Pump size 130 mm vs 180 mm: which fits my boiler

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Pump size 130 mm vs 180 mm: which fits into my boiler?

When it is time to replace the circulation pump in the ATTACK boiler – whether due to failure, wear, or a switch to a more energy-efficient model – most customers run into the same problem: pumps look identical at first glance, have the same hydraulic parameters, and even the same designation 25/6, but differ in one seemingly minor detail. Construction length. 130 mm versus 180 mm. And it is precisely this minor detail that determines whether the pump will simply click into place in the boiler or whether you will end up with a leak, improper flange tension, and an unnecessary complaint.

This article will explore the topic in depth – from what the construction length actually is and where it comes from, through specific ATTACK boiler models and their requirements, to practical tips on how to verify the correct size before ordering without having to disassemble the entire boiler.

What is the construction length of a pump and why does it matter

The construction length (sometimes also called "axial length" or "mounting length") is the distance between the outer surfaces of both connecting flanges of the pump – that is, between the inlet and outlet flanges measured along the flow axis. The pump is installed into the pipe or into the hydraulic block of the boiler by inserting it into a pre-prepared hole of a given length and then fastening it. If the pump is shorter than the hole in the boiler, the flanges will not fit tightly together and the connection will be leaky. If the pump is longer, it will simply not fit into the hole.

The standard DIN EN ISO 228 and standards for circulation pumps distinguish several standardized construction lengths. For small circulation pumps of class 25 (nominal diameter DN 25, i.e. 1 inch), the common lengths are 130 mm and 180 mm. These are the two most widespread variants, which you will find with all manufacturers – Grundfos, WILO, DAB, Laing and, of course, also with pumps of the brand ATTACK.

The difference of 50 mm seems small, but in a compact hydraulic block of a boiler it is crucial. Boiler manufacturers design the internal hydraulic circuit precisely for one of these lengths, and it is not possible to interchange them without mechanical modifications, which are not feasible in practice (and would void the warranty).

Why there are two lengths: historical and technical context

The length of 180 mm is historically older and dates back to the time when circulation pumps were larger, heavier, and less compact hydraulically. In the 80s and 90s of the last century, practically all boilers in the CSSR and later in Slovakia and the Czech Republic were designed for 180 mm – this length was the "default" for an entire generation of boilers.

With the advent of more compact electronically controlled pumps (especially pumps with EC motors and integrated electronics), manufacturers began to reduce the mounting dimensions. The length of 130 mm became the standard for newer, modernized boiler constructions – including some ATTACK models from the last decade. It is not a rule that shorter = worse or cheaper. Often it is the opposite: the 130 mm version can be modernized and more energy-efficient (class A) in the same or better performance than the older 180 mm version.

Which ATTACK boilers use 130 mm and which use 180 mm?

This is the key practical question. In short: older ATTACK boilers (mainly from production before around 2012–2015) usually use the length of 180 mm. Newer and revised models of the hydraulic block have switched to 130 mm. But beware – it is not an absolute rule. There are exceptions in both directions and even within one product line the mounting length may have changed depending on the serial number or year of manufacture.

Therefore, it is always worth verifying the specific information according to the boiler's type plate and service documentation. Below we present an approximate division based on the practice of service technicians and the manufacturer's documentation:

ATTACK boilers with 180 mm pump (approximately)

ATTACK DP – older series of combined boilers for solid fuel with electric heating
ATTACK PELLET (older series before around 2015)
ATTACK DPX (original series)
ATTACK Lambda Touch (certain series)
ATTACK WOOD (wood boilers, older production series)
Boilers delivered with Grundfos UPS 25-60 or similar older models with a length of 180 mm

ATTACK boilers with 130 mm pump (approximately)

ATTACK DP (newer series from around 2015–2016)
ATTACK Pellet Automat (newer generation)
ATTACK FOCUS (compact version)
Boilers delivered with WILO Yonos PARA RS25/6 or Stratos PARA pump
Certain versions of ATTACK LAMBDA with revised hydraulic block

We emphasize again: this division is approximate. Before ordering a replacement pump, always measure the mounting hole directly on the boiler or look at the type plate of the pump you have in the boiler now. On the plate you will find the production number and in the documentation for the given pump model the construction length is always specified.

How to correctly measure the mounting length before ordering

If you don't have the boiler documentation or the pump's nameplate at hand, you can measure the mounting length directly. The procedure is simple, but it requires physical access to the pump (which is usually not a problem with a functioning boiler):

Turn off the boiler and let it cool down to a safe temperature.
Use a flashlight or your phone to illuminate the pump. Most ATTACK boilers have the pump located on the front or side of the hydraulic block.
Place a sliding caliper or measuring tape against the outer surface of the inlet flange and measure to the outer surface of the outlet flange – along the axis of flow (horizontally).
The result will be either 130 mm (±1–2 mm) or 180 mm (±1–2 mm). Any other value would indicate a non-standard situation – modified piping, reductions, or a special version.

If the pump is still running (you just want a spare or it is switching to class A), you can read the mounting dimension directly from the nameplate of the running pump. The nameplate always includes: nominal performance (e.g. 25/6), power supply, and the mounting length in mm. If the mounting length is not listed directly, enter the pump's model number into a search engine – you will always find the manufacturer's technical data sheet with this value.

Overview of available ATTACK pumps by dimension

In the ATTACK pumps range on atria.sk, you will find products for both dimensions. Here is an overview with comments on usage:

Pumps with a mounting length of 130 mm

ATTACK Heating circulation pump 25/6, 130 mm, class A is a universal heating pump for boilers with a 130 mm mounting opening. It is certified in energy class A, meaning it meets the requirements of the European ErP (Energy-related Products) regulation valid since 2015. It is suitable as a replacement for the original pump in newer ATTACK boilers, as well as an upgrade for an older pump during a renovation, if the mounting opening has been modified to 130 mm.

ATTACK Pump WILO Yonos PARA RS25/6, 130 mm, spare part for ATTACK boilers is an original spare part for ATTACK boilers that were originally equipped with the WILO Yonos PARA pump. This pump has integrated electronics with automatic performance adjustment – it is not a simple pump with fixed speeds, but a full-featured EC motor with proportional or constant regulation. It is a direct replacement without any modifications for boilers where this pump was originally installed.

Pumps with a mounting length of 180 mm

ATTACK Heating circulation pump PR 25/6, 180 mm, class A is the equivalent of the 130 mm version, but for boilers with a longer mounting opening. Class A ensures low electricity consumption and is suitable as a replacement for older, less efficient pumps in boilers with 180 mm openings.

ATTACK Pump Yonos PARA RS25/6 RKC, 180 mm, spare part for ATTACK boilers is the 180 mm version of the original spare part WILO Yonos PARA, intended for older ATTACK boilers. The suffix "RKC" in the designation indicates a specific variant with a control card compatible with the boiler's control electronics.

ATTACK Spare part – pump RS 25/6-3-PR, 180 mm is another 180 mm variant for ATTACK boilers, where the designation "3-PR" refers to a 3-speed version with a manual switch. Such a pump is a typical solution for older boilers, where electronic regulation was not part of the original and where a simple three-stage speed control is sufficient.

Can a 130 mm pump be replaced with a 180 mm one using a reducer or an extension piece?

This is a question we receive quite often, and the answer is clear: no, we do not recommend it and it usually does not work well in practice.

There are so-called extension kits (adapters) that can physically bridge the difference between a 130 mm and 180 mm opening, but their use brings several critical problems:

Sealing: Every additional connection is a potential leak point. In a pressurized heating system with operating pressure of 1.5–3 bar and temperatures up to 90 °C, any poorly made reducer will eventually show a leak.
Warranty conditions: The use of an adapter or non-standard accessory usually voids the warranty on the pump and the boiler.
Space limitations: There is very little space inside the boiler. An extension piece would have to be mounted on a very small surface and mechanical stress on the connections due to pump vibrations is likely.
Hydraulic losses: Every unnecessary bend or narrowing in the hydraulic circuit increases resistance and reduces the overall circulation performance – which is exactly the opposite of what we want from a pump.

The correct solution is always to order a pump with the correct mounting length. If you are unsure, it pays off to wait an extra day and verify the dimensions, rather than later dealing with returning the item and placing a new order.

Can I install a newer class A EC pump into an older boiler (180 mm)?

This is a very good question with practical implications – and the answer depends on what type of pump you originally have and what you expect from the new one.

If your boiler has a 180 mm mounting hole and the original pump is an old three-speed high-consumption model, this is an ideal opportunity to upgrade to a modern class A pump with a 180 mm mounting length. Such a pump is physically compatible (same flange size DN 25, same mounting length 180 mm) and brings significantly lower electricity consumption.

Practical example: A customer with an ATTACK DP 25 boiler from 2008 replaced the original pump UPS 25-60 (consumption approx. 45–65 W depending on the speed) with a modern EC class A 180 mm pump. Result: consumption dropped to 3–18 W depending on the current system demand. Over a heating season of 7 months, this means saving several tens of euros per year just on electricity – the pump pays for itself in 2–4 seasons.

The replacement is usually straightforward (plug-and-play) provided that the new pump has the same flange size (G 1½" or flanged DN 25) and the same mounting length. The electrical connection is standard 230V/50Hz, so there are no complications here either.

For more information on energy class A and its real impact on consumption, read our article Energy efficiency class A in circulation pumps: what it means in practice and also Original vs universal replacement pump for ATTACK boiler: is it worth saving.

Parameters 25/6 – what do they mean and how do they relate to size

The designation 25/6, which is used by almost all the pumps mentioned here, refers to hydraulic parameters – not the mounting length. The number 25 refers to the nominal pipe diameter in mm (DN 25 = 1 inch), the number 6 is the maximum head in meters of water column (i.e., the maximum pressure of the pump at zero flow). These parameters are the same for the 130 mm and 180 mm versions – the difference is only in the mounting length, not in the performance.

It is important to understand that the 25/6 pump is not always operating at maximum performance. Modern EC pumps (such as WILO Yonos PARA) automatically adapt to the needs of the system and most of the season they run at a fraction of the nominal power. In practice, this means that for a standard family house with system pressure losses of 1–3 m, the 25/6 pump is oversized and automatically selects the optimal operating point on its own.

We elaborate on this topic in the article Pump parameters 25/6: what the numbers mean and how to read them.

Practical scenarios from practice: how we handled it with customers

Scenario 1: Pump failure in an ATTACK DP 45 boiler, production year 2010

The customer called saying the boiler was not heating, but the burner was working. Diagnosis: blocked rotor (bearing corrosion after 13 years of operation). The boiler had a Grundfos UPS 25-60 pump with a length of 180 mm installed. The customer wanted an original ATTACK replacement pump – we recommended ATTACK Spare Part – Pump RS 25/6-3-PR, 180 mm as a direct replacement. The customer replaced the pump himself in half an hour following the procedure in the article Installation of a circulation pump in an ATTACK boiler: procedure and common mistakes.

Scenario 2: Pump upgrade during boiler room reconstruction, ATTACK PELLET 25 from 2017

The customer was replacing an ATTACK PELLET boiler, which had a built-in WILO Yonos PARA RS25/6 with a length of 130 mm. The old boiler was going into retirement, and the new one (same model) also had a 130 mm mounting. The customer wanted to keep a spare pump in stock for certainty. We recommended ATTACK Pump WILO Yonos PARA RS25/6, 130 mm, spare part for ATTACK boilers – an original part that fits without any modifications.

Scenario 3: The customer ordered the wrong size and what followed

A classic situation: the customer had a boiler with 180 mm mounting, but the photo in the pump description confused him (pumps look similar in photos) and he ordered the 130 mm version. When he tried to install the pump, the flange did not fit – it was exactly 50 mm short. The return went smoothly, but the customer lost an additional 3 days. Lesson: always verify the size physically before ordering – a sliding tape measure, 5 minutes, no stress.

Scenario 4: The seller could not help, the customer helped himself

The customer had an ATTACK boiler without documentation (he bought an older house). There was no readable type plate on the boiler. Procedure: he photographed the pump, read the type designation from the pump's own plate (not the boiler), entered it into the search engine, and from the technical data sheet of the pump he found out the mounting length. It was the 180 mm version, so he ordered the correct pump on the first try. Practical lesson: if you know nothing about the boiler, at least read the pump's plate.

Further technical details that affect the selection

Flanged or threaded connection?

Most circulation pumps for ATTACK boilers use threaded connections G 1½" (6/4 inch) with a nut connection – not flanged connections with bolts. This type of connection is standard for the entire segment of small boilers. The sealing is a rubber O-ring or flat gasket, which is replaced with every pump replacement. Never use an old gasket again – leakage is almost certain.

Head orientation of the pump

Modern pumps (WILO Yonos PARA, ATTACK EC class A) have a rotatable head – the electric motor with electronics can be rotated by 90° or 180° as needed. This is practical because it allows you to position the control elements and display in an accessible location regardless of the mounting orientation. Check that rotating the head does not cover a service valve or thermostat.

Power supply and control

All pumps mentioned here are single-phase powered at 230V/50Hz. EC pumps can also have an input for a PWM signal or a 0–10V analog signal, by which the boiler control unit controls them. Not every ATTACK boiler uses this input – some models simply turn the pump on/off without speed regulation. In such cases, the EC pump operates in automatic proportional mode, which is still significantly more efficient than a three-speed version.

Frequently asked questions (FAQ)

How can I find out which pump I have in my ATTACK boiler, if I don't have the documentation?

The easiest way is to read the type designation directly from the label on the pump. The label is located on the pump body (usually on the motor head or on the side of the electronics housing). If you have the type designation (e.g. RS25/6 or UPS 25-60), simply enter it into a search engine – the manufacturer always publishes a technical sheet with the mounting length specified. Alternatively, you can measure the distance between the flanges as described in this article.

Can I install a 130 mm pump into a boiler with 180 mm mounting using an extension piece?

Technically, it is not directly impossible, but we strongly advise against it. Every additional connection is a potential leak point, using non-standard adapters voids the warranty, and in a compact boiler mounting, such a solution is very difficult to implement reliably. It is always better to order a pump with the correct mounting length, even if it means waiting an extra day.

Is a class A pump really better than an old three-speed pump? Is it worth the investment?

In the vast majority of cases, yes. Old three-speed pumps consume 40–80 W, and most of the season they run at medium or higher speed. EC class A pumps consume 5–20 W in real operation because they automatically adapt to the hydraulic conditions of the system. The difference in consumption is around 100–200 kWh per year, which at current electricity prices means 25–60 € per year. The return on investment is typically 2–5 years, after which you save purely. In addition, EC pumps usually last longer because they do not unnecessarily run at full power.

Can I replace the pump in my ATTACK boiler myself, or do I need to call a service technician?

The pump replacement itself is mechanically simple and can be done by a skilled DIYer without special certifications. It is not an intervention into a gas appliance or into an electrical installation above 25V, so it is not subject to legal restrictions for unqualified persons. It is important, however, to follow the correct procedure: close the service valves, drain the system, disconnect the power, remove the old gasket, install the new one, tighten the nuts to the correct torque, refill the system, and purge the air. The entire process including common mistakes can be found in the article Installation of a circulation pump into an ATTACK boiler: procedure and common mistakes. If you are unsure, call a technician – the hourly rate is significantly lower than the cost of repairs due to incorrect installation.

What if the mounting length of my boiler is something other than 130 or 180 mm?

It is unusual, but not impossible – some boilers had non-standard mountings in the past (e.g. 150 mm, 160 mm), or the hydraulic block was previously modified by a previous owner. In such a case, pump selection is more complicated and you should contact a service technician. Standardized ATTACK pumps for 130 mm and 180 mm should not be installed in such a boiler without modifications.

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

The lifespan of a circulation pump depends on the quality of the water in the system, the operating conditions (temperature, pressure, presence of corrosion inhibitors), and the production design itself. In practice, a quality pump can last 12–20 years under proper conditions. EC pumps are generally more durable because motors with permanent magnets and no brushes have fewer moving and wearing parts. More about signs of an approaching end of life can be found in the article When to replace a circulation pump in a boiler: signs and lifespan.

Summary: how to choose the right pump

Selecting the right circulation pump for an ATTACK boiler is not complicated if you proceed systematically. The mounting length of 130 mm vs. 180 mm is an absolutely key parameter that must be correct – hydraulic parameters and energy class are important, but the mounting length is a necessary condition. Without it, the pump simply will not fit.

Steps for correct selection:

Determine the mounting length – from the pump's type label, boiler documentation, or by direct measurement.
Decide whether you want an original spare part (to preserve the original function and compatibility with the boiler's control electronics) or a universal heating pump (cheaper, suitable if the boiler only turns the pump on/off without regulation).
Choose the energy class – class A is always a better choice from a long-term economic perspective.
Check the flange connection size – for ATTACK boilers, the standard is G 1½" (6/4").

If you are still unsure about the selection, check out other articles in our Knowledge Center – specifically How to choose a circulation pump for an ATTACK boiler, Replacing the WILO Yonos PARA pump in an ATTACK boiler: what you need to know, or Common questions about circulation pumps for ATTACK boilers. These articles complement the information in this text and together form a comprehensive guide for every situation – from a standard replacement to an upgrade to a modern EC pump.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.