Frequently Asked Questions About Circulation Pumps for ATTACK Boilers
Common questions about circulation pumps for ATTACK boilers – a comprehensive guide
The circulation pump is the heart of every heating system with an ATTACK boiler. Despite being a relatively small and inconspicuous component, it is precisely on it that the boiler's ability to distribute heat, whether the heat exchanger overheats, and ultimately how much you pay for electricity during the entire heating season depends. Over the years of experience in customer practice, we repeatedly encounter the same questions – whether from people who have bought a new ATTACK boiler and are looking for a replacement pump, or from those whose existing boiler has suddenly started to make noise, not heat, or whose service technician said the pump "has gone". This article is an overview of the most common questions we receive, expanded with practical explanations and technical context.
Why is choosing the right pump for an ATTACK boiler so important?
ATTACK boilers are designed so that the circulation pump is an integral part of the hydraulic circuit. It is not like in some older systems, where the pump was "somewhere on the manifold" and could be replaced with almost anything with similar flanges. In ATTACK boilers – specifically in models DP, DPX, DP PELLET, SL, PELLET and others – the pump is mounted directly in the boiler body or in the manifold, and its parameters must match not only hydraulically, but also physically in terms of dimensions.
When a customer comes with the question "which pump for my ATTACK", the first two things we address are: the mounting length (130 mm or 180 mm) and the pump's performance point (25/6 or other). Without these two basic parameters, it is not possible to give reliable advice. A more detailed look at the dimensional issue can be found in the article Mounting length of the pump 130 mm vs 180 mm: which fits into my boiler, where specific models of ATTACK boilers are also listed according to the mounting dimension.
What do the numbers 25/6 on the pump mean?
This is by far the most common question we receive from people who are not yet very familiar with pumps. The designation 25/6 consists of two numbers:
- 25 – maximum diameter of the connecting thread, or the nominal bore of the flange in millimeters (DN 25, which corresponds to a G 1½" thread)
- 6 – maximum pumping head (pumping pressure) in meters of water column (m w.c.), which is 0.6 bar for water
In practice, this means that the 25/6 pump can overcome the hydraulic resistance of the circuit up to 6 m w.c. at a flow rate approaching zero, or at zero pressure it achieves the maximum flow. The actual operating point lies somewhere between these extremes on the pump curve. For ATTACK boilers with a power of 15–35 kW (which is the most common configuration in family homes), the 25/6 parameter is usually sufficient – this power range covers most installations with floor heating or radiators. A deeper look into the issue of parameters is offered by the article Pump parameters 25/6: what do the numbers mean and how to read them.
What is the difference between a 130 mm and 180 mm pump?
The mounting length (distance between the centers of the connecting threads) is a purely dimensional parameter – it says nothing about the performance or energy efficiency class of the pump. Both dimensions can have the same hydraulic performance of 25/6. The reason both exist is historical and production-related: different generations of ATTACK boilers had different spatial arrangements of the hydraulic block, and depending on that, either a shorter (130 mm) or longer (180 mm) variant of the pump was installed.
From the customer's point of view, it is essential: a 180 mm pump simply cannot be installed in a place intended for a 130 mm pump and vice versa. The distance between the flanges of the boiler is precisely defined and cannot be overcome without interfering with the hydraulics – which even experienced technicians do not do in practice, as it would unnecessarily complicate the installation and could violate the warranty conditions. Therefore, it is essential to measure the existing pump or check the boiler's technical documentation before ordering a replacement pump.
Both variants are available in the range: for example, ATTACK Heating circulation pump PR 25/6, 180 mm, class A for boilers with a longer mounting, or ATTACK Heating circulation pump 25/6, 130 mm, class A for a shorter arrangement.
What is energy efficiency class A and why does it matter?
Energy efficiency class A for circulation pumps is today the mandatory minimum standard in the European Union for new boilers and for the sale of separate pumps since 2015. However, in older installed equipment – and thus also in many ATTACK boilers from the years 2010–2014 – pumps of class C or D were installed. These pumps consume significantly more electrical energy: a comparison shows that a class D pump with a power consumption of 40–60 W will cost about 48–72 € per year in operation (at 6000 hours per year and an electricity price of 0.20 €/kWh). A modern class A pump (electronically commutated motor, automatic regulation) typically has a power consumption of 5–25 W depending on the current load – annual electricity costs thus drop to 6–30 €.
Savings of 30–50 € per year may seem small, but with an assumed pump lifespan of 10–15 years, we are talking about 300–750 € over the entire lifetime. And this does not include cases where a pump with manually set speed is forgotten to be switched from winter to summer mode and runs at full power throughout the year. Class A also brings another advantage: automatic regulation according to the hydraulic resistance of the circuit (so-called proportional or constant pressure mode), which means the pump automatically reduces speed with changes in thermal load – the system is quieter and more comfortable.
More on this topic is discussed in the article Energy efficiency class A for circulation pumps: what it means in practice.
Are WILO Yonos PARA pumps directly interchangeable with original ATTACK pumps?
Yes – and no. The WILO Yonos PARA series pumps are electronically controlled class A pumps that are physically mounted in the same flange positions as the original pumps. As long as they fit dimensionally (130 or 180 mm construction length) and in terms of performance (25/6), they are a direct hydraulic replacement. The difference may be in the electrical wiring – some ATTACK boilers have a control unit that communicates with the pump via a PWM signal (Pulse Width Modulation) or via a 0–10 V analog signal. If the old pump is a three-speed model with an external selector switch, the new electronic WILO Yonos PARA pump operates autonomously and communicates with the boiler differently – or does not communicate at all and is controlled by its own logic.
In practice, this is usually not a problem: the pump operates safely even without active communication with the boiler's control unit, simply ensuring the circulation of the medium and regulating itself according to its own pressure sensor. Problems arise exceptionally in boilers with advanced hydraulic control, where the control unit expects feedback from the pump. In such cases, it is necessary to consult with the ATTACK technical department or an authorized service. Replacement WILO pumps directly for ATTACK boilers can be found, for example, here: ATTACK Pump WILO Yonos PARA RS25/6, 130 mm or ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm.
How can I tell that the pump in my ATTACK boiler is broken or has reached the end of its life?
Failures of the circulation pump are expressed in several ways and are not always immediately obvious. From practice, we know that customers often come with the complaint that "the boiler heats, but the house is cold" or "the boiler turns on and off every few minutes" – both of these situations can be caused by the pump, not the boiler itself.
The most common signs of a faulty pump:
- Noise – knocking, tapping, squeaking, or rumbling. It may be air in the system, worn bearings, or cavitation (underpressure in the suction part).
- The boiler switches to a "high temperature" fault – the pump is not running or is running too slowly, the heat exchanger overheats and the thermostat shuts off the boiler.
- Uneven heating – some radiators or floor heating circuits are cold, others are warm. The pump is not delivering sufficient flow.
- The pump is not running – the motor is jammed (blocked by limescale or rust after a period of inactivity), the rotor is not turning. Sometimes it is enough to unscrew the cover and free the shaft with a screwdriver.
- Leakage
- High electricity consumption – older class C/D pumps with a damaged rotor or magnetized bearings may have higher consumption than usual.
This topic is discussed in more detail in the article Common circulation pump failures in ATTACK boilers and their causes. Information on when it is the best time to replace the pump can also be found in the article When to replace the circulation pump in a boiler: symptoms and lifespan.
Is it worth buying an original ATTACK pump or is it better to go for a cheaper alternative?
This question is asked by almost every customer who decides to replace the pump. The answer is not black and white – it depends on the age of the boiler, warranty conditions, and what you expect from the replacement.
Original (OEM) pumps that ATTACK supplies as spare parts (including RS 25/6 variants marked with the manufacturer WILO) are physically identical to those that were in the boiler from the factory. They guarantee 100% dimensional and electrical compatibility, do not raise any questions regarding warranty, and service technicians from ATTACK work with them without reservations. For example, ATTACK Spare Part – Pump RS 25/6-3-PR, 180 mm is a direct industrially produced replacement with full compatibility for the relevant boiler models.
Universal alternatives (typically cheaper Asian-made pumps) may work, but the risks are real: different electrical connections for the thermistor or fuse, different H/Q curve characteristics (the pump may be labeled as "25/6" on paper, but in reality achieves maximum head only at a lower flow rate), and especially the issue of boiler warranty. If the boiler is still under warranty, installing a non-original pump may be a reason to reject the warranty repair – just like in a car. This topic is also covered in a separate article Original vs universal replacement pump for the ATTACK boiler: is it worth saving.
Can I replace the pump myself, or do I need a technician?
Replacing the circulation pump in the ATTACK boiler is considered a medium-complexity plumbing task – it's not rocket science, but it's not as simple as changing a light bulb. For someone without experience with heating technology, there are several risks: incorrect pump orientation (the rotor must be in a horizontal position or in positions allowed by the manufacturer), insufficient flange sealing, forgetting to bleed the system, or damaging the control electronics by unqualified handling of electrical wires.
In ATTACK boilers, you also need to drain at least part of the heating system content during pump replacement – which requires knowing where the drain valve is, how to work with the expansion tank, and how to properly refill and bleed the system after replacement. If you know all this, you can replace the pump yourself. If not, we recommend hiring a technician – the investment in a service call will pay off in the form of a properly functioning system and the certainty that you won't miss anything. A complete step-by-step guide with warnings about common mistakes can be found in the article Installation of a circulation pump in the ATTACK boiler: procedure and common mistakes.
How long does a circulation pump in the ATTACK boiler last?
The lifespan of a circulation pump depends on the quality of the power supply, the quality of the water in the system, the intensity of operation, and the quality of the pump itself. Original pumps in ATTACK boilers – typically from manufacturers such as WILO, Grundfos, or Laing – are designed for 10,000 – 20,000 operating hours, which at an average heating season (6–7 months, 12–16 hours of daily operation) corresponds to 7–15 years of real use.
In practice, however, we also encounter pumps that run without problems for 20 years (usually in systems with soft and chemically treated water), as well as pumps that fail after 4–5 years (hard water with high calcium content, corrosion due to insufficient venting, or simply a cheaper production line). A preventive replacement after 12–15 years is therefore a reasonable idea, especially if you are planning a complete heating system renovation or boiler replacement.
If you plan to replace the pump preventively (not after a failure), we recommend taking the opportunity to switch to a class A pump – the investment will pay off in lower electricity consumption and more comfortable regulation.
What are the most common mistakes when choosing a replacement pump?
Over the years of practice, we have identified several recurring mistakes that cost customers time and money:
- Mixing up the mounting length – ordering a 180 mm pump instead of a 130 mm one or vice versa. This usually happens because the customer measures from the edge of the cover to the edge, not from the center of one flange thread to the center of the other. Correct measurement: from the center of one thread to the center of the other.
- Ignoring the energy efficiency class – purchasing the cheapest class C pump just because "it has the same 25/6 parameters". From a long-term perspective, this is the more expensive option.
- Purchasing a pump without verifying compatibility with the boiler's control unit – in newer models of ATTACK with advanced electronics, it may be important whether the pump supports PWM control or not.
- Choosing a pump based on price, not parameters – there are pumps that appear to be 25/6, but their actual H/Q characteristic is weaker. The result is insufficient flow and repeated issues with the boiler entering a fault state.
- Forgetting about pump orientation – most modern class A pumps can be mounted in various positions, but some older models have limitations. A pump with a vertical rotor may overheat and shorten its lifespan.
What to do if the pump is noisy or vibrating?
Noise from a circulation pump is most often caused by one of the following: air in the system, excessively high or low filling pressure, mechanical bearing wear, or cavitation. Diagnostic steps:
First, check the pressure in the system on the boiler's pressure gauge. It should be 1.0–1.5 bar in a cold system. If the pressure is too low (below 0.8 bar) or too high (above 2.5 bar), bleed the radiators and possibly adjust the expansion tank. A large portion of "noisy pumps" can be resolved by bleeding the system – air in the pipes causes characteristic knocking or water noise.
If the noise persists after bleeding the system, check whether the pump is set to too low a performance (classes C/D with manual speeds) with too closed regulating valves – this causes cavitation. Set the pump to a higher speed or slightly open the thermostatic heads.
If that doesn't help and the noise sounds like squeaking or metallic tapping – the bearings are worn out and the pump needs to be replaced. During a long operational break (e.g., summer months), the rotor can sometimes get stuck due to deposits – try to loosen it by unscrewing the hanger screw and gently turning the shaft with a screwdriver, then screw it back in.
Most Frequently Asked Questions (FAQ)
Do I need to drain the entire heating system when replacing the pump?
Not necessarily the entire system. If there are ball valves (isolation valves) installed at the pump, it is sufficient to close both and drain only a small amount of water between them – usually 0.5–2 liters. Most ATTACK boiler installations have these valves installed by default during assembly. If you don't have them, the system must be partially drained at least to the level below the boiler.
Can I use a WILO Yonos PARA pump from another boiler (e.g., an old Grundfos or Junkers) in an ATTACK boiler?
Hydraulically, yes – provided the construction length (130 or 180 mm) fits, it has G 1½" and a performance of 25/6. WILO Yonos PARA pumps are standardized products and their physical dimensions are regulated. The electrical connection is also standard (230 V, 50 Hz, three-core cable L/N/PE). We still recommend checking whether the pump was used correctly in the original boiler and is not worn out – reusing a used pump from another installation carries risks.
The pump in the ATTACK boiler is running, but the boiler reports a high-temperature fault – what is wrong?
The pump may be running, but it may not be providing sufficient flow – for example, due to cavitation, a clogged filter (pump screen), or because a pump with insufficient performance was installed. Check whether the filter basket before the pump is clean, whether all regulating valves in the system are open, and whether the pump is circulating properly – the flow at the boiler outlet should be warm, and the return should be cooler. If both sides are equally warm, the pump is not running or is running too slowly.
What is the difference between the designations RS 25/6 and PR 25/6 for ATTACK pumps?
Both designations describe a pump with the same hydraulic parameters (25 = DN25, 6 = 6 m head), but from different manufacturers or product lines. The designation RS is typical for WILO pumps (e.g., Yonos PARA RS25/6), while PR may indicate another production line or generation intended for a specific ATTACK boiler. Always verify the construction dimensions and electrical parameters before replacement – hydraulic compatibility is likely, but not automatically guaranteed.
How much does replacing the pump in an ATTACK boiler cost, including the technician's work?
The price of the pump alone ranges from 60–200 € depending on the variant (OEM spare part vs. a heating workshop pump of class A). Installation by an authorized technician usually costs 50–120 € including travel, bleeding, and adjustment – so total costs are typically 130–300 €. When you compare these costs with what you would pay for an incorrectly installed pump (overheating, boiler shutdowns, possible additional service visits), it becomes clear why it's worth doing it right from the start.
Can the pump in an ATTACK boiler be replaced under warranty if the original one failed?
Yes – if the ATTACK boiler is within the warranty period and the pump failed without user intervention (i.e., not due to mechanical damage or improper operation), the warranty repair should include the pump replacement. The condition is that the claim is handled by an authorized ATTACK service and the procedure is followed according to the manufacturer's warranty terms. A self-replacement of the pump during the warranty period – even if done correctly – may affect the remaining warranty period of the boiler, so we recommend contacting the service during the warranty period.
Conclusion: the right pump = a reliable boiler
The circulation pump is relatively inexpensive compared to the price of the entire boiler, but its correct selection, installation, and operation significantly affect heating comfort, electricity consumption, and the lifespan of the entire heating system. For ATTACK boilers, always check the construction length (130 or 180 mm), performance (25/6 for most domestic installations), and energy class (today it should be at least class A). If you don't know what pump you have in your boiler, take a photo of the type plate on the side of the pump and contact us – we will advise you quickly and without unnecessary searching. An overview of available pumps can be found in the ATTACK pump category on atria.sk.
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.