What connection thread do I need – 1" or 6/4" for the Avansa pump
What thread size do I need – 1" or 6/4" for the Avansa pump?
When choosing an Avansa circulation pump, one of the first technical questions you will encounter is the size of the connecting thread. In practice, we come across two dimensions: 1 inch (1") and 6/4 inch (6/4"). The difference between them is not large – we are talking about a few millimeters – but in the field, this difference causes big problems when a customer orders a pump without verifying the actual size of the existing pipe or pump body. Returns, additional orders, unnecessary technician visits – all of this happens because people underestimate this seemingly simple technical detail.
In this article, we will go into depth: what these dimensions mean, how to reliably measure them, which Avansa models have which thread, when the choice matters and when the situation can be solved with reductions. If you are still considering the pump itself and want to know more than just the connecting thread, also see our article How to choose an Avansa circulation pump for your heating system or What do the numbers in the name of the Avansa pump mean – power, head and pitch.
What is an inch thread and why is it used in inches?
Inch threads (marked with the symbol ") are a historically rooted standard in plumbing, heating and water installations throughout Europe. Although the Slovak regulatory framework has long been moving towards metric dimensions, in the practice of thermal engineering and pump technology, the inch thread remains the standard. The reason is simple: most of the pipe, fittings, manifolds, distribution systems and installation components – from local manufacturers as well as international suppliers – are still standardized to these dimensions. That is why we encounter them with Avansa pumps as well.
The inch thread referred to in this context is the so-called BSP thread (British Standard Pipe), which is identical to the G thread according to ISO 228. It is a cylindrical (not conical) thread, which is important to know for the tightness of the connections – a cylindrical thread is not self-sealing by itself, it requires sealing tape (Teflon) or hemp fiber with paste. The conical BSP thread (BSPT, marked R) is not commonly used in this application.
Dimensions 1" and 6/4" – specific millimeters and what it means in practice
To avoid unnecessary abstraction, let's look at the specific dimensions:
| Thread designation | External thread diameter (mm) | Thread pitch (mm) | Typical use |
|---|---|---|---|
| G 1" (1 inch) | 33,25 mm | 2,309 mm (11 threads/inch) | Smaller pumps, small circuits, simple installations |
| G 6/4" (6/4 inch = 1½") | 47,80 mm | 2,309 mm (11 threads/inch) | Larger pumps, longer distribution systems, higher flow |
The thread pitch is the same, but the external diameter differs by more than 14 mm. This means that it is not possible to replace one connection with another without a reduction. The threads are not compatible and cannot be physically connected without an adapter. This is the number one source of problems when replacing pump models.
Note also that 6/4" is equivalent to 1½" (one and a half inches) – in some catalogs and manuals you may encounter both designations for the same dimension. It is the same thing.
Overview of Avansa models and their connecting thread
Let's look specifically at the currently available Avansa pump models and their thread dimensions. This is information you need to verify before you order – not after the package has been delivered.
Pumps with a connecting thread of 1"
The smaller, 1-inch thread category in the Avansa range includes the model AVANSA 15-6/130 – circulation pump with a connecting thread of 1". This pump is intended for smaller circuits, typically for family homes with lower thermal load, or for secondary circuits (e.g. floor heating as a separate loop). The number 15 in the name indicates a maximum flow of 15 l/min under standard conditions, the number 6 indicates a maximum head of 6 m of water column and 130 is the axial distance of the connections (pitch) in millimeters.
The 130 mm pitch is an important parameter for this model – just like for all other models with the same pitch, it must match the distance of the existing threads in the installation. If you are replacing an old pump and its pitch was different, it is not enough to just replace the pump, you also need to address the piping connections.
Pumps with a connecting thread of 6/4"
The larger group consists of models with a 6/4" thread, which cover a wider performance range:
- AVANSA 25-4/130 – circulation pump, connecting thread 6/4" – pitch 130 mm, head 4 m, flow up to 25 l/min
- AVANSA 25-4/180 – circulation pump, connecting thread 6/4" – same performance as the previous one, but pitch 180 mm for installations with a greater distance between connections
- AVANSA 25-6/130 – circulation pump, connecting thread 6/4" – higher head of 6 m at a flow of 25 l/min, pitch 130 mm; suitable for longer and more complex distribution systems
- AVANSA AUTO 25-4/180 E – electronic circulation pump, connecting thread 6/4" – electronically regulated model with automatic performance adjustment, pitch 180 mm
As you can see, all the more powerful models in the Avansa range have a 6/4" thread. This is not a coincidence – the larger connection diameter directly corresponds to the larger pump flow and higher flow capacities. If your system requires a flow of around 25 l/min or more, you automatically move towards a 6/4" model.
How to measure an existing thread in the installation – step-by-step guide
The most common mistake we encounter in practice is ordering a pump without actually verifying the size of the existing thread. Customers rely on memory, an old invoice, or what a previous plumber said. The result? An incorrect thread.
Here is a reliable method to determine what thread you have in the installation:
Method 1: Measuring the outer diameter with a caliper
If the installation has a male thread (external – the body with the thread), measure the outer diameter of the threaded part with a caliper. Not the smooth pipe, but the thread itself. If the measured value is approximately 33 mm, it is 1". If it is around 48 mm, it is 6/4". You can measure accurately on a disconnected pipe or a cut-off fitting.
Method 2: Identification based on the old pump
If you are replacing an existing pump, the connection thread should be indicated on the label or in the documentation of the old pump. If the documentation is missing, try to identify the manufacturer and model and look up the technical specifications online. Alternatively, measure the old pump physically.
Method 3: Template or test fitting
In any good plumbing store, they will give you a test fitting G1" or G6/4" – simply place it against the thread and you will immediately know which one fits. This method is 100% reliable and will eliminate any doubts.
When the type of thread matters and when a reducer can be used
From practice, I know that many customers ask: "What if I have a 1" connection in the wall, but I need a 6/4" pump?" Or the other way around. The answer is not straightforward – it depends on the specific situation.
Case 1: Replacing the pump with the same thread size
This is the ideal scenario. If you are replacing an old pump with a 6/4" thread with a new Avansa pump with a 6/4" thread (and the same pitch), the replacement is straightforward. You unscrew the old one, fit the new one, seal it, and you're done. This case makes up the majority of standard replacements in practice.
Case 2: New installation – the pump determines the connection size
In a new installation, where the piping is not yet laid out, the customer can choose the pump based on performance parameters and then adjust the piping accordingly. In this case, the thread is secondary – the more important factors are the head, flow rate, and pitch. For more information on how to correctly choose a pump's performance, see the article What do the numbers in the Avansa pump name mean – performance, head, and pitch.
Case 3: The customer has existing 1" connections, but needs a more powerful 6/4" pump
This is a more problematic scenario. Technically, a reducer G6/4" – G1" is readily available. Theoretically, you can therefore fit a 6/4" pump into a 1" connection using a reducing bushing. However, in practice, we rarely recommend this and it is not advised as a long-term solution.
Why? Because the internal diameter of the pipe and fittings is significantly smaller with a 1" connection – flow resistance increases, a hydraulic bottleneck is created, and the pump operates inefficiently. If the system requires a flow rate for which a 6/4" pump is dimensioned, then reducing to a 1" connection means the actual flow will be lower than the system needs. It's like connecting a highway to a narrow country gate.
The correct solution in this case: replace the relevant section of the pipe and connections with 6/4" dimensions. Yes, it is a bigger job, but it is the correct solution. Alternatively, consider whether you really need a more powerful model – perhaps a model with a 1" thread is sufficient if the performance parameters are adequate.
Case 4: The customer has 6/4" connections, but is considering a smaller 1" pump
This is the opposite scenario – you have larger connections and want to fit a smaller pump. This is less problematic from a hydraulic point of view, but still requires a reducer. The potential issue is performance-related: if your system has 6/4" connections, it was likely designed for a higher flow rate, and a smaller pump with a 1" thread will not be able to meet that demand. Check the performance parameters before making a decision.
Spacing 130 mm vs. 180 mm – how it relates to thread selection
Along with the thread size, you should also pay attention to another key dimension: the spacing of the connections (the axial distance between the pump's inlet and outlet). In the Avansa range, we encounter spacings of 130 mm and 180 mm. This number must exactly match the distance of the existing connections in the installation; otherwise, the pump simply won't fit – either the connection will be too short or the piping will have to be "stretched".
Replacing a 1" thread with a 6/4" thread and changing the spacing from 130 mm to 180 mm are thus two separate dimensions in which you must be precise. It is entirely possible to have the correct thread but the wrong spacing – and the pump still won't fit. Always measure both before ordering.
In practice, most older installations (from the 90s and the beginning of the 2000s) have a spacing of 130 mm, while newer installations are sometimes designed for 180 mm. If you are unsure, measure the distance between the axes of both threaded connections using a ruler or a caliper. This value must match the spacing of the selected pump model.
Concrete examples from practice – jobs we remember
Job 1: Family house, pellet boiler, replacing an old pump
The customer had a boiler room with a pellet boiler and an old three-speed pump. The connections were G6/4", spacing 180 mm. He needed to replace the old pump with a more modern, energy-efficient model. After consultation, we recommended the AVANSA AUTO 25-4/180 E – an electronic circulation pump with automatic regulation, connection thread 6/4", and spacing exactly 180 mm. The replacement took half an hour, including unscrewing the old one, installing the new one, and bleeding the system. The customer saved on electricity and didn't have to deal with any adapters.
Job 2: Low-energy house, floor heating, new installation
In a new build with floor heating and a total thermal load of around 6 kW, the designer proposed using a separate pump for the secondary loop of the floor heating. Since the loop was short and the flow requirements were modest, the AVANSA 15-6/130 with a 1" thread was selected. The connections in the manifold were designed precisely for a 1" G thread, spacing 130 mm. The solution works without any problems.
Job 3: The customer ordered the wrong model
Unfortunately, this happens. The customer had 6/4" connections in the boiler room and ordered (without verification) a model with a 1" thread, because he remembered that "the pump was smaller". After delivery, he found out that the thread didn't fit. We had to arrange the exchange for the correct model – AVANSA 25-4/130 with a 6/4" thread. The whole process delayed the installation by two days and the customer had to pay the return shipping. Lesson learned: always measure before ordering.
Practical decision criteria – a brief overview
To quickly orient yourself, here is a simplified decision framework:
- If you are replacing a pump with the same one – check the thread and spacing of the old pump and choose the same new model.
- If you are replacing it with a more powerful model – check if the new model has the same thread; if not, arrange for pipe adaptation.
- If you are doing a new installation – choose the pump according to performance and adapt the piping accordingly.
- If you have any doubts – measure. A caliper is a cheaper mistake than returning the product and waiting for a new one.
- Use reductions only exceptionally – and only if reducing the diameter does not create a hydraulic limitation for the system.
Connection between thread selection and pump performance parameters
The size of the connection thread is not random – it directly relates to the internal diameter of the impeller and the pump body. The larger the thread, the greater the possible flow without excessive hydraulic losses. Therefore, it is not possible to simply take a model with a small 1" thread and expect the same flow as from a larger 6/4" model – even if you have the same head (the number after the dash in the name).
If this topic interests you in depth and you want to understand what the numbers 15, 25, 4, 6 in the names of individual models specifically mean and how to derive performance and suitability for a particular system from them, I recommend the article What do the numbers in the name of the Avansa pump mean – performance, head and spacing in the Knowledge Centre.
Sealing of the threaded connection – what not to forget during installation
Once you know which thread you need and have the correct pump model, one more thing determines whether the installation will be trouble-free: sealing material for the threaded connection. A G thread (BSP cylindrical) is not self-sealing. Every connection must be supplemented with:
- PTFE tape (Teflon) – wound in the direction of the thread, usually 3–5 layers for a G1" thread, 4–6 layers for a G6/4" thread; an inexpensive, simple and reliable solution
- Hemp fiber with sealing paste – a traditional method, still used by experienced plumbers; works well with metal threads; a mixture of hemp and paste (e.g. Fermit, Loctite 55) fills the thread and, after hardening, creates a reliable sealing layer
- Liquid sealant (e.g. Loctite 577) – for more demanding applications or where the thread has a finer surface finish; applied directly to the thread and polymerizes after tightening
Not sealing the thread at all is the most common cause of dripping after pump replacement. The second most common cause: over-tightening with a wrench, which can damage the thread or crack the pump body. A G thread is not tightened to the maximum with a wrench – you tighten it by about 2–3 turns after manual tightening and that's it. More about the correct procedure can be found in the article Installation of the Avansa circulation pump step by step.
Material aspects of the thread – cast iron, brass, stainless steel
The body of the threaded connection of Avansa pumps is mostly made of cast iron (grey cast iron, EN-GJL-200 or similar). This should be kept in mind when selecting the opposite fitting – the paired metals should not differ too much in electrochemical potential in order to minimize galvanic corrosion. A brass fitting on a cast iron connection is a common and proven combination. A steel connection on a cast iron thread does not present a problem in a standard installation with boiler water.
In more aggressive environments (for example, in refrigeration applications or with water containing high levels of chlorine and oxidants), pay attention to the materials – but this is more of a topic for special applications, not for standard heating of a family home.
Most frequently asked questions (FAQ)
Is 6/4" the same as 1½"?
Yes, exactly the same. Both notations refer to the same dimension – a one and a half inch thread with an outer diameter of the threaded part of 47.80 mm. European manufacturers commonly use the notation 6/4" in their catalogs, while English-speaking documents often use the notation 1½". If you encounter both notations in different documents for the same installation, it is not a contradiction – it is the same thing.
Can I use a 6/4" pump in a system with 1" connections using a reducer?
Technically yes, a G6/4" to G1" reducer exists and is readily available. Practically, we do not recommend it if the system requires higher flow rates for which the 6/4" pump is designed. Reducing the internal diameter to 1" significantly increases hydraulic resistance, which reduces the actual flow in the system. The result is inefficient pump operation and potential problems with heating/distribution. A better solution is to adapt the connections to 6/4".
How can I determine the thread size of my old pump if I don't have the documentation?
Measure the outer diameter of the threaded part directly on the old pump or on the opposite fitting in the installation using a caliper. A value around 33 mm = 1", a value around 48 mm = 6/4". Alternatively, place a test fitting (bought or borrowed from an installation store) and physically check which one fits. A third option: find the manufacturer and model according to the label on the pump and look up the technical specifications online.
Do I need to replace the sealing material of the threaded connection when replacing the pump?
Yes, always. The old sealing material (Teflon, hemp) is usually damaged or remains on the opposite thread after disassembly and is not reusable. Always clean the threaded surface of the opposite connection before installing the new pump and apply fresh sealing material. Attempts to save money on Teflon are the source of many unnecessary leaks. A Teflon tape costs just a few cents and will save you a lot of trouble.
I bought an Avansa pump with the wrong thread – can this be fixed without replacement?
It depends on the situation. If you bought a pump with a 6/4" thread and have a 1" connection – you can use a reducer, but see the limitations mentioned above. If you bought a 1" pump and have a 6/4" connection – a reducer will not help at all, because the internal thread of the connection is larger than the external thread of the pump and direct fittings in this direction are reducing sleeves with internal 6/4" and external 1", which still does not solve the problem functionally. In both cases, the clean solution is to replace the pump with the correct size.
Where can I find information about the thread type of a specific Avansa model when ordering?
The connecting thread is always mentioned directly in the product name – for example, "AVANSA 15-6/130 – circulation pump, connecting thread 1"" or "AVANSA 25-4/130 – circulation pump, connecting thread 6/4"". Additionally, this information is available in the technical specifications of each model on the product page. If you are unsure, check both sources before placing your order.
Conclusion – a few words before you click on the order
The question of the connecting thread – 1" or 6/4" – may sound technically trivial, but in practice it accounts for an unexpectedly large number of incorrect orders and unnecessary complications during installation. Yet verifying the correct size takes literally five minutes with a caliper in your hand.
To summarize what you should have taken away from this article: Avansa models with higher flow rates (range 25) have a 6/4" thread, while the smaller model (range 15) has a 1" thread. Reducers are possible, but hydraulically disadvantageous. The spacing of 130 or 180 mm is a separate dimension that must also match. Always measure before ordering, not after delivery. And during installation, never forget to use sealing material for the threaded connection.
If you are also interested in what to do after a successful installation – how to set the correct speed level or adjust the flow – we recommend the article "Setting and adjusting the flow of an Avansa pump after installation". And for those considering an electronic model with automatic regulation, a comparison is available: "Electronic vs. standard Avansa circulation pump – is it worth paying extra for the AUTO version".
Do you have a question about this topic?
Are you unsure or dealing with a specific situation in your home? Write to us – we are happy to help.