How to Choose a Circulation Pump for an ATTACK Boiler

We are still preparing products.

But you can look at other categories.

How to choose a circulation pump for the ATTACK boiler: a complete guide

The circulation pump is the heart of any heating system. When it stops working, the boiler may still burn, but the heat simply won't spread through the house. In practice, we see that it's precisely the selection of a replacement pump that causes the most confusion for customers – not because the pumps are complicated, but because there are multiple types, sizes, and performance variants on the market, and every detail matters. This article will help you navigate and choose the right pump for your specific ATTACK boiler, without unnecessary guesswork and without the risk of ordering something that doesn't fit.

ATTACK boilers are very common on the Slovak market – they are a domestic manufacturer from Martin, producing solid fuel, pellet, gas, and combined boilers. The circulation pumps in these boilers have standardized dimensions and performance, but there are differences between individual models and series that you need to be aware of. We'll go through everything from the basics – what the pump actually does, which parameters are important, how to distinguish dimensions, when to replace it, and what to buy specifically.

What does the circulation pump do in an ATTACK boiler

The circulation pump ensures forced circulation of water in the primary circuit of the heating system. Without a pump, hot water from the boiler would move through the pipes only by the thermosiphon effect – very slowly and inefficiently. The pump creates overpressure that pushes the heated water to the radiators or floor heating and returns the cold water back to the boiler. In practice, this means that the pump runs almost continuously throughout the entire heating season, making it one of the most stressed components of the entire system.

In ATTACK boilers, the pump is either located directly in the body of the boiler (internal pump) or on the output pipe outside the boiler. Most modern ATTACK boilers have an integrated internal pump – this simplifies installation and protects the pump from external influences, but it also places requirements on precise adherence to dimensional parameters when replacing it.

As you can see on the diagram, the pump is located in the return pipe (return line) – that is, where the water is the coldest. This is standard practice, as colder water is denser and reduces the risk of cavitation in the pump. In some older installations, you may also encounter a pump on the output, which is not ideal, but it works.

Key parameters of a circulation pump: what do the numbers mean

When you look at a pump designation such as 25/6, each number tells you something. The first number (25) indicates the diameter of the thread of the connection in millimeters – that is, 25 mm, which corresponds to a G 1" (inch) thread. The second number (6) indicates the maximum pumping head of the pump in meters of water column. This parameter directly affects what hydraulic resistance the pump can overcome – the larger the house, the longer the system, and the more radiators, the greater the resistance and the higher the pumping head required.

For typical family homes with an ATTACK boiler, the combination of 25/6 is standard. A performance of 6 meters of water column is sufficient for most standard installations up to an area of around 200–250 m². If you have a larger building or a complex system with long pipe runs, it is advisable to have a hydraulic calculation done by a specialist. A more detailed explanation of the parameters can be found in the separate article Pump parameters 25/6: what do the numbers mean and how to read them in this Knowledge Center.

Another important parameter is the axial distance between the connections – that is, the distance between the centers of the pump's inlet and outlet ports. In ATTACK boilers, two values are used: 130 mm and 180 mm. This is probably the most critical dimension when selecting a replacement pump, as a mismatch means the pump simply won't fit into the boiler – the screws won't reach the place, or you may have to make adaptations, which is not recommended.

A detailed guide on how to determine which dimension fits your specific boiler can be found in the article Boiler Pump Size 130 mm vs 180 mm: Which Fits My Boiler. Quick tip: always measure the distance between the centers of the connections on the existing pump or in the boiler body before placing an order.

Overview of available pumps for ATTACK boilers

At atria.sk, you will find several variants of pumps for ATTACK boilers, each with its specific purpose. Let's go through them systematically.

Pumps with 130 mm center distance

ATTACK Heating Circulation Pump 25/6, 130 mm, Class A is a standard heating pump primarily intended for newer series of ATTACK boilers, where the center distance of the connections is 130 mm. Performance 25/6 (connection diameter 1", head 6 m) is sufficient for typical family homes. Classification Class A means that the pump meets the strictest energy requirements of the ErP (ecodesign) directive valid since 2013 – more specifically, the pump has an integrated electronic speed control that automatically adapts to the current needs of the system. In practice, this means significantly lower electricity consumption compared to older pumps with fixed speeds.

For those looking for an original part from Wilo – a renowned German pump manufacturer – there is ATTACK Pump WILO Yonos PARA RS25/6, 130 mm, replacement part for ATTACK boilers. Yonos PARA is a proven design with an electronically commutated motor (EC motor), which is among the most popular and reliable pumps on the Central European market. This pump is certified by the manufacturer as a replacement part for ATTACK boilers – that is, it is an original, not a clone or a cheap substitute. The WILO Yonos PARA pump is characterized by quiet operation (under 30 dB at lower speeds), smooth regulation, and long service life.

Pumps with 180 mm center distance

ATTACK Heating Circulation Pump PR 25/6, 180 mm, Class A is intended for boilers where the center distance of the connections is 180 mm. The designation "PR" in the name refers to a specific mounting type (also used in some commercially distributed models of ATTACK). It is again a Class A pump with electronic regulation. This dimension typically appears in older models of ATTACK boilers or in models of higher performance classes, where there is more space in the boiler body.

The second option for 180 mm is ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm, replacement part for ATTACK boilers. The designation "RKC" in the name identifies a specific type of connector – it is a version with an integrated regulating connector adapted to the control unit of certain models of ATTACK. This is an important point: if your ATTACK boiler communicates with the pump electrically (speed control via boiler control), you must use a compatible connector. If you install a pump without the correct connector, the boiler will either not start the pump at all or will control it incorrectly – an error code will appear on the display. More on this is discussed in the article Replacing the WILO Yonos PARA Pump in an ATTACK Boiler: What You Need to Know.

The last product in this category is ATTACK Replacement Part – Pump RS 25/6-3-PR, 180 mm. This model is explicitly marked as a replacement part – that is, it is intended for specific series of ATTACK boilers that originally contained this type of pump. The number "3" in the designation RS 25/6-3 usually indicates a three-speed version, meaning that the pump does not have smooth electronic regulation, but three fixed performance levels (1-2-3). This is a technically older approach, but still functional and sufficient for certain simple installations. When replacing this pump, it is crucial to maintain the same type (three-speed), if the boiler control unit does not support EC motors.

Energy class A: why it matters

Pumps of class A have been mandatory for new installations within the EU since January 1, 2013 (ErP 641/2009 directive). An old pump without electronic regulation consumed 60 to 120 W of electrical energy continuously throughout the entire heating season. A modern class A pump with an EC motor consumes an average of 5 to 25 W – depending on the current load of the system. For a 6-month heating season (4,380 hours), the difference in consumption is as follows:

Old pump (80 W average): 80 × 4,380 = 350 kWh/year, at a price of 0.25 €/kWh = 88 €/year
New class A pump (15 W average): 15 × 4,380 = 66 kWh/year, at a price of 0.25 €/kWh = 16.5 €/year
Annual savings: ~71 € – so the pump pays for itself in 2–3 years

This is a real calculation, not a marketing figure. For more information on what class A means technically in practice, read the article Energy efficiency class A for circulation pumps: what it means in practice.

Original vs. universal pump: the dilemma when replacing

This is a question that arises almost with every order: "Do I need to buy an original pump, or is a cheaper universal replacement sufficient?" The answer depends on several factors.

Original pumps (such as WILO Yonos PARA marked directly as a replacement part for ATTACK boilers) are tested on specific boiler models, have the corresponding connector, and are guaranteed to be compatible with the control unit. This is especially important for boilers with modern electronics, where the boiler control actively communicates with the pump – it monitors the pump’s operating status, performance, and possibly error messages. If you install a pump with a different connector or communication protocol, the boiler may report a fault even though the pump is physically running.

A universal pump (e.g., a standard Grundfos MAGNA1 or an unbranded copy) may have the correct dimensions and performance, but the connector may not fit, or the boiler will only operate in a backup mode (without speed regulation via the boiler). This is not the end of the world, but you will lose some functions and energy efficiency will not be optimal.

Our recommended strategy from practice: if the ATTACK boiler is under warranty or it is a newer model with advanced control, always choose the original spare part. If it is an older boiler without electronic communication with the pump (the pump is simply switched on via a relay), you can use a quality universal type – but still pay attention to the dimensions and performance. A more detailed look at this decision is provided in the article Original vs. universal replacement pump for the ATTACK boiler: is it worth saving.

How to determine which pump you need: step by step

In practice, we often see people coming in with a photo of the old pump, the boiler’s type plate, or the knowledge that "the boiler is an ATTACK from 2012." Let's go through a systematic procedure for this.

Step 1: Determine the model of the ATTACK boiler. The type plate is usually on the front panel or on the side of the boiler. Record the full model – for example, "ATTACK DPX 35." This information will allow you to verify directly with the manufacturer or seller which pump was used in the original configuration.

Step 2: Measure the center-to-center distance of the connections. If the boiler is still in operation and the pump is physically in the boiler, measure the distance between the centers of the inlet and outlet ports. If the pump is removed, measure the pump itself. A value of 130 or 180 mm is a critical parameter.

Step 3: Check the electrical connector. Examine how the pump is electrically connected. If you see a standard three-pole plug (L, N, PE), it is a simple connection without communication – you can use most compatible pumps. If you see multi-pin connectors or a special connection, it is likely a communication connector (such as RKC), and you need a specific type.

Step 4: Verify the performance and connection size. On the type plate of the old pump, there should be something like "RS 25/6" or "25-6." These numbers must remain the same – the diameter G1" (25 mm) and the head 6 m.

Step 5: Decide between original and replacement. Based on the age of the boiler, the warranty status, and the type of control, choose the appropriate option from the range.

Typical scenarios from practice: what customers most often deal with

From the dozens of orders we see each year, certain typical situations repeat themselves. Let's go through the most common ones.

Scenario 1: ATTACK DPX boiler from 2010 – pump is humming and not circulating. The customer has an older boiler model with a 180 mm pump, three-speed transmission. The pump is humming because its bearings are worn out or it is clogged. Solution: ATTACK Spare part – pump RS 25/6-3-PR, 180 mm – a directly compatible spare part without the need for electrical modifications. Symptoms of pump failures are described in detail in the article Common circulation pump failures in ATTACK boilers and their causes.

Scenario 2: New ATTACK PELLET 25 boiler from 2021 – planned pump replacement after warranty ends. In this case, the boiler communicates with the pump, has an EC motor and 130 mm connections. The customer wants to buy a spare pump for storage. The clear choice: ATTACK Pump WILO Yonos PARA RS25/6, 130 mm – an original spare part that the customer can keep in stock and replace in case of failure without waiting for delivery.

Scenario 3: The customer does not know which boiler model they have, and just wants a "pump for ATTACK". This is the riskiest situation. In such a case, we always recommend measuring the shaft distance before ordering, ideally also taking a photo of the boiler's nameplate and the existing pump. Without this information, there is a risk of ordering the wrong size.

Scenario 4: The boiler is running, but radiators in a certain part of the house are cold. The customer does not immediately think about the pump – rather, about clogged radiators. In practice, however, the problem may be reduced pump performance – for example, a clogged filter, worn rotor, or the pump running only on the first stage. Diagnosis: check the pump's power setting and possibly bleed the system. The pump does not necessarily need to be replaced immediately.

Scenario 5: After house renovation, additional radiators were added and the system cannot circulate properly. In this case, the capacity of the original 25/6 pump may be insufficient if the pipes are long and many new circuits were added. In such a case, a hydraulic calculation is needed and possibly a pump with a higher head (e.g. 25/8 or 32/8). This is, however, not a typical order for a standard family house.

What to pay attention to during installation

Installing a circulation pump into an ATTACK boiler is not technically complex, but there are mistakes that customers repeatedly make. A detailed procedure can be found in the article Installation of a circulation pump into an ATTACK boiler: procedure and common mistakes, here we list the most important points.

Always close the filling/draining valves before removing the old pump. If the boiler does not have them directly at the pump, drain the system. Underestimating this step = flooding the boiler room.
Pump position: The pump must be mounted so that the motor axis is horizontal. Vertical mounting (axis up or down) is prohibited for most models – it shortens the bearing life and can cause air accumulation.
Sealing: Use new O-rings or sealing tape, never the old/hard sealing from the old pump.
Deairing: Always bleed the system after installation. A pump with air can overheat and fail the same day.
Electrical connection: Check the polarity and connector before the first start. Wrong connection of an EC motor can cause boiler error codes.
First start: Let the pump run for 10–15 minutes and check for leaks at the connections. Tighten the nuts only after the water temperature has dropped (hot nuts tighten differently).

Pump lifespan and when to replace it

A quality circulation pump from a reputable manufacturer has a lifespan of 10–15 years with proper operation. In practice, we encounter pumps that last 20 years, as well as those that fail after 5 years – it mainly depends on the water quality in the system, frequency of service and correct settings.

Signs that indicate the pump is nearing the end of its life or about to fail:

Noisy operation – humming, tapping, squeaking (worn bearings, air in the system)
The pump is running, but heating is uneven – reduced rotor performance
Increased electricity consumption without an obvious reason
The pump stops and restarts – electrical failure or overheating
Water leakage at the shaft – worn sealing sleeves
The boiler displays an error code related to circulation (depends on the ATTACK model)

More symptoms and their interpretation can be found in the article When to replace a circulation pump in a boiler: symptoms and lifespan.

From a preventive point of view, we recommend: if the pump is older than 12 years and shows the first signs of wear, do not wait for a complete failure in the middle of winter. Preventive replacement is always cheaper than an emergency repair on a freezing weekend with a service technician called in outside working hours.

Most frequently asked questions (FAQ)

How do I find out whether I need a pump with a 130 or 180 mm shaft distance?

The most reliable method is direct measurement. Remove the boiler covers so you can see the pump and measure the distance between the centers of both connection flanges (input and output). Alternatively, measure the pump itself from the center of one flange to the other. If you cannot reach the pump in the boiler, the boiler's nameplate with the model number will help – based on it, the seller (or the ATTACK manufacturer) can determine which type of pump was originally installed. General rule: newer series of ATTACK boilers (after approx. 2015) mostly use 130 mm, older series and higher power classes commonly use 180 mm. This is not an absolute rule – always measure.

What does the RKC connector mean and do I need it?

RKC (Remote Control Connector) is a special multi-pin connector that allows the ATTACK boiler's control unit to communicate directly with the pump – monitor its status, set performance and receive error messages. If your ATTACK boiler originally had a pump with an RKC connector (you can tell by the fact that two separate cables run from the pump – power and control), you must use a replacement pump with the same type of connector. If you use a pump without RKC, the boiler will either display an error code or will control the pump only via power supply without regulation, which reduces energy efficiency.

Can I use any 25/6 pump, or must it be specifically for ATTACK?

From a hydraulic point of view, any 25/6 pump with the appropriate shaft distance is physically compatible – the flange size G1" is standardized. The problem arises with electrical connection: if your boiler has a communication connector (RKC or similar), a generic pump may not be compatible and the boiler may report a fault. For older boilers without communication (the pump is switched on only via a relay), any quality pump of the correct size and performance is functional. For newer boilers, we recommend original spare parts or pumps explicitly certified for ATTACK.

Why is the pump buzzing, even though it was recently replaced?

Buzzing of a new pump most often has these causes: air in the system (most common – bleed the system and the pump itself via the bleed screw on the rotor head), setting too high performance (try reducing to stage 1 or automatic mode), mechanical stress in the piping (piping must not be attached directly to the pump without compensators), or incorrect installation (pump in the wrong position – rotor axis must be horizontal). If buzzing persists after bleeding and checking the position, contact service.

How long does a circulation pump in an ATTACK boiler last?

Under normal operation and good water quality in the system (properly degassed, without corrosion), you can expect a quality pump from WILO or Grundfos to last 10–15 years. In practice, we most often see a lifespan of around 8–12 years – depending on whether the system was regularly maintained, whether the pump filter was regularly cleaned, and whether the water in the system was aggressive (acidic, with high calcium content). Pumps in boilers where water is drained and refilled every summer (which is not recommended) have a shorter lifespan due to the corrosive effect of fresh water.

Is pump installation something I can do myself, or do I need a heating technician?

The physical replacement of a pump is manageable even for a layperson with manual dexterity and the correct procedure – it involves loosening two nuts, replacing the gasket, and re-screwing. The more problematic part is the electrical connection in boilers with communication connectors – in these cases, we recommend assistance from a technician, as incorrect wiring can damage the control unit. Before any installation, read the procedure in the article Mounting a circulation pump in the ATTACK boiler: procedure and common mistakes and check whether replacing the pump yourself could void the warranty of your boiler.

Conclusion: Choosing the right pump saves time, nerves, and money

Selecting a circulation pump for the ATTACK boiler may seem complicated at first glance, but in reality, it's enough to know three key parameters: the distance between the connections (130 or 180 mm), the type of electrical connection (standard power supply or communication connector RKC), and the performance (25/6 for standard applications). Once you know these three things, the selection is simple.

From the perspective of quality and reliability, we recommend using original spare parts for ATTACK boilers from the last 10 years – specifically the WILO Yonos PARA pumps, which are directly certified for these boilers and guarantee full compatibility. For older boilers without electronic communication, the selection is a bit more flexible.

In case of doubts, it is worth consulting with a seller before placing an order – a few minutes on the phone can save you from returning the wrong product and waiting for the correct one. On the page atria.sk/attack, you will find the complete current range of pumps for ATTACK boilers with current prices and availability in stock.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we're happy to help.