Installation of the circulation pump into the ATTACK boiler: procedure and common mistakes
Installation of circulation pump in the ATTACK boiler: complete procedure and most common mistakes
The circulation pump is the heart of any heating system. If it stops working properly, the boiler may still burn, but the heat simply won't be distributed around the house. In practice, we encounter two situations: either you need to replace the original pump that has reached the end of its life or has failed, or you are installing a new pump during a reconstruction or service intervention. In both cases, correct installation determines whether the pump will work for years without problems or whether you will be dealing with leaks, noise, and a shortened lifespan sooner than expected.
This article will guide you through the entire process – from preparation and choosing the right pump, through the step-by-step installation procedure, to the most common mistakes we see among customers and less experienced installers. We will focus specifically on ATTACK boilers, where pump installation has its own specifics.
Why correct pump installation is so important in ATTACK boilers
ATTACK boilers are robust devices designed for Slovak and Central European conditions. Most models have the pump mounted directly inside the boiler, integrated into the hydraulic circuit. This brings advantages – compactness, better conditions for the control electronics – but also higher demands on installation accuracy. Unlike a pump mounted externally on the distribution pipes, here you work in the confined space of the boiler body, with precisely defined connection geometry and a specific range of acceptable dimensions.
If you are wondering which pump fits your specific ATTACK model and how to choose the right alternative or spare part, see our guide article How to choose a circulation pump for an ATTACK boiler. The topic of dimensions – specifically the difference between the 130 mm and 180 mm mounting length – is covered in detail in the article 130 mm vs 180 mm pump dimensions: which one fits my boiler. Without the correct selection, there is no point in starting the installation.
What you will need: tools, accessories, and spare parts
Never underestimate preparation. From experience, we know that most interruptions and delays occur precisely because the installer starts disassembling the boiler and only then realizes that they are missing a gasket, a key of the right size, or the new pump has different dimensions than expected.
Basic tools
- Adjustable wrench or a set of ring spanners (sizes 24–36 mm depending on the boiler model)
- Flat and cross-head screwdrivers (for electrical terminals and covers)
- Multimeter (for checking power supply and terminals)
- Container for collecting water (at least 5–10 liters)
- Hose or pump for draining the system
- Teflon tape or hemp fiber with sealing compound (for threaded connections)
- New gaskets (O-rings or flat gaskets depending on the flange type)
- Cleaning cloth and WD-40 for possibly corroded threads
What spare parts to have ready in advance
Before installation, make sure you have the correct pump available. The following models are commonly used for ATTACK boilers:
- ATTACK Heating circulation pump PR 25/6, 180 mm, Class A – for boilers requiring a mounting length of 180 mm
- ATTACK Heating circulation pump 25/6, 130 mm, Class A – for more compact models with a mounting length of 130 mm
- ATTACK Pump WILO Yonos PARA RS25/6, 130 mm – original spare part Wilo for ATTACK boilers with a 130 mm flange connection
- ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm – 180 mm version with RKC control connector for integration with the boiler electronics
- ATTACK Spare part – pump RS 25/6-3-PR, 180 mm – direct replacement pump for older ATTACK models
Also, check in advance that you have enough gaskets. The gasket you remove is usually a one-time use item – it deforms after compression and must not be reused.
Installation Procedure Step by Step
Step 1: Safety Preparation – Turn Off the Boiler and Disconnect Power
Before any intervention in the boiler, it is absolutely necessary to turn off the boiler and disconnect it from the electrical network. It is not enough to simply turn off the boiler via the thermostat or control panel – unplug the power cord from the socket, or turn off the circuit breaker that powers the boiler. The reason is simple: modern ATTACK boilers have electronics active even in standby mode and 230 V near a wet environment is a serious risk.
Allow the boiler and system to cool down to a maximum of 30–40 °C before starting any manipulation. Hot water under pressure is a serious hazard during careless disassembly. If it is necessary to work faster, at least close the gas supply and wait until the temperature on the manometer drops below 40 °C.
Step 2: Closing the Ball Valves and Draining Water
Most ATTACK boilers have service ball valves on the hydraulic circuit – one on the output (hot water to heating) and one on the return (cold water back to the boiler). Close these valves. This will isolate the boiler's hydraulic circuit from the entire heating distribution and you will not have to drain the whole house.
Place a catch container under the pump. The pump has a bleed/drain screw at the bottom – carefully loosen it and let the water drain. There may be several liters of water in the system even with closed valves, which must be drained. Proceed slowly – if you feel resistance or water is spraying under pressure, something is not properly closed.
Practical experience: On older installations, we have often encountered ball valves that were corroded and could not be fully closed. In such cases, you need to drain at least part of the system – open the drain valve at the lowest point in the system and fill the expansion tank with drained water. Remember that you will need to replenish water and possibly a heating system additive during refilling.
Step 3: Disassembly of the Old Pump
After draining the water, proceed to the actual loosening of the pump. Pumps in ATTACK boilers are typically mounted on flanged connections or threaded fittings (thread size G 1 ½" is typical for pumps of the 25/6 series). Loosen both connections with a wrench – first note the original orientation of the pump (motor direction, position of the bleed screw).
The electrical wiring is typically secured by a terminal block or connector at the top of the pump. Unfasten the terminal block cover and note (or photograph) the wire connections – usually L (phase), N (neutral) and PE (ground). For pumps with a control connector RKC (e.g., model Yonos PARA RS25/6 RKC), there is also a connector for communication with the boiler's control electronics. This connector is usually secured by a plastic clip – do not force it out, press the locking tab and gently pull it out.
Always discard the old gasket without exception – never reuse it when installing a new pump.
Step 4: Cleaning the Seats and Flanges
Before installing the new pump, it is necessary to clean the flange contact surfaces. On older boilers, corrosion, limescale or remnants of the old gasket can settle here. Use a plastic scraper or fine sandpaper (grit 120–180) to clean the contact surfaces until they are clean and flat. Metal chips or limestone fragments in the seat are the most common reason why a new gasket does not seal even after proper tightening.
Also check the condition of the threads on the flange nuts. If you see stripped threads or heavy corrosion, the problem must be resolved before installing the pump – otherwise leakage or even flange cracking during tightening is likely.
Step 5: Mounting the New Pump and Gasket
Gently grease the new gaskets (O-rings or flat gaskets depending on the flange design) with clean water or glycerin to ensure proper seating. Do not use oil or hydrocarbon-based lubricants – they may degrade the rubber material of the gasket.
Mount the pump in the correct motor orientation. For horizontal piping, the motor axis must be horizontal (motor sideways or upwards) – not downwards, because the oil in the bearing would not perform its function. To verify, look at the pump's nameplate or installation instructions – most Wilo and standard ATTACK pumps have an arrow on the pump body indicating the allowed position.
Hand-tighten the flanges, then with a wrench – alternately (crosswise), not on one side. Tighten to a torque that is not too strong to avoid damaging the gasket. For steel flanges with a G 1½" thread, we usually work within the range of 30–40 Nm. Do not over-tighten – this is the most common mistake of beginners who try to eliminate leaks by force instead of using proper gaskets.
Step 6: Electrical Connection
The electrical connection must be implemented according to the manufacturer's diagram. Standard single-phase supply 230 V / 50 Hz is connected to the trio of terminals L, N, PE. If you are replacing a pump of the same type, simply repeat the original connection (which is why we emphasized photographing before disassembly). If you are installing a pump with an RKC control connector into a boiler that supports this system, also connect the control cable – it is usually a four-pin connector; the coding is mechanical, so the connector cannot be connected in the reverse direction.
Warning: if the old boiler does not have wiring for the RKC control connector, the pump will still operate – but only in manual mode without automatic power regulation. For full integration with the boiler control, the wiring must be added or a pump without an RKC connector must be selected.
Ensure that the terminal box cover is properly closed and the sealing of the cover is not damaged. Any moisture entering the electrical part can cause a short circuit or insulation failure.
Step 7: Filling the System, Bleeding and Pressure Test
After completing the mechanical and electrical installation, slowly open the filling valve or the ball valve on the return. Let the water slowly fill the circuit – monitor the pressure gauge. The pressure in the cold system should be set according to the expansion tank setting, usually 1.0–1.5 bar. If not, adjust it using the filling valve.
As soon as water starts to flow out of the bleed valves on the radiators and from the pump's air vent, you know the system is filling correctly. Loosen the pump's bleed screw by half a turn until water flows out without air bubbles, then tighten it. This step is critical – a pump operating without water will run dry and be destroyed within a few minutes.
Visually check all flanged connections and threads you have touched. If you see water droplets, tighten them carefully (not forcefully!) and monitor whether the seal is working. Most minor leaks can be resolved by slight tightening even before the system reaches operating pressure.
Step 8: Boiler Start-up and Function Check
Now you can reconnect the boiler to the electrical grid and start it. Let the boiler heat up to operating temperature (at least 60–70 °C). The pump should start automatically – with Wilo Yonos PARA models, you will notice this by the blue display backlight and the pump performance display. With simpler models without a display, it is sufficient to check whether the pump hums and the pipe is warm.
After 15–20 minutes of operation, check the sealing again at operating pressure and temperature – the sealing material expands slightly due to temperature and may close a minor leak that was present with cold water. If the leak persists, turn off the boiler, disconnect the power supply, cool it down, and resolve the issue.
Also check the noise. A new pump operates quietly – a soft, even hum is normal. Knocking, noise, vibrations, or intermittent sounds may indicate air in the system (air pocket at the rotor), incorrect orientation, or installation problems. Air in the system should be resolved by repeated bleeding of the system when it is cold and hot.
Most Common Errors When Installing a Pump into the ATTACK Boiler
Over years of practice and hundreds of customers, we have identified several errors that repeat again and again. If you read them before installation, you will save yourself a lot of problems.
Error No. 1: Incorrect Construction Length
This is by far the most common error, especially when ordering online without prior consultation. Pumps of the 25/6 series for ATTACK boilers exist in two construction lengths – 130 mm and 180 mm. These dimensions are not interchangeable. If you install a pump with a length of 180 mm in a place intended for 130 mm, it will physically not fit, or the flanged connection will be misaligned. It does not take much for a customer to come in with a pump that cannot be installed. Always measure the construction length of the original pump – it is the distance between the contact surfaces of both flanges.
Error No. 2: Reusing Old Seals
Never reuse seals after disassembly. Rubber O-rings or flat seals are permanently deformed after the first tightening. If you reinstall them, they will not fill the space evenly and a leak is almost certain – sometimes immediately, sometimes only after the first heating of the system. Seals are a cheap item; never save money on seals.
Error No. 3: Incorrect Motor Orientation
As we have mentioned – the motor must be horizontal or upwards, never downwards. In the cramped space of a boiler, it is sometimes tempting to rotate the pump to make it easier to install. Result: after several months, the bearings fail due to insufficient lubrication. If the space geometry does not allow the correct orientation, solve it by modifying the pipe connections, not by incorrectly installing the pump.
Error No. 4: Over-tightening Flange Bolts
Amateurs try to eliminate leaks by over-tightening. Over-tightened bolts will damage the seal, crack the flange sheet or strip the thread. The result is the exact opposite – a larger leak and potential permanent damage to the boiler flange. Work with the appropriate torque and correct sealing.
Error No. 5: Starting the Pump Without Water (Dry)
Starting the pump before filling the system with water is fatal for the bearings and mechanical sealing surface. The rotating rotor without cooling and lubrication from water overheats within 2–3 minutes, the seal cracks and the bearings are damaged. Always ensure that the system is full and bled before the pump starts. Note: some ATTACK boilers will automatically start the pump after connecting to electricity – be prepared for that.
Error No. 6: Cable Swapping During Electrical Connection
Swapping the phase and neutral (L and N) is functionally acceptable for simple pumps, but with electronically controlled pumps (Wilo Yonos PARA and similar), it can cause a failure of the control electronics. With pumps with an RKC control connector, it is also necessary to correctly connect the signal cable – with incorrect connection, the communication protocol does not work and the boiler may not regulate the pump performance correctly.
Specifications for Replacing the Wilo Yonos PARA Pump in ATTACK Boilers
Wilo Yonos PARA pumps are a standard original equipment in some models of ATTACK and are also available as spare parts. A detailed discussion of this specific series is provided in the article Replacing the WILO Yonos PARA Pump in the ATTACK Boiler: What You Need to Know. Here we highlight the most important differences compared to simpler pump models.
Yonos PARA is an electronically controlled pump with automatic performance regulation (energy efficiency class A – see the article Energy Efficiency Class A for Circulation Pumps: What It Means in Practice). This means that after connecting to power, it will go through an automatic diagnostic cycle – the rotor will briefly rotate in both directions, the pump will check for blockage, and only then will it start in normal mode. This process takes 10–30 seconds and is normal. Do not worry if the pump starts immediately.
With the model Yonos PARA RS25/6 RKC, 180 mm, pay attention to the control connector. RKC (Room Controller Connector or boiler control connector) transmits a digital signal between the boiler control unit and the pump. Without it, the pump will operate at a fixed power – you will lose full regulation and energy optimization. The connector is four-pin with a locking tab – when installing, insert it until you hear the tab click into place.
Original vs. Universal Pump: What to Install and Why
When choosing a pump for replacement, customers often consider whether to buy an original spare part or a cheaper universal pump with the same parameters. This topic is comprehensively discussed in the article Original vs. Universal Spare Pump for the ATTACK Boiler: Is It Worth Saving?. From a practical installation perspective, we add a few observations.
Original pumps (for example, spare part RS 25/6-3-PR, 180 mm) are specifically designed for a given boiler model – they have matching flange shapes, electrical outlet positions, and current parameters that fit without any adaptations. Universal pumps with the same hydraulic parameters may have a different position of the terminal block, different type of gasket, or different dimensions of mounting holes, which can cause problems during installation in the confined space of the boiler.
General recommendation from practice: for ATTACK boilers with warranty and within the first 10 years of life, we always recommend an original or manufacturer-approved spare part. For older boilers, where it is purely about extending life without warranty, a universal pump may be an acceptable alternative – but always verify the dimensions and electrical compatibility before purchase.
Post-Installation Procedure: Inspection, Service Record, and Preventive Maintenance
Many installers (including professionals) often forget one important thing at the end of the installation – recording the service intervention. Record the date of replacement, the type of the new pump, the installer's name, and a brief description of the condition in the boiler service sheet or in the manufacturer's application. This is important both for claiming the warranty on the new pump and for future diagnostics and service interventions.
Preventive maintenance of the pump is minimal, but it does exist. Once a year (typically at the beginning of the heating season), check:
- System pressure (a cold system should have 1.0–1.5 bar, up to 2.0–2.5 bar when heated)
- Noise from the pump – a change in sound may indicate air in the system or beginning bearing wear
- Visual sealing of the flanges – any moisture or water marks indicate a leak
- Function of automatic air venting – if the boiler has an automatic air vent, check that it is not stuck
If air problems in the system persist (you have to bleed the system multiple times during the season), the issue is not the pump, but usually the expansion tank, micro-leakage, or improper placement of air vents. These issues need to be addressed separately.
For more on the signs that it is time to replace the pump, see the articles When to Replace the Circulation Pump in a Boiler: Signs and Lifespan and Common Failures of Circulation Pumps in ATTACK Boilers and Their Causes.
Most Frequently Asked Questions (FAQ)
Do I need to drain the entire heating system in the house when replacing the pump in an ATTACK boiler?
No, if the boiler has service ball valves on the hydraulic circuit (supply and return), it is sufficient to close these valves and drain only the content of the boiler circuit itself. The system in the distribution and radiators remains full. The amount of water to be drained and then refilled is usually only 2–5 liters. If the boiler does not have service valves (older installations), a larger part of the system must be drained.
How can I determine whether my pump is 130 mm or 180 mm?
The construction length of the pump is the distance between the contact seating surfaces of both flanges (not the total length including the motor). Measure it with a ruler or caliper on the original pump. Most pumps have the construction length engraved or printed directly on the nameplate or in the catalog number. If you are unsure, read our article Pump Size 130 mm vs 180 mm: Which Fits My Boiler.
I replaced the pump, but the boiler still heats unevenly – what should I do?
Uneven heating after pump replacement is most often caused by air in the system. Bleed all radiators in the house (start with the highest floors and continue downward), check the pump's air vent, and let the boiler run for at least an hour before further bleeding. If the problem persists, check whether the pump's performance setting matches the size of the heating system. Pumps of the 25/6 series have adjustable performance – if set too low, heat will not reach distant branches. The parameters of 25/6 pumps and how to read them are discussed in the article Pump Parameters 25/6: What the Numbers Mean and How to Read Them.
The pump buzzes and vibrates after installation – is that normal?
Light, even buzzing is normal – it is the sound of the rotating rotor. If the pump clicks, makes an intermittent noise, or vibrates unevenly, it is most often due to air in the system (solution: bleed the system), or the pump is in the wrong orientation (the motor is pointing downward), or the piping is mechanically stressed and transfers vibrations to the boiler. Also check whether the flange nuts are evenly tightened and whether the piping is causing mechanical stress on the flange connection.
Can I install the pump myself, or do I need a professional?
Hydraulic pump replacement is a relatively simple intervention that a skilled DIY enthusiast can handle. The electrical part – connecting to the terminal block – is technically also feasible for a layperson, but according to current legislation, electrical connections to a fixed installation (not a plug) should be done by a person with electrical qualifications. If the pump has a plug into a standard 230 V socket, there is no problem. For a boiler connected via a fixed installation (circuit breaker in the distribution board), we recommend leaving the electrical connection to an electrician. In addition to legislation, it is also a matter of warranty – some manufacturers require professional installation for warranty coverage.
How long does it take to replace a pump in an ATTACK boiler?
An experienced installer can replace a pump in 45–90 minutes, including draining, replacement, and refilling and bleeding. If it is your first time with a similar intervention, plan for 2–3 hours – especially bleeding and checking for leaks require patience. Additional time is added if there is corrosion on the flanges or other complications (frozen valves, missing tools). Therefore, we always recommend starting the job in the morning so you have the whole day to address any problems and so your home does not remain without heating into the night.
Conclusion: Reliable Installation as the Basis for Long Pump Lifespan
A circulation pump, when properly installed and with basic maintenance, is a device with a long lifespan – high-quality models of class A from Wilo or original ATTACK pumps can last 10–15 years without major problems. Every installation error, however, dramatically shortens this lifespan: incorrect sealing leads to leaks, poor orientation causes bearing wear, insufficient bleeding leads to cavitation and premature rotor failure.
If you are unsure about choosing the right pump for your ATTACK boiler model, please check our overview of available models in the ATTACK pumps on atria.sk category, where you will find a complete range of original and replacement pumps with detailed technical specifications. You will also find answers to further practical questions in the article Frequently asked questions about circulation pumps for ATTACK boilers in our Knowledge Centre.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.