Pump Parameters 25/6: What Do the Numbers Mean and How to Read Them
Pump parameters 25/6: what do the numbers mean and how to read them
When you look at a catalog of circulation pumps or the label on a pump you just pulled from a boiler, you will encounter a designation in the form 25/6, or RS 25/6, PR 25/6, and similar. For the average customer, these are just two numbers separated by a slash – but for a technician or an experienced salesperson, they represent the basic technical identification of the pump, from which they can immediately determine whether it is suitable for a specific system or not. In this article, we will examine these parameters in depth – what they exactly mean, how they relate to physical quantities, what practical consequences they have when selecting and installing, and where you can get confused with incorrect interpretation.
We will also show how these numbers work in the context of specific products that are commonly available for ATTACK boilers – that is, boilers where you will encounter the designation 25/6 most often.
Where does the designation 25/6 come from – a brief history and standardization
Labeling circulation pumps with a numerical code in the format diameter/pressure head is not a new thing – this convention arose in Western European industrial practice somewhere in the 70s–80s of the last century and is now practically worldwide among manufacturers such as Grundfos, Wilo, DAB, Lowara or Laing. The manufacturer ATTACK uses Wilo pumps in its boilers – specifically the Yonos PARA RS series, or other types in the PR series – and these pumps also carry the same nomenclature.
Standardization of the nominal connection diameter is advantageous because it allows simple interchangeability between manufacturers – as long as the pump has the same nominal diameter and the same center distance (130 mm or 180 mm), it is mechanically compatible with the mounting location regardless of whether it is a Grundfos or Wilo. Of course, electrical compatibility and performance parameters must be addressed separately.
The first number: 25 – connection diameter (nominal bore diameter)
The number 25 in the designation 25/6 refers to the nominal bore diameter (DN) of the pump nozzles, i.e. the diameter of the inlet and outlet pipes to which the pump is connected. In this case, it is DN 25, which corresponds to a diameter of ¾ inch (internal diameter of the thread G 1½" external vs. pipe bore 25 mm – note, do not confuse the bore with the external diameter of the thread).
In practice, this means that the pump 25/6 has G 1½" external (i.e., "male") threads on both nozzles – the inlet and the outlet. This is the most common size for medium-power boilers (up to 30–50 kW), which form the basis of residential heating in family homes. For larger boilers, diameters DN 32 (number 25 in the designation) or DN 40 are used, and for smaller apartment pumps DN 20 (number 20).
Interchangeability between pumps with the same first number is direct – the pump 25/4, 25/6, and 25/8 all have the same nozzles and thus the same possibility of connecting to the piping. They differ only in hydraulic performance, which brings us to the second number.
It is important to realize that the number 25 does not say anything about the physical size of the pump body – specifically, the distance between the centers of the nozzles (center distance). This is determined by the second parameter, or a separate dimensional specification of 130 mm or 180 mm, which we will address in a separate article Pump size 130 mm vs 180 mm: which fits into my boiler.
The second number: 6 – maximum delivery head (Hmax) in meters of water column
The number 6 in the designation 25/6 represents the maximum delivery head of the pump expressed in meters of water column (m.w.c. or m H₂O). In English literature, this is denoted as Hmax or head. It is a key hydraulic parameter that determines what hydraulic resistance the pump can overcome.
Specifically, 6 m w.c. means that the pump at zero flow (closed valve) generates a pressure equivalent to a water column of 6 meters – which corresponds to a pressure of approximately 0.6 bar or 60 kPa. The conversion is simple: 1 m w.c. = 0.0981 bar ≈ 0.1 bar. At full flow (no resistance), the pressure is zero and the flow is maximum.
For a layperson, another interpretation is more useful: the number 6 determines how much hydraulic resistance the entire heating system (boiler + manifold + pipes + valves + radiators) can overcome for the water to circulate. If your system has a calculated pressure loss greater than 6 m w.c., the pump 25/6 is insufficient and a more powerful one, such as 25/8 or 32/8, should be selected.
Why this number is important when selecting
A typical family house with underfloor heating may have a pressure loss of 2–4 m w.c., and thus the pump 25/6 is unnecessarily powerful – but still usable, because modern class A pumps (which are exactly the products of the ATTACK series with the designation class A) automatically adjust their performance according to the system's needs. On the other hand, an old house with rusty radiator piping and partially clogged pipes may have a pressure loss of 5–6 m w.c., where the pump 25/6 is working right at the edge of its capabilities.
It is also important to realize that the numerical value 6 represents the maximum delivery head – that is, the point on the characteristic curve where the flow drops to zero. The actual operating point of the pump always lies somewhere in the middle between maximum flow (at zero resistance) and zero flow (at maximum delivery pressure).
What is not encoded in the numbers 25/6 – what to pay attention to
The designation 25/6 refers to the diameter of the flanges and the maximum pumping head. It does not refer to the following parameters, which can be equally important:
- Power consumption and energy efficiency class: A 25/6 pump can be an older model with power consumption of 60–80 W (classes C or D), or a modern ECM pump of class A with power consumption of only 5–20 W. The difference in energy consumption is huge. More on this in the article Energy efficiency class A for circulation pumps: what it means in practice.
- Center distance (130 mm vs. 180 mm): The physical distance between the centers of the flanges, which determines whether the pump fits into a particular boiler. We cover this topic in detail in the article 130 mm vs 180 mm pump dimensions: which fits into my boiler.
- Maximum flow rate Qmax: The number 6 says nothing about how much water the pump can move under low resistance. A typical 25/6 pump has a Qmax of around 2.5–3 m³/h, but this varies according to the specific model.
- Maximum medium temperature: Most heating circulation pumps can handle 110 °C, but there are exceptions.
- Material of the seal and impeller: Important for resistance to corrosive media or media with high salt content.
- Type of regulation: Manual speed switching vs. automatic pressure regulation vs. proportional regulation.
This is one of the reasons why the designation 25/6 alone is not sufficient to fully identify a pump. When ordering a replacement part, you also need to know the center distance and the model designation of the series (e.g. RS 25/6, PR 25/6, Yonos PARA RS25/6, etc.).
Practical reading of the designation: breakdown of specific ATTACK products
Let’s look at how the designation appears in reality on specific products available for ATTACK boilers:
ATTACK PR 25/6, 180 mm, class A
The designation PR 25/6 breaks down as follows: PR is the model series (an internal designation by the manufacturer for a specific range), 25 is the nominal diameter of the flanges (DN 25), 6 is the maximum pumping head (6 m w.c.). Added to this is the center distance 180 mm and the energy efficiency class A. ATTACK Heating circulation pump PR 25/6, 180 mm, class A is therefore a pump with a larger center distance (for boilers with a wider mounting space), hydraulically identical to other 25/6 models, but physically larger.
ATTACK 25/6, 130 mm, class A
Here the PR series is omitted – this is the basic model designation without reference to a specific series. The center distance 130 mm is more compact and better suited for boilers with less space for the pump. ATTACK Heating circulation pump 25/6, 130 mm, class A is hydraulically equivalent to the 180 mm version – same pumping head, same flange diameter.
Wilo Yonos PARA RS25/6
In the designation RS25/6, RS is the model series by Wilo (Roto Speed – i.e., pumps with speed control). 25 again refers to DN 25, 6 to the maximum pumping head of 6 m. ATTACK Pump WILO Yonos PARA RS25/6, 130 mm is an original Wilo pump installed by ATTACK in its boilers. The Yonos PARA series is one of the most widespread and reliable series on the Slovak market.
The variant ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm adds the suffix RKC – this indicates a specific hardware configuration (type of terminal block, power supply method and signal wiring for communication with the boiler control). Hydraulically, this pump is identical, but the electrical wiring may differ.
Replacement part RS 25/6-3-PR, 180 mm
In the designation ATTACK Replacement part – pump RS 25/6-3-PR, 180 mm, we are also interested in the number 3 after the slash in the extended designation. This number indicates the number of manual speed settings (for traditional pumps without electronic regulation) – but in a modern context, it is more of an internal production code. The hydraulic characteristics remain at 6 m of maximum pressure with DN 25 flanges.
Hydraulics in practice: how to estimate whether 25/6 is sufficient for your system
The pressure loss in a heating system depends on several factors: the length and diameter of the pipes, the number and type of valves, the resistance of the boiler itself, the resistance of the radiators and control elements. For an approximate calculation, simplified formulas are used in practice, but in custom work, technicians usually rely on experience and manufacturer catalog data.
For a typical family house up to 150 m² with radiator heating and modern thermostatic valves, the pressure loss usually does not exceed 2.5–4 m w.c. The pump 25/6 therefore has sufficient reserve, which is the reason why this performance is standardized in heating boilers – it covers most standard installations.
In floor heating systems, the situation is more complex: long circuits (over 100 m per one circuit) can generate a pressure loss of 3–5 m w.c., and if the system is properly designed with hydraulic balancing, the total resistance should not be higher. In practice, however, we encounter systems where balancing was not done during the initial installation, connections are partially clogged, and the pump is running at maximum speed – in such cases, the 25/6 may be barely sufficient.
Practical example No. 1: Older family house, circulation problem
The customer has a house from 1985, steel pipe distribution, 12 radiators. The boiler ATTACK with an integrated 25/6 pump – and despite that, the last radiators in the system are cold. The problem is not in the 6 m w.c. parameter (which is sufficient), but in the fact that the pump was running only at the first stage and the system was not hydraulically balanced. After setting the pump to a higher speed (or switching to automatic mode), the circulation balanced out. Conclusion: the number 6 was correct, the problem was elsewhere.
Practical example No. 2: New construction with floor heating
House 200 m², 8 floor heating circuits, on average 80 m each. The system was installed with one 25/6 pump directly in the boiler. The calculation showed that at maximum flow (all thermostats open), the system pressure loss is around 5.2 m w.c. – the pump is operating at the extreme point of the characteristic curve. Recommendation: adding a secondary pump after the collector or replacing it with a 25/8 model. Here it is evident that the number 6 is not unlimited.
Comparison of 25/4 vs. 25/6 vs. 25/8 – when to use which
Manufacturers’ standard logic offers several performance variants for the same pipe diameter. For DN 25, these combinations are common:
| Designation | Hmax [m] | Typical Qmax [m³/h] | Typical use |
|---|---|---|---|
| 25/4 | 4 | ~2.0 | Small apartments, simple system, short piping |
| 25/6 | 6 | ~2.5–3.0 | Family houses 80–200 m², radiators and floor heating |
| 25/8 | 8 | ~3.0–3.5 | Larger houses, long circuits, combined systems |
| 32/8 | 8 | ~4.5–5.0 | Multi-functional buildings, boiler rooms, larger installations |
The 25/6 variant is installed by default in ATTACK boilers, because the performance range of Attack boilers (from 15 to 45 kW for standard residential boilers) corresponds to the need for a pump with exactly these hydraulic parameters. Choosing a different parameter would require modifying the boiler or an external hydraulic circuit.
Measurement units and conversions – to avoid confusing bar and meters
Head of pumping is expressed in various units in different documents, and this often leads to misunderstandings. A basic overview:
- 1 m w.c. (meter water column) = 9.81 Pa·m⁻¹ · 1 m = 9810 Pa ≈ 0.0981 bar ≈ 0.1 bar
- 6 m w.c. = 6 × 0.0981 bar ≈ 0.59 bar ≈ 0.6 bar = 60 kPa
- In technical data sheets you may also encounter the value in kPa: 6 m w.c. = 58.9 kPa ≈ 59 kPa
- Older literature sometimes uses at (technical atmosphere): 1 at = 1 kp/cm² ≈ 0.981 bar, so 0.6 bar ≈ 0.612 at
This is practically important, for example, when setting up an expansion tank, checking the manometer on the boiler, or comparing technical data sheets from different manufacturers. The number 6 in the pump designation always means 6 meters of water column – not 6 bar (which would be a huge pump for industrial use).
Service Reading: How to Identify Parameters When Replacing a Pump
When a pump fails and you need to replace it, the identification process is as follows:
Step 1 – Read the nameplate from the existing pump. On the body of the pump there is always a label or stamped plate with the type designation. Look for a sequence of numbers in the format XX/Y (e.g. RS 25/6, 25/6-130, etc.). Take a photo of the entire plate.
Step 2 – Measure the center distance. Measure the distance between the centers of the flange axes – that is, the distance between the center of the inlet and outlet thread. For ATTACK boilers, the result will be either 130 mm or 180 mm. Measure twice.
Step 3 – Check the electrical connection. Check whether the pump is powered by 230 V single-phase or 400 V three-phase (for 25/6 it will always be 230 V) and how the signal wiring is routed (modern ECM pumps may use a PWM signal or 0–10 V analog).
Step 4 – Compare with available spare parts. Based on steps 1–3, select the correct pump. For ATTACK boilers, several variants are available – for example, the original ATTACK Spare Part – pump RS 25/6-3-PR, 180 mm for models with a larger center distance, or Wilo Yonos PARA for boilers requiring this specific pump.
A detailed replacement procedure can be found in the articles Installation of a Circulation Pump in an ATTACK Boiler: Procedure and Common Mistakes and Replacement of a WILO Yonos PARA Pump in an ATTACK Boiler: What You Need to Know.
Original Spare Part vs. Universal Pump – The Range of Options
The designation 25/6 is standardized, so it should be true that any 25/6 pump with the same center distance is interchangeable. In practice, this is not always the case, and it is precisely here that parameters hidden behind the basic numerical designation come into play:
- Communication protocol: ATTACK boilers have electronic control that can communicate with the pump via PWM or a proprietary protocol. An incorrect pump (even hydraulically identical 25/6) can cause error messages from the boiler because the control unit does not receive feedback.
- Energy class: If the boiler reports consumption and the energy class of the system, replacing a class A pump with a class C one will change the monitoring results.
- Physical dimensions of the body: Some 25/6 pumps with a 130 mm center distance have a wider body that physically does not fit into the boiler despite the correct center distance.
Therefore, it is always safer to choose an original spare part or a verified compatible one. More on this topic can be found in the article Original vs. Universal Spare Pump for an ATTACK Boiler: Is It Worth Saving?.
Most Frequently Asked Questions (FAQ)
What exactly does the number 25 in the designation 25/6 mean?
The number 25 indicates the nominal inside diameter (DN 25) of the pump’s flanges – that is, the inlet and outlet connections. In practice, this corresponds to a G 1½" thread and tells you what diameter of piping system connects to the pump. Nothing else – the physical size of the pump body, center distance, or performance class – is derived from this number.
What does the number 6 in the designation 25/6 mean – is it pressure in bars?
No, it is not 6 bars – that would be an industrial pump. The number 6 expresses the maximum head in meters of water column (m w.c.), which corresponds to a pressure of approximately 0.59 bar or 59 kPa. It is the maximum value the pump can generate at zero flow. In real operation, the pump always works at a lower point on this characteristic curve.
Can I replace a 25/6 pump with another 25/6 from a different manufacturer?
Hydraulically yes – the same flange diameter and the same maximum head. Mechanically, you must check the center distance match (130 mm vs. 180 mm). Electrically, you must verify compatibility with the boiler control – modern ATTACK boilers communicate electronically with the pump and not every universal 25/6 pump supports these signals. Therefore, for ATTACK boilers, it is always a safer choice to use an original spare part.
Why do 25/6 variants with 130 mm and 180 mm center distances exist – isn’t it an unnecessary complication?
No – the 130 mm vs. 180 mm center distance depends on the physical construction of the boiler and the space where the pump is installed. Some models of ATTACK boilers have a more compact layout and require a shorter pump (130 mm), while others have more space and a standard 180 mm center distance. From a hydraulic perspective, both variants are identical – they only differ in the physical length of the pump body between the flanges.
Is a 25/6 pump sufficient for my home, or do I need a more powerful one?
For most family homes up to 200 m² with standard radiator or floor heating, a 25/6 pump is sufficient if the system is properly designed and hydraulically balanced. Problems arise in large systems with long circuits (over 100 m per circuit), old unbalanced piping, or when multiple heating circuits are used without a collector. In these cases, a 25/8 pump or an additional secondary pump may be necessary.
What do suffixes like "RKC" or "-3-PR" after the basic designation 25/6 mean?
These suffixes are production or configuration codes for a specific model. "RKC" in Wilo Yonos PARA pumps indicates the type of connector and signal interface, which affects electrical compatibility with the boiler. The suffix "-3" may indicate the number of manual steps or hardware version. "PR" is a model series. The hydraulic parameters (25/6) remain the same for all these variants – only the control method and physical connector design change.
Conclusion: 25/6 numbers are the foundation, but not the whole story
The parameters 25/6 are the basic hydraulic identity of the pump – they indicate the connection diameter (DN 25) and the maximum head (6 m w.c.). For correct selection and replacement of a pump in an ATTACK boiler, they are essential, but not sufficient on their own. You must always add the center distance (130 mm or 180 mm), energy class (for modern installations always class A), and the model series designation, which determines the electrical and communication compatibility with a specific boiler.
Understanding these parameters will save you time and money – whether when selecting a new pump, during custom installation, or when troubleshooting. If you need to go deeper – for example, how to choose the correct model for a specific ATTACK boiler – we recommend the article How to Choose a Circulation Pump for an ATTACK Boiler and Common Circulation Pump Faults in ATTACK Boilers and Their Causes.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we're happy to help.