primary pump: Q = 22 ÷ (1.163 × 20) = 0.95 m³/h, H estimated at 3 m w.c. (short primary loop, only boiler heat exchanger and balancer). For radiators, secondary pump: Q = 14 ÷ (1.163 × 20) = 0.60 m³/h, H = 5 m w.c. (thermostatic heads + longer loop).
Solution: For the primary loop Everline 25/4/180, for the secondary radiator loop Everline 25/6/180. For the floor loop, they installed a special pump with a low flow (floor loops have very high pipe resistance, but low flow). Overall three pumps, each dimensioned precisely for its loop.
Scenario 3: Old house 240 m², steel pipe ¾", long piping
A house from 1975, boiler 32 kW. Piping from galvanized steel pipe ¾" and ½", length of the furthest loop 65 meters, 10 radiators, some with third-generation thermostatic heads (high resistance when partially closed).
Calculation Q: Q = 32 ÷ (1.163 × 20) = 1.37 m³/h. Hydraulic resistance: galvanized steel pipe ¾" after 30 years has increased resistance due to corrosion, we estimate R = 200 Pa/m, length 65 m, coefficient 1.3: H = 200 × 65 × 1.3 ÷ 10 000 = 1.69 m w.c. only from friction. Thermostatic heads when partially closed add another 1.5–2 m. Total estimated H = 4–5 m w.c.
Solution: Everline 25/6/180, speed 2 in normal operation. Model 25/8 would be unnecessarily oversized here and would operate in an inefficient point on the curve.
What else influences the choice besides the actual power?
Spacing of connections (130 mm vs. 180 mm)
Spacing is the distance between the centers of the inlet and outlet ports of the pump. You must measure this when replacing an old pump – literally place a ruler on the existing pump or the fittings in the piping. If the spacing is 130 mm, take the /130 model; if 180 mm, take the /180 model. If you have fittings on the walls that are close together, and the original pump had a different spacing, you must install extension nipples or change the fittings.
Mounting direction (horizontal vs. vertical rotor axis)
Standard Everline circulation pumps are designed for horizontal installation of the motor axis (motor pointing up or sideways, rotor horizontal). Never install the pump so that the motor points downward – this will cause the seal to fail and damage the bearings. A detailed installation procedure can be found in the article Mounting an Everline circulation pump – step by step.
Water quality in the system
Dirty, sludgy water full of magnetite (black sludge) is one of the most common causes of premature pump failure. Before installing a new pump, always check the water condition and if it is dark and muddy, have the system flushed and install a magnetic filter before the pump. This will extend the life of the pump by years.
Adjustability vs. fixed speed
Everline pumps operate at fixed speeds (usually 3 levels). If your system has thermostatic heads, it is recommended to set the pump to medium speed and let the system self-regulate. For systems with thermostatic heads on all radiators, an ideal pump today is one with electronic pressure regulation (so-called ECM pumps), but these are significantly more expensive and for smaller systems are not always economically justified. For most family houses with 4–8 radiators and a properly dimensioned Everline at medium speed, the result is fully satisfactory.
Most Common Mistakes in Selection and Their Consequences
Mistake 1: "I'll buy the same one that was there before." This works only if the original pump was correctly dimensioned and the system has not changed since its installation. If thermostatic valves have been added, a new circuit has been added, or the piping has been replaced – the original parameters may no longer be suitable.
Mistake 2: Choosing based on price, not parameters. A cheaper model 25/4 in a system where 25/6 is needed will constantly run at maximum capacity, overheat, reduce its lifespan, and some rooms may remain insufficiently heated.
Mistake 3: A large pump = a better pump. An oversized pump circulates water too quickly, causing noise in the pipes (cavitation, turbulence), unnecessary electricity consumption, and reduced valve lifespan. Moreover, it operates at an inappropriate point on its characteristic curve, where hydraulic efficiency is low.
Mistake 4: Neglecting system cleanliness during replacement. Old bearings and rotors often fail because magnetic sludge from old steel pipes has entered them. A new pump without a magnetic filter in front of it may fail within a year.
Mistake 5: Incorrect spacing. A customer buys a model online without verifying the spacing and at home discovers that the connections are 50 mm further apart. Result: purchase of adapters, delayed installation. Always measure the spacing of the existing pump before ordering.
Electricity consumption – practical comparison
A circulation pump runs almost continuously during the season – in basic mode for up to 200 days a year, 24 hours a day, which is approximately 4,800 hours per year. Even a small difference in power consumption becomes noticeable on electricity bills over the years.
Old single-speed pumps from the 90s had a power consumption of up to 100–150 W. Modern three-speed pumps such as Everline typically have a power consumption of 40–80 W depending on the setting. ECM pumps with electronic regulation achieve a power consumption of 5–25 W, but their price is three times higher. For a typical family house, a three-speed Everline pump is the right balance between cost and operating expenses.
Example: At a power consumption of 60 W and 4,800 hours of operation per year, the consumption is 288 kWh/year. At an electricity price of 0.20 €/kWh, this is about 58 € per year. The difference in power consumption between models 25/4 and 25/6 is usually only 5–10 W, which amounts to barely 2–5 € per year – a negligible factor when choosing the right model.
Frequently Asked Questions (FAQ)
How can I find out the power of my old pump?
Look for the type label on the old pump (usually an aluminum or plastic plate on the body of the pump). On it, you will find the model designation, from which you can read out Q and H, or directly the maximum head. If the label is unreadable, measure the spacing of the connections and the diameter of the flange – these values are key to selecting a replacement. Alternatively, check the original documentation for the boiler or project, where the pump may be specified.
Can I use the Everline 25/6 pump where a 25/4 is sufficient? Won't it be too powerful?
Yes, you can, and it works well in practice. The Everline pump has three speed levels – simply choose a lower speed (level 1 or 2), and the pump will operate with a lower flow and pressure. The operating point will shift to a lower part of the curve, where the model 25/6 is hydraulically efficient. It is not ideal if you want a precisely optimized system, but for everyday use, it is a fully acceptable solution – for example, exactly when replacing, if you do not know the exact parameters of the original pump.
What if I have floor heating and radiators in my house at the same time?
A mixed system is more complex hydraulically, as both circuits have different operating conditions (temperatures, pressure losses). The correct solution is a hydraulic balancer or a mixing unit, where each circuit has its own pump individually dimensioned. Do not try to "tame" one circuit with regulating valves and fit it to a single pump – the result is usually an unbalanced system, where either the floor or the radiators operate incorrectly.
Why is my new Everline pump noisy, even though it is correctly dimensioned?
Noise from a new pump is usually caused by something other than incorrect dimensioning. The most common causes are air in the system (solution: bleeding), a dirty filter or check valve behind the pump, or an excessively high pump speed in a system with almost closed thermostatic valves (solution: switching to a lower speed). Detailed diagnostic tips can be found in the articles Noisy Everline circulation pump – causes and solutions and Common Everline circulation pump faults and how to eliminate them.
How long is the lifespan of an Everline pump and when should it be replaced?
With correct dimensioning, a clean system, and basic maintenance (bleeding, checking seals, greasing after long downtime), the typical lifespan is 8–15 years. Signs that it is time for a replacement are: a significant drop in flow (radiators do not heat evenly), constant increased noise without a clear cause, water leakage at the seals, or increased electricity consumption at the same performance. More information can be found in the article Maintenance and servicing of a circulation pump – what to check and how often.
Is it necessary to replace the pump when switching from a gas boiler to a heat pump?
Very likely yes. Heat pumps operate with a lower ΔT (typically 5–8 °C instead of 20 °C), which, for the same thermal output, requires a significantly higher water flow (see the Q calculation formula). In addition, some heat pumps have an internal pump for the primary circuit, but the secondary circuit (heat distribution in the house) may require a more powerful external pump with a higher flow, which a standard 25/4 or 25/6 pump can provide. Consult an installer or the heat pump manufacturer, who will recommend the correct parameters for you.
Conclusion – a few practical rules to remember
Selecting the right pump is not black magic, but it does require at least a basic understanding of what the pump does and what conditions your system imposes on it. For most standard family homes in Slovakia, a few simple rules apply:
- Always calculate based on the actual thermal output of the boiler, not the area of the house.
- Do not forget about ΔT – floor heating and low-temperature systems require a higher flow.
- Hydraulic resistance can be estimated, but for more complex systems, leave the calculation to the designer.
- For the vast majority of family homes up to 200 m² with radiator heating, the correct choice is the Everline 25/6 model – Everline 25/6/130 or Everline 25/6/180 depending on the spacing of your connections.
- During renovations, always measure the spacing and check the condition of the water in the system before installing a new pump.
- Do not forget that the pump is not the only component – a properly functioning system also includes an expansion tank, air vent, magnetic filter, and correct pressure setting in the system.
If after reading this article you are still unsure which model is right for your specific case, also check out other topics in our Knowledge Center – especially How to choose an Everline circulation pump for heating your home and Everline vs other brands of circulation pumps – a comparison of quality and price, where you will find additional context for your decision.
Do you have a question about this topic?
Still undecided or dealing with a specific situation in your household? Write to us – we are happy to help.