Dimensional compatibility of circulation pumps – shaft distance and connection

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Dimensional compatibility of circulation pumps – shaft distance and connection

When a circulation pump in a heating system breaks down, the customer's first question is almost always: "Can I get the same size?" A seemingly simple question. In practice, however, it hides a whole range of dimensional parameters that must match – shaft distance, thread or flange diameter, position of the terminal block, flow direction, and type of pump head. If even one parameter is incorrect, the pump will either physically not fit or the installation will cost extra hours of work and additional fittings, which the customer certainly did not expect.

In this article, we will go into the issue in depth – from the basic explanation of the term "shaft distance" through standardized dimensional series, types of hydraulic connections, all the way to practical scenarios from installation practice. If you are replacing or upgrading a pump and want to avoid unpleasant surprises on site, read on.

What is the shaft distance of a pump and why is it key

The shaft distance (in English installation length, in German Baulänge) is the distance between the front faces of both hydraulic connections of the pump – that is, between the inlet and outlet flanges or between the seating surfaces of threaded fittings. It is measured along the axis of flow, i.e., in the direction in which the water/medium flows through the pump.

Why is this dimension so important? Because the pump is installed into an existing pipe section. If the new pump is shorter than the original one, a gap will remain that must be filled with a filler piece, an extension nipple, or sealed in some other way – and these are unnecessary costs. If it is, on the other hand, longer, the piping simply will not fit, or it will be necessary to cut and weld, or at least extend the hose line on the supply or return side.

In practice, the most common shaft distances for circulation pumps in single-family homes and apartment buildings are as follows:

130 mm – historically the most widespread dimension, used from the 1970s until today, you will find it in Grundfos UPS, Wilo Star-RS, DAB Evoplus, and hundreds of other models
180 mm – common for larger pumps DN 32–40, or for some older three-speed pumps of size 25/6 and 25/8
200 mm – less common, occurs in industrial circulation pumps or in dimensions DN 40–50
110 mm – some compact models, mainly in the 15/x and 25/x classes, or newer ECM pumps with shortened hydraulics
160 mm, 220 mm, 280 mm – special dimensions for larger industrial applications or inline pumps

When replacing a pump, always measure the shaft distance directly on the existing pump or read the value from the type plate. It is not enough to measure the distance between the pipe connections in the wall, because there are still gaskets and seals that add millimeters. An error of 5–10 mm can cause a leak.

Standardized dimensional series and what that means when replacing a pump

The good news is that manufacturers of circulation pumps basically stick to a few established dimensional series. The bad news: "established" does not mean "uniform" – there are exceptions, regional habits, and historical models that deviate from the standards.

The most important international dimensional standard for small circulation pumps is EN 1151-1, or the older DIN 24255. These standards define the mounting dimensions for pumps with threaded connections DN 15 to DN 50. The key point is that the standard also defines the position of the mounting holes – so, at any manufacturer that follows the standard, the motor should sit in the same place.

In practice, this works as follows: if you have an old Grundfos UPS 25-60 with a shaft distance of 130 mm and a G 1½" thread, you can replace it with practically any pump of the same size group – for example, Wilo Star-RS 25/6, DAB Evosta, IMP Pumps GHNA, Taco, or dozens of other less well-known brands you can find in the Other circulation pump brands category. All these pumps have a shaft distance of 130 mm and a G 1½" (or Rp 1½") thread, so they are physically interchangeable.

Where does the problem arise? Primarily when:

The old pump is of a non-standard origin (e.g., Soviet or Eastern European production with a metric thread system)
The pump has a different type of flange (e.g., PN6 vs. PN10, different bolt spacing)
Special housing configurations – e.g., eccentric outlets, pumps for expansion tanks, or solar pumps with three outlets
The replacement is being carried out with a pump of a different size class (e.g., from DN 25 to DN 32) for hydraulic reasons

Types of hydraulic connections – thread, flange, coupling

Shaft distance is one dimension, but the type and size of the hydraulic connection are equally important. Here we have several fundamentally different concepts:

Threaded connection (most common for DN 15 – DN 32)

This is by far the most common type for circulation pumps in single-family homes and apartment buildings. The pump has an external or internal pipe thread to which a flange with a gasket is bolted. Common sizes:

G 1" (Rp 1") – small pumps, typically for DN 15–20 systems
G 1¼" (Rp 1¼") – less common, some older models
G 1½" (Rp 1½") – the most widespread dimension for standard domestic pumps of size 25/x
G 2" (Rp 2") – larger pumps of size 32/x and 40/x

Important detail: there is a difference between G thread (ISO 228, cylindrical, sealing is ensured by a sealing surface or O-ring) and Rp thread (ISO 7, conical, sealing is ensured by the thread itself). On the piping, it is usually Rp (conical external – R), the pump has an internal G or Rp thread. Most standard flanges are compatible with both types, but with special materials (brass, stainless steel, plastic), this must be verified.

Flange connection (DN 32 and larger)

For larger pumps – typically from DN 32 upwards – a flange connection according to EN 1092-1 (PN6, PN10, PN16) is used. The flange has a precisely defined diameter, bolt circle and number of bolt holes. Key parameters:

Nominal diameter (DN): 32, 40, 50, 65, 80, 100 mm...
Pressure class (PN): PN6, PN10, PN16 – differ in flange dimensions and bolt spacing
Number of bolts: DN 50 / PN10 typically has 4 M16 bolts, DN 80 / PN10 has 8 M16 bolts

Replacement between different manufacturers is mostly straightforward here, as the standard is stricter and manufacturers usually adhere to it – but always check the center distance, as it is not strictly defined by the standard and may vary.

Union connection with a seating plate (screwed connection)

Most small pumps for domestic heating are sold either with or without unions attached. A union (technical term "screwed connection with a seating plate" or colloquially "union") consists of a nut, a cast iron or brass washer, and a gasket. The gasket function is critical – for threaded connections PN 6 / 10 the following are used:

Flat rubber gasket (EPDM, NBR) – for G threads (cylindrical)
Flax fiber + sealant or PTFE tape – for conical Rp threads (although modern pumps rarely have conical threads)
O-ring in a groove – in newer designs, typically EPDM 70 ShA

The gasket must be replaced with a new one during every pump replacement – old rubber gaskets become brittle and crack after years in hot water. This is one of the most common causes of leakage after pump replacement, which the customer then complains about.

Dimension codes in pump designation – how to read them

Most manufacturers encode dimensional parameters directly into the pump's trade name. Typical examples:

Grundfos UPS 25-60 130 – DN 25, max. head 6 m, center distance 130 mm
Wilo Star-RS 25/6 – DN 25, max. head 6 m (center distance in the catalog, usually 130 mm)
DAB Evoplus 40/180 XM – DN 40, center distance 180 mm
IMP Pumps GHNA 25/60-130 – DN 25, max. head 6 m, center distance 130 mm

Note that not all manufacturers include the center distance directly in the name – for some less well-known brands, it is necessary to look into the technical data sheet or catalog. This is where customers most often make a mistake: they select a pump based on performance and price, forget to check the center distance, and then wonder why the new pump does not fit in the place of the old one.

If you are interested in how to assess the performance parameters and hydraulics of a pump, we recommend the article Hydraulic parameters of a circulation pump: flow rate, head and power in this Knowledge Center.

Presence of insulation and terminal cover – dimensional complications

Along with purely hydraulic dimensions, there are two additional factors that affect whether a new pump will physically fit during replacement: the insulation cover and the position of the terminal box (electrical housing).

Modern ECM (electronically commutated motor) pumps are usually more compact in size than their predecessors with three-phase asynchronous motors – however, the electronic display and housing may protrude in different directions. If the pump is installed in a technical room with a wall that is tightly adjacent, the manufacturer-recommended minimum side clearance of 50–80 mm may not be met.

The terminal box (electrical housing) on classic pumps is usually rotatable in 90° or 360° increments, allowing the installer to direct the electrical input so that it does not interfere with other components. This is an advantage that some cheaper models do not have – their terminal box is fixed, which can complicate installation in confined spaces.

Related topics can be found in the articles Installation of a circulation pump – procedure, orientation and most common errors and Setting up a circulation pump after installation – manual vs. automatic mode.

Replacing a pump without changing dimensions – practical scenarios from practice

Let's look at a few specific situations that we regularly encounter in customer orders:

Scenario 1: Replacing a 20-year-old three-speed pump with an ECM pump

The customer has a Grundfos UPS 25-60 130, center distance 130 mm, thread G 1½". They want to replace it with a more modern ECM pump with automatic regulation. The dimensional situation is simple – most modern ECM pumps in this class (Grundfos Magna1, Wilo Yonos MAXO, DAB Evoplus, various less well-known brands) have the same center distance of 130 mm and the same thread. The replacement is straightforward: unscrew the old unions, replace the gaskets, and install the new pump. Electrically, add a new cable if the old one lacks grounding.

What can complicate the situation: a new pump is physically larger in diameter (larger motor, electronics) – it may interfere with walls, radiator valves or expansion tank in a tight technical room. Always check the maximum outer diameter of the new pump before ordering.

Scenario 2: Boiler room in an apartment building – pump DN 32, flange PN6

In the boiler room of an apartment building, we have a DN 32 pump with a PN6 flange connection and an axial distance of 180 mm. An old pump (e.g., Grundfos CM or an old ITT Bell & Gossett) needs to be replaced. Problems may arise if the new pump has a PN10 flange – the bolt spacing differs, the gasket sits differently, and the connection is not straight. Solution: use transition flanges or choose a pump with the same PN rating.

Scenario 3: Floor heating pump with a shorter axial distance

The customer has a manifold for floor heating with a built-in pump. DN 25, but the axial distance is only 110 mm – some compact manifold assemblies work with this shorter distance. If the customer buys a standard 130 mm pump, it simply won’t fit into the manifold. Solution: look for a pump specifically designed for this application, or extend the piping and install the pump externally.

Scenario 4: Garden/well pump repurposed as a circulation pump

This is a scenario we encounter rarely, but still – the customer has a small submersible or garden pump installed as a replacement for a circulation pump. Dimensional compatibility practically does not exist here, the hydraulics are poorly dimensioned (the pump is designed for high pressure and low flow, whereas a circulation pump is for low pressure and high flow), and the entire solution must be redesigned from scratch. More about choosing the right pump is covered in the article How to choose a circulation pump for heating – what to focus on.

Flow direction and motor orientation – overlooked details

A circulation pump is not a symmetrical component – the flow always goes in one direction. An arrow or label indicating the flow direction is usually engraved on the pump body. If you install the pump rotated by 180°, it will not function properly – most small circulation pumps are designed for only one flow direction, from inlet to outlet. Reversed installation may work mechanically, but hydraulic efficiency drops rapidly, and in some types (e.g., pumps with axial inlet), it may cause damage to bearings or seals.

In addition to the flow direction, the orientation of the motor head is also important. Circulation pumps can be installed in the following ways:

Horizontally with a horizontal motor axis (most common case)
Horizontally with a vertical motor axis (motor up or down)
Vertically with a horizontal motor axis

Most manufacturers allow all these orientations, with one exception: the motor must not be rotated so that the electrical terminal box points downward. In such a case, condensation could drip into the electrical components and cause a short circuit. This is a safety instruction included in the manual of every pump – and it is also one of the most common installation errors, which are discussed in more detail in the article Installation of a circulation pump – procedure, orientation and most common errors.

Adapters, reductions and other compromise solutions

What to do if an ideally compatible pump is not available? There are several commonly used solutions:

Transition bushings and nipples

If the axial distance of the new pump is 10–20 mm shorter, the difference can be compensated with an extension nipple or longer bushing. This solution is common and cost-effective – a metric extension nipple G 1½" costs just a few euros. Disadvantage: each connection is a potential leak point, so the quality of processing matters.

Reducing bushings for different thread diameters

If you want to install a pump with a different nominal diameter (e.g., DN 25 instead of DN 20), a reducing bushing allows physical connection. But be careful – this solution only ensures mechanical compatibility, not hydraulic. Reducing from DN 20 to DN 25 is fine (higher flow), but the reverse reduction from DN 32 to DN 25 can create a hydraulic bottleneck and increase pressure loss. Always assess the hydraulics of the entire system.

Extension pieces and compensators

If the axial distance of the new pump is larger, it is sometimes possible to shift the piping slightly – but only if space allows and the piping is not rigidly fixed. Otherwise, flexible hose compensators (anti-vibration hoses made of EPDM or rubber) are used, which also reduce the transmission of pump vibrations to the piping. These hoses are 150–300 mm long and can compensate for a distance difference of 20–30 mm in each direction.

Dimensional differences between less known and premium brands

One of the questions we receive quite often is: "I have a Grundfos, can I replace it with a cheaper brand and will it fit?" The answer is usually yes – as long as it comes to dimensions. As explained above, standardized dimensional series (130 mm, G 1½") are maintained across manufacturers.

More on whether it is worth choosing a less known brand in terms of quality and reliability can be read in the article Less known circulation pump brands vs. Grundfos and Wilo – is it worth it?. Here we just note that dimensional compatibility alone is not a guarantee of the same quality of processing of the hydraulic part or electronics.

In practice, we have encountered the following dimensional deviations with less established manufacturers:

The axial distance is 130 mm, but the flange mounting surface has a different slope or is deformed – the gasket does not sit properly and the connection leaks
The thread is technically G 1½", but the tolerances are larger – the bushing is harder to screw in or the centering is not precise enough
The mounting holes for the bushing are slightly offset, making it difficult to mount bushings from another manufacturer

This does not mean that cheaper pumps are generally bad – it just means that it is better to have a few spare gaskets of different thicknesses (1.5 mm, 2 mm, 3 mm EPDM) on hand during the first installation, in case the mounting surface is not perfectly flat.

Special cases: solar, storage, and geothermal pumps

Circulation pumps for special applications – solar thermal systems, storage loops, geothermal heat pumps – may have non-standard dimensions or special fittings:

Solar pump units: these are often 1-way or 2-way configurations with built-in valves, flow meters, and drain cocks. The pump is integrated into the assembly and is not directly interchangeable – the entire pump unit must be replaced or it must be verified whether the pump is removable.
Storage tank pumps: some hot water storage tanks have an integrated connection block for a circulation pump. Dimensions may be non-standard (DN 15 with a shorter center distance of 110 mm). Always verify with the tank manufacturer.
Geothermal loops: here, the pumps are resistant to non-freezing mixtures (glycol, propylene glycol). Seals must be made of a material compatible with these media – standard EPDM seals are mostly compatible, while NBR (nitrile rubber) may degrade when in contact with some glycols.

How to correctly measure and document dimensions before selecting a pump

To conclude this section, we offer a practical guide on how to proceed with diagnosing and planning a pump replacement from a dimensional perspective:

Photograph the nameplate of the existing pump – it contains the manufacturer, model, power, flow rate, and sometimes the center distance. If the pump is too dusty or unreadable, clean it before taking the photo.
Measure the center distance using a caliper or precise measuring tool – do not confuse it with the total length of the pump including the flange, it is measured from the mounting surface of one flange to the mounting surface of the other flange.
Identify the type and size of the thread – most threads are marked on the body of the pump (G 1½" or Rp 1½"). If not, a thread gauge is the most reliable way to identify it.
Document the orientation – take photos of the pump from multiple angles, including the direction from which the electrical cable comes and where the air vents are located.
Measure the available space in the technical room – height, width, distance from the wall behind the pump, distance from the wall above the pump. Compare it with the maximum dimensions of the new pump.

Such documentation before ordering saves time, unnecessary transportation costs from returning goods, and most importantly, the nerves of the installer who is waiting for the parts.

Most frequently asked questions (FAQ)

Must the new pump have exactly the same center distance as the old one?

Ideally yes, but small differences of up to 5–10 mm can be solved with an extension nipple or compensator. A larger difference usually requires work on the piping – cutting, extending, or repositioning. If you want a trouble-free replacement, always look for the same center distance – most commonly 130 mm for domestic pumps.

Can I use the old flanges and gaskets from the original pump?

Flanges yes, if they are in good condition and their thread diameter matches the new pump. Gaskets no – always install new ones. Rubber gaskets lose elasticity after years in hot water, discolor, and become brittle. A new gasket costs just a few cents, while a leak and repeated installation cost hours of labor.

What if the new pump has a different type of thread – G instead of Rp or vice versa?

In practice, standard flanges are mostly compatible with both types – cylindrical thread G and conical thread Rp fit together, with the sealing being ensured by a rubber gasket, not the thread itself. Problems arise with hard plastic or exotic metal flanges, where the shape of the mounting surface may not match. If you have doubts, use flanges from the same manufacturer as the pump, or verified brass flanges of standard quality.

What is the difference between PN6 and PN10 flanges and can they be interchanged?

PN indicates the maximum operating pressure (PN6 = 6 bar, PN10 = 10 bar). The more important difference is in dimensions: for DN 32 and larger, the bolt hole spacing and the outer diameter of the flange differ between PN6 and PN10 – they are not directly interchangeable without transition flanges or adapters. For standard heating systems (operating pressure 1.5–3 bar), PN6 is sufficient, but when replacing, you must match the same PN level as the existing piping.

How can I determine if the pump is installed correctly in terms of flow direction?

Each circulation pump has an arrow on its body indicating the flow direction. Flow goes from the boiler (pressure output) through the pump into the heating circuit, or on the return – it depends on the wiring diagram. If in doubt, check the boiler wiring diagram or have an installer check it. A pump installed in the wrong direction will show significantly reduced performance and increased noise.

Is it safe to install a circulation pump without a professional plumber?

Replacing a pump in a closed heating system is not a restricted trade – if you know how to work with tools, drain the relevant section of the system, and properly seal the threads, it is not a technically demanding task. However, for electrical connections beyond just replacing a plug (hardwired 230 V without a plug), the services of a qualified electrician are required. If you have doubts about the hydraulics, hydraulic balancing, or the overall system design, we recommend consulting a specialist – this is discussed in more detail in the article Common circulation pump faults and how to recognize and fix them.

Conclusion: dimensional compatibility is not a detail, it is the foundation

As we have shown, dimensional compatibility of circulation pumps is a topic with multiple layers. It is not enough to know just the center distance – the type and size of the thread or flange, motor orientation, available space at the installation site, and material compatibility of the gaskets are equally important. Most standard replacements in the class DN 25, 130 mm, G 1½" are trouble-free, as manufacturers stick to established dimensional standards. Complications arise with non-standard dimensions, special applications, or when switching to a different nominal diameter.

The good news is that with the correct measurement and documentation process before ordering the pump, most problems can be avoided. And if you are still unsure, the nameplate of the pump or the technical data sheet of the model you want to buy contains all the necessary dimensions – you just need to know how to read and compare them correctly.

For a deeper understanding of selecting the right pump not only from a dimensional, but also from a performance and energy efficiency perspective, we recommend reading the articles Energy efficiency classes of circulation pumps – what A, B, C means and how much you can save and Maintenance and air venting of circulation pumps – how to extend their lifespan, which complete the overall picture of how to properly manage heating technology.

Do you have a question about this topic?

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