Replacement of WILO Yonos PARA pump in ATTACK boiler: what you need to know

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Replacement of the WILO Yonos PARA pump in the ATTACK boiler: what you need to know

The circulation pump is the heart of every heating system. If it stops working properly in the ATTACK boiler, the entire system stops – radiators remain cold, floor loops do not transfer heat, and the boiler may start indicating faults. The WILO Yonos PARA pump, which ATTACK installs in its boilers as an original part, has a service life of 10 to 15 years under proper operation. However, sooner or later comes the moment when it needs to be replaced. This article will guide you through the entire process – from recognizing symptoms through selecting the correct replacement part to the actual installation and the first operation after replacement.

If you are considering selecting a pump for the ATTACK boiler from the perspective of parameters or dimensions, I recommend first reading the related articles How to choose a circulation pump for the ATTACK boiler and Pump size 130 mm vs 180 mm: which fits my boiler. Here we will focus primarily on the practical replacement procedure and everything related to it.

Why exactly WILO Yonos PARA and not another brand?

Many customers ask why ATTACK decided on WILO pumps and why it is important to reach for the same type during replacement. The answer is practical: when designing the boiler hydraulics, ATTACK calculates with specific hydraulic characteristics of the pump – with its Q/H curve (flow vs. pressure difference), with the range of adjustable speeds, with the physical dimensions of the mounting space, and with the electrical connection. Yonos PARA is an electronically commutated pump (ECM motor) of energy efficiency class A, which means it operates with variable speeds according to the actual system needs.

If you were to install an old three-speed pump without regulation instead of the original part, you would pay five to eight times higher electricity consumption for pump operation and at the same time you could disrupt the boiler's hydraulic balance. More on the topic of energy efficiency class A can be found in the article Energy efficiency class A for circulation pumps: what it means in practice.

When is replacement really necessary?

Before ordering a new pump, it is important to confirm that the problem is really in it. From practice, I know that a fairly large portion of "pump faults" have a completely different origin – clogged filter, closed ball valve, air in the system, or a fault in the boiler's control board that does not give the pump a signal to start.

Typical symptoms of a real fault in the WILO Yonos PARA pump in ATTACK boilers:

The boiler burns, but the radiators are cold – the combustion chamber produces heat, but the water does not rise into the circuits.
Buzzing or knocking from the boiler area – cavitation or mechanical wear of the rotor bearings.
Error code on the boiler display – most newer ATTACK boilers indicate a circulation fault with code E06, E07 or similar (the exact code depends on the model).
The pump does not start at all – you can check this by placing your hand on the pump body; if it is completely cold during operation and has no vibration, it is most likely not rotating.
The pump rotates, but does not pump water – it may be manifested, for example, by cracking and air hammering in the radiators, even though you have been bleeding them for a long time. It may be a worn rotor or a clogged diffuser.
Leakage at the flange or body of the pump – if water is leaking around the pump, the mechanical seal or O-rings are damaged.

More on diagnostics can be found in the article When to replace the circulation pump in the boiler: symptoms and lifespan and Common circulation pump faults in ATTACK boilers and their causes.

Pump size: 130 mm or 180 mm?

This is the first and most important technical question you must resolve before ordering. The WILO Yonos PARA pumps are supplied to ATTACK boilers in two basic construction lengths:

130 mm – shorter version, common in smaller ATTACK boilers (Lambda, DPX, PELLET series up to approx. 25 kW)
180 mm – longer version, typical for larger boilers and special series (DP series, larger outputs)

The construction length is the distance between the axes of the flange threads (input/output of the pump). If you replace a 130 mm pump with an 180 mm one or vice versa, the pump simply does not fit into the boiler – the connection opening is either too long or too short. Verify the measurement directly on the old pump before disassembly.

An exact procedure for determining the correct size for your specific boiler model is described in the article Pump size 130 mm vs 180 mm: which fits into my boiler. If you are unsure, the simplest way is to measure the center-to-center distance of the flange holes of the old pump using a sliding caliper or a regular ruler.

Selecting the correct replacement part

In the category of replacement parts for ATTACK boilers, you will find several specific variants of pumps. Let's take a closer look:

ATTACK Pump WILO Yonos PARA RS25/6, 130 mm – original replacement part specifically designed for ATTACK boilers with a 130 mm mounting length. Typical application: ATTACK Lambda, DPX 12–25 kW, PELLET 15–25 kW boilers. The parameter designation 25/6 means DN25 (1 inch) flange ports and a maximum transport pressure (Hmax) of 6 m w.c. Learn more about reading pump parameters in the article Pump parameters 25/6: what do the numbers mean and how to read them.
ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm – variant for boilers with a longer axial spacing of 180 mm. The suffix RKC denotes a specific WILO product line with certain differences in the electrical terminal block and communication with the boiler control. If your ATTACK boiler has the 180 mm version, always look for this exact part.
ATTACK Replacement part – pump RS 25/6-3-PR, 180 mm – special version for the PR series boilers (see below). A three-speed version with a different electrical connector, suitable for older models of the ATTACK DP/PR series.

If you are also looking for alternatives in the form of high-quality universal class A pumps, also consider:

These variants are intended for ATTACK boilers, but they are type-approved pumps that may have slightly different electrical connector wiring compared to the boiler's circuit board. Always verify the type of connector and the orientation of the motor's output terminal box before ordering (the rotatable head of the WILO allows different positions, but the connector must be compatible). The topic of original versus universal replacement pumps for ATTACK boilers is covered in a separate article Original vs universal replacement pump for ATTACK boiler: is it worth saving.

What to prepare before replacement

Replacing the WILO Yonos PARA pump in an ATTACK boiler is not an especially complex procedure, but you need to have the right materials and tools on hand. If you are an experienced heating technician or a technically skilled homeowner, you can complete it in 30 to 60 minutes. Here is a list of what you will need:

Adjustable wrenches (spanner), or a 30–36 mm open-end wrench for tightening flange connections
Flat screwdriver (for loosening the pump terminal cover and the drain screw)
Crosshead screwdriver PH2
Bucket, hose, or catch container for water (the system will be partially drained during replacement)
Clean rags or towels
New gasket / O-rings – most new WILO pumps include them in the packaging, but double-check
Teflon tape or heating hemp fiber + sealant (if the installation involves threaded ports, not flanges)
Electrical tester / multimeter (to confirm the correct voltage before connecting)
Mobile phone for taking photos of the original wiring before disassembly

Photographing the original wiring before disassembly is a habit that can save a lot of nerves – especially when it comes to the orientation of the pump's electrical terminal box and the direction of water flow (the pump is unidirectional).

Step-by-step replacement procedure

The following procedure applies to most ATTACK boilers where the pump is mounted in the primary circuit of the boiler (between the boiler and the manifold, or directly in the boiler body). For integrated boilers (where the pump is part of a sealed hydraulic section), the procedure may differ slightly – I recommend always consulting the service manual for the specific model.

Step 1: Safety shutdown

Always start by turning off the boiler via its control (not just the Stand-by button, but a full shutdown). Then switch off the boiler circuit breaker at the distribution board. For boilers with thermal storage or UPS backup, wait until the system stabilizes. Use a multimeter to confirm that there is no voltage at the pump terminals.

Step 2: Closing service valves

Most ATTACK boilers have service ball valves (short red or black levers) installed near the pump. Turn both to the "closed" position (perpendicular to the pipe). If your boiler does not have these valves, you will have to partially drain the entire system. In practice, this situation occurs only in very old installations or in cases of poor installation quality.

Step 3: Draining residual water

On the side of the pump impeller (plastic cover with a slot for a screwdriver), there is a small drain / air vent screw. Place a catch container underneath, carefully loosen it, and let the residual water drain from the pump body. If the valves are sealing well, only a few deciliters will drain. If more water is flowing, one of the valves is not sealing properly and needs to be closed in another way.

Step 4: Electrical disconnection

The WILO Yonos PARA pump has either a fixed cable with faston connectors on the boiler terminal block or a plug-in connector (depending on the ATTACK boiler series). In both cases, before disconnecting, take a photo of the wire color coding – L (phase, usually brown), N (neutral, blue), and PE (ground, yellow-green). For models with a communication interface (bus cable), pay special attention to the thinner signal cable as well.

Step 5: Mechanical Disassembly

Flange connections of the pump are typically sealed with flat rubber gaskets and tightened with screws (M6 or M8, usually 4 pieces on each neck). When loosening, proceed in a crisscross pattern – just like when changing car tires. Never loosen the screws only from one side, as you might twist the flange. Discard the old gaskets – never reuse them.

Step 6: Installation of the New Pump

Before installation, check the direction of rotation / flow direction – there is an arrow on the pump body indicating the direction of water flow. In ATTACK boilers, water flows from the output of the heat exchanger (hot side) through the pump to the distributor or to the heating circuit. The motor’s swivel head allows you to set the output box into 4 positions (every 90°) without changing the hydraulic layout. Use this to direct the electrical connector upwards or sideways – never directly downwards (condensation could leak into the terminal box).

Slide the new gaskets into the grooves, insert the pump into the flange connection and tighten the screws evenly, in a crisscross pattern, with a torque of approx. 6–8 Nm (for M6 screws). Do not overtighten – flat rubber is easily deformed.

Step 7: Filling, Bleeding and First Operation

After electrical connection, open the service valves and let the system fill up. Check the pressure in the expansion tank – it should be 1.0 to 1.5 bar when the system is cold (depending on the building height and the expansion tank setting). Start the boiler and check whether the pump is running (vibration, sound, warm motor). Bleed the radiators, or the bleed valve on the distributor. After 10–15 minutes, visually check the seals and retighten if necessary.

A more detailed description of the installation, including typical errors, can be found in the article Installation of a Circulation Pump in an ATTACK Boiler: Procedure and Common Mistakes.

Setting the Pump After Replacement

The WILO Yonos PARA pump does not need to be adjusted in the same way after replacement as the old three-speed pumps. It operates in an automatic mode (Δp-v or Δp-c), where it regulates its performance according to the system’s needs. Despite this, there are settings that affect comfort and energy efficiency:

Δp-v mode (variable pressure difference) – recommended for most ATTACK boilers with thermostatic valves on radiators. The pump reduces performance when the valves are closed and increases it when they are open. Result: quiet operation, lower consumption, longer lifespan.
Δp-c mode (constant pressure difference) – more suitable for boilers with floor heating without thermostats or for systems with very short circuits.
Maximum performance setting – the pump has a button and an LED display where you can set the max. Hmax in a typical range of 2–6 m w.c. For a standard family house, a setting of 3–4 m w.c. is sufficient.

The exact setting depends on the hydraulic configuration of your system. If you have doubts, I recommend starting with the automatic Δp-v mode and setting Hmax to 4 m w.c. Then monitor the temperatures on the distributor and the system behavior during the first three heating days. In most cases, further correction is not needed.

Most Common Mistakes When Replacing a Pump in ATTACK Boilers

Experience from dozens of customer cases has shown that the same mistakes are repeated. Here are the ones to watch out for:

Wrong flow direction – the pump is mounted backwards. It results in extremely low flow and buzzing. There is always an arrow on the pump body – follow it.
Old gasket left in place – some technicians install a new pump with the old gasket “just temporarily.” Result: immediate or delayed water leak.
No bleeding after replacement – air trapped in the pump or circuits causes cavitation and noisy operation. The WILO pump has a bleed screw – use it.
Over-tightening of flange screws – the rubber gasket is crushed and starts to leak. Tighten by hand and with a wrench only slightly.
Wrong position of the output box – the terminal box is turned downwards. Condensation and short-circuit risk.
Connection without pressure check – after replacement, the system pressure was not refilled. The pump cavitates and the boiler reports a fault.
Using different types of connectors – the original connector of the ATTACK boiler has a specific pin layout. Using a differently connected connector may cause a fault in the boiler’s electrical board.

When to Call a Professional Instead of Doing the Replacement Yourself?

Although pump replacement is a relatively simple service task, there are situations when it is better to leave the work to a professional:

The boiler is still under warranty – any unauthorized intervention may void the warranty.
There are visible signs of corrosion or pipe damage at the pump connection point.
You cannot safely identify whether the service valves are functional and tight.
The boiler has an integrated hydraulic section with multiple branches and it is not clear where exactly the primary circuit pump is located.
When disconnecting the electrical connector, you see signs of melting or burning of cable insulation.

Lifespan of the New Pump and How to Extend It

WILO Yonos PARA pumps are designed for a minimum lifespan of 15 years under standard operating conditions. However, in practice, the lifespan is reduced by several factors:

Water quality in the system – hard water (over 2.5 mmol/l) causes limescale deposits on the rotor. The ideal pH of the water in the system is 7.0–8.5.
Magnetite sludge – black deposits from corrosion of steel radiators and pipes accumulate in the rotor and increase wear. Solution: installation of a magnetic filter (sludge separator) before the pump.
Operation without water – the WILO Yonos PARA pump is cooled and lubricated by liquid. If the system runs dry (e.g., due to a leak), the pump can be damaged within minutes.
Long periods of stagnation – during summer, when the pump is idle for months, the rotor may corrode on the shaft. I recommend turning the pump on manually for a few minutes once a month (the boiler has a “test run” function for this).

Most Frequently Asked Questions (FAQ)

How can I determine which type of WILO pump is in my ATTACK boiler if I don't have the documentation?

The pump type is always engraved or printed on a label attached to the pump housing (usually on the side of the motor's output box). Look for markings such as "Yonos PARA RS25/6" or similar. Another option: check the service manual of the boiler – it is available on the manufacturer's ATTACK website or can be sent to you by an authorized service. The mounting length is either 130 or 180 mm – measure it directly on the pump with a caliper before disassembly.

Can I replace the pump myself, or do I need a licensed heating technician?

Legally, the replacement of a circulation pump in a closed heating system is not a licensed activity, and the property owner can perform it themselves – unlike, for example, gasification of a boiler. In practice, a technically skilled person with basic tools can handle it. If the boiler is under warranty, document the work and use original parts to be able to prove the intervention in case of a claim.

The replaced WILO Yonos PARA pump is making noise – is that normal?

During the first 1–2 hours of operation, the pump may emit slight noise caused by air in the system. Bleed all radiators and also bleed the pump itself using a bleed screw. If the noise persists for more than 24 hours in the form of buzzing or knocking, check the system pressure (must be at least 1 bar), the pump's connection direction, and whether any valves are only partially open.

Difference between RS25/6 and RS25/6 RKC variants – does it matter when replacing?

Yes, it does matter. The suffix RKC (or other variants such as ST, RKD) indicates a specific pump version with a different electrical connector and sometimes different communication functions for the boiler's control electronics. If your ATTACK boiler requires the RKC version, using a standard RS25/6 without this suffix may cause the control board to not operate the pump correctly or to report its status incorrectly. Always follow the marking on the original pump or the boiler's service documentation.

Is it worth replacing only the rotor of the pump instead of the whole pump?

Theoretically yes – WILO sells rotor kits as separate spare parts, and in some cases (e.g., rotor jamming due to sludge), replacing only the rotor module is sufficient. In practice, however, this operation requires more skill and the correct tools. Moreover, if the pump is 8–10 years old and the rotor is worn, the bearings and seals are likely worn as well – replacing the entire pump is usually more economical and practical. The price of a new original ATTACK pump is not significantly higher than the price of a rotor plus the labor for its replacement.

Can I use a pump with a longer mounting length and compensate with an adapter?

Theoretically, this can be done using reducing or extending couplings, but I strongly advise against it from a practical standpoint. Every additional connection is a potential leak point. Moreover, in the cramped space of the ATTACK boiler, the coupling may simply not fit. Always order a pump with the correct mounting length.

Conclusion: A proper replacement is worth it

Replacing the WILO Yonos PARA pump in an ATTACK boiler is a task you can handle without special professional certifications, provided you proceed systematically and respect safety rules. The key to success lies in three things: first, correctly identifying the type and dimensions of pump your boiler needs; second, using an original or verified compatible spare part; and third, following the installation procedure, including bleeding and pressure checks.

Investing in an original replacement pump – whether Yonos PARA RS25/6 in the 130 mm variant or Yonos PARA RS25/6 RKC in the 180 mm variant – pays off in the form of trouble-free, quiet, and energy-efficient heating system operation for the next 10 to 15 years. The savings on electricity (class A vs. old three-speed pump) represent dozens of euros per year in a typical family home. Therefore, the replacement pays off economically as well as in terms of heating reliability.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.