What thread does a circulation pump need – the difference between 1" and 6/4" connection

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What thread does a circulation pump need – the difference between 1" and 6/4" connection

At first glance, a trivial question – what thread does my pump have – can cause serious problems when you return home from the store with a new circulation pump and find out that it simply doesn't fit into the piping system. Or, on the contrary: the thread fits, but the pump is weak and you don't understand why. The size of the connection thread is not only about whether the nut "fits" – it is directly related to hydraulic performance, flow rate, suitability for a given type of installation, and ultimately to the proper functioning of the entire heating system.

In this article, we will take a detailed look at both common standards – the 1" thread and the 6/4" thread – and explain what these designations actually mean, how the dimensions differ, where each is used, what risks come with mixing them up, and how to choose the right pump for your specific case. We base our insights on real experiences from installations, where thread errors appear surprisingly often – not only among amateurs, but also among experienced tradespeople who work with a different standard than the specific installation has.

What does an "inch thread" actually mean and why do we talk about 1" and 6/4"

Circulation pumps for central heating and hot water heating use threads according to the BSP (British Standard Pipe) standard, specifically its parallel version known as BSPP or G (according to ISO 228). This designation comes from the British imperial system, where dimensions are given in inches (symbol "). One inch = 25.4 mm. The thread is therefore not "metric" in the sense of M16 or M20 – it has its own pitch and profile.

An important paradox that confuses everyone who encounters it for the first time: the number in the thread designation does not correspond to the outer diameter of the thread or the inner diameter of the pipe. It is a historically defined nominal dimension that originated as a derivative of the inner diameter of old cast iron pipes. Therefore, when we talk about the G 1" thread (i.e., one inch), its actual outer thread diameter is 33.25 mm – not 25.4 mm as you might expect. And the G 6/4" thread (i.e., one and a quarter inches) has an outer diameter of 41.91 mm.

These numbers are fixed, defined by the standard, and do not depend on the manufacturer. Every customer who buys a pump with the designation 6/4" will get exactly the same thread regardless of whether it is a Grundfos, Wilo, Avansa or other brand.

Actual thread dimensions – numbers you need to remember

To be specific, here are the key dimensions of both threads according to the ISO 228 (BSPP/G) standard:

Parameter	G 1" (one inch)	G 6/4" (six quarters)
Outer thread diameter	33.249 mm	41.910 mm
Mean thread diameter	31.770 mm	40.431 mm
Thread pitch	2.309 mm (11 threads/inch)	2.309 mm (11 threads/inch)
Flow cross-section (approximation)	≈ 5.47 cm²	≈ 9.68 cm²
Equivalent in metric pipes	DN 25	DN 32

From the table, it is clear that the 6/4" diameter is almost 8.7 mm larger than the 1" diameter. This difference is not negligible – it is almost an additional centimeter in diameter, which in a circular cross-section means that the 6/4" thread allows almost twice as much water to flow at the same pressure. That is why these two types of threads are not used as interchangeable alternatives – each has its own hydraulic role.

Where is the 1" thread used – typical applications in practice

The G 1" thread is historically the most widespread for circulation pumps in smaller and medium-sized residential heating systems. You will find it primarily:

In family homes with underfloor heating – where flows are typically lower and pressure losses in the distribution system are not dramatic. Pumps with a 1" thread are fully sufficient here.
In apartments and apartment buildings with classic radiator circuits – typically 2-3 circuits, heating water volume up to 100-150 liters, boiler power up to 20-25 kW.
For circulation of hot domestic water – where it is necessary to maintain the temperature of hot water in the distribution system and prevent it from cooling down in summer. Flows are low here, and the 1" thread is ideal.
In solar installations – where the circulation pump circulates fluid between the collector and the storage tank.

A typical example of a product for these applications is AVANSA 15-6/130 – circulation pump with a 1" connection thread. This pump has a maximum head of 6 meters of water column and a shaft length of 130 mm, making it ideal for typical home installations. Customers use it mainly in new family homes with gas boilers up to 20 kW, where underfloor heating is combined with a few radiators on the upper floor. The flow of the 1" thread is more than sufficient for these power levels.

Where is the 6/4" thread used – and why bigger does not always mean better

The G 6/4" thread appears everywhere where the system requires a higher flow rate or where it is necessary to overcome greater pressure losses in the system. We are talking about these cases:

Larger family homes and villas – living area over 200 m², multiple heating circuits, boiler with a power of 30-50 kW.
Bivalent systems – combined system with a heat pump and a backup boiler, where a higher nominal flow rate is required for hydraulic balancing.
Industrial and commercial applications – smaller businesses, restaurants, hotels with central heating.
Mixing nodes and distributors – where the pump operates in the primary circuit and must supply water to a distributor with 4-6 branches.
Systems with long pipe runs – where pressure losses in the pipes are large and the pump must work with a higher head and flow rate at the same time.

These situations are suitable for pumps such as AVANSA 25-4/130 with a 6/4" thread, AVANSA 25-6/130 with a 6/4" thread, or AVANSA 25-4/180 with a 6/4" thread – the last mentioned is interesting for installations where a larger construction length of 180 mm is needed, typically when installing in an existing position after another pump or with specific types of heating modules.

An important practical note: customers sometimes assume that a pump with a 6/4" thread will automatically be more powerful than one with a 1". This is not always true. A 6/4" thread is only a physical condition for a higher flow rate – the hydraulic performance itself depends on the pump parameters (maximum head, nominal flow rate, Q-H curve characteristic). A 25-4 pump with a 6/4" thread has a maximum head of 4 meters of water column, while the AVANSA 15-6 with a 1" thread has a maximum head of 6 meters. A smaller thread, a higher head – that is the reality to keep in mind. For more information on how to read these parameters, see the article What do the numbers in the Avansa pump name mean – performance, head and pitch.

Physical difference – what the thread affects hydraulically

Flow cross-section is a key parameter. If water flows through a smaller opening at the same pressure drop, it must flow faster. Flow velocity directly affects local pressure losses and noise. For a properly functioning heating system, a water flow velocity in the pipes of 0.3 to 0.7 m/s is recommended – higher values cause noise and erosion, while lower values risk sedimentation.

Specifically: if the system requires a flow rate of 2.5 m³/h (a typical value for a 20-25 kW boiler with a temperature drop of 10 K), water flows through a 1" thread (flow cross-section ~5.5 cm²) at a speed of about 1.26 m/s – that is too fast and the 1" thread becomes a hydraulic throttling point. The same amount of water through a 6/4" thread (flow cross-section ~9.7 cm²) flows at only ~0.72 m/s – that is on the edge, but acceptable. For boiler outputs over 30 kW, a 6/4" thread is practically essential.

This hydraulic dependency is the reason why pump manufacturers always specify the maximum recommended flow rate for a given thread type. If you were to install a pump with a 1" thread in a system requiring a flow rate of 3 m³/h, the pump would physically run, but the thread would be the bottleneck of the entire system and pressure losses at the inlet/outlet would dramatically reduce efficiency.

How to determine the thread type in your existing installation

This is the number one practical question when replacing an existing pump. It is not enough to simply measure the diameter with a wrench – this will give you the outer dimension, but not information about the thread. The correct procedure:

Check the documentation of the existing pump – any label, card, or technical sheet usually states the thread type.
Measure the outer diameter of the thread with a caliper – if the measured value is 33–33.5 mm, it is G 1". If it is around 41.5–42 mm, it is G 6/4".
Look at the current pipe dimension – if the supply pipe is copper or steel DN25, you have a 1" thread. If it is DN32 or larger, it is 6/4".
Check the type of fitting – in practice, we most often encounter unions (screwed couplings). Most standard unions for circulation pumps have either a 1" or 6/4" nut part – simply place the replacement part next to it.

Practical warning: in older installations, especially in panel buildings from the 80s and 90s, corroded threads occasionally appear, where the dimension is difficult to measure. In such a case, I recommend taking the old threaded part (union or valve) to the store and comparing it physically. The flow of customers who "measured" the thread visually and bought the wrong type of pump is a real phenomenon at every store with heating technology.

Thread reduction – is it possible and at what cost

Yes, brass reductions G 6/4" → G 1" and vice versa exist. But be careful – this solution has its pitfalls and is recommended only as a temporary or emergency solution. Why?

First, when reducing from 6/4" to 1", the flow cross-section is reduced to less than 60% of the original value. This means that the hydraulic performance of a pump dimensioned for 6/4" is significantly limited at a 1" port – the local pressure loss at the reduction can be equivalent to an additional 0.5–1 meter of water column. If your pump is dimensioned precisely for the hydraulic conditions of the system, this loss can bring you below the critical value needed for circulation in all branches.

Second, each additional mechanical connection is a potential leak point. Brass reductions are reliable, but each additional connection increases the likelihood of a leak, especially during temperature cycles, when materials are in motion.

Third, in some countries and for certified installations (e.g., heat pumps with subsidies), the inspection technician will refuse to approve such improvisations.

Practical conclusion: if it is technically possible, always choose a pump with the correct type of thread from the beginning. Reduction is a solution only when the correct variant is not available in stock and the system needs to be running urgently – not as a permanent solution.

Electronic pumps and thread – does the same apply

For electronic circulation pumps (inverter, with automatic flow regulation), the same rules for the thread apply as for standard pumps. The difference between an electronic and a conventional pump is in the electric motor and control electronics – not in the hydraulic part. For example, AVANSA AUTO 25-4/180 E – electronic circulation pump with 6/4" thread has the same physical thread as its non-electronic brother AVANSA 25-4/180, only it is equipped with automatic flow regulation according to the current load of the system. If you are considering switching from a conventional to an electronic pump and both models have the same type of thread, the replacement is simple. However, if you are also changing the thread type, you need to adapt the piping.

More about the advantages of electronic pumps can be found in the article Electronic vs. standard Avansa circulation pump – is it worth paying extra for the AUTO version.

Selection rules – a simple decision-making process

Based on practical experience and technical parameters, I recommend the following procedure for selecting the correct thread:

Step 1: Determine the boiler or heat source power. Up to 25 kW: a 1" thread is sufficient. Above 25 kW: a 6/4" thread is more suitable or necessary.
Step 2: Calculate the required flow. Approximate formula: Q (m³/h) = P (kW) / (1.163 × ΔT), where ΔT is the temperature difference between supply and return (typically 10–20 K). For P = 30 kW, ΔT = 10 K: Q ≈ 2.58 m³/h – this is on the edge of a 1" thread, a 6/4" thread is recommended.
Step 3: Check the existing pipe dimensioning. DN25 → 1" thread. DN32 or larger → 6/4" thread.
Step 4: Check the existing pump. If you are replacing it with an equivalent replacement, keep the same thread type – you will save on installation work and materials.
Step 5: In case of doubts, choose a larger thread. An oversized flow diameter will not cause problems; an undersized one can restrict the system hydraulically.

A more complex selection process for the entire pump – not just the thread, but also the head, construction length, and power – can be found in the article How to choose an Avansa circulation pump for your heating system.

Special case: construction length and its relation to the thread

When purchasing a replacement pump, customers sometimes confuse two dimensions: thread type and construction length (center-to-center distance). These are two completely independent values. You can have a G 6/4" pump with a construction length of 130 mm (e.g., AVANSA 25-4/130) and a G 6/4" pump with a construction length of 180 mm (AVANSA 25-4/180). Both have the same thread, but different lengths – this means they will not fit into the same space in the piping.

The construction length of 130 mm is the most common standard in Europe for domestic installations; 180 mm is common in older installations or in industrial applications. When replacing a pump, always measure the distance between the center of the inlet and outlet thread – this is the construction length you need to maintain. Changing the construction length requires mechanical modification of the pipe segment, which increases costs and installation time.

For more information about construction length parameters, see the article What do the numbers in the Avansa pump name mean – performance, head, and construction length.

Most Frequently Asked Questions (FAQ)

Can I connect a pump with a 6/4" thread to a 1" threaded pipe without an adapter?

No, these threads are not mutually compatible. G 1" and G 6/4" have different outside and inside thread diameters – although the pitch (number of threads per inch) is the same (11 threads per inch), the threads themselves will not match. You need either a brass reducing nipple or you must replace the flanged part of the piping system. Never try to force the threads together – you will damage the thread surface and the connection will never be tight.

Why does the 6/4" pump have a lower head than the 1" pump?

The type of thread does not determine the head – this depends on the hydraulic design of the impeller. A larger thread allows for a higher flow rate, but hydraulic efficiency at low flows may be lower. Specific models such as AVANSA 25-4 have a head of 4 m, while AVANSA 15-6 with a 1" thread has a head of 6 m – this is an intentional design choice, not a shortcoming of the larger thread.

What if I accidentally buy a pump with the wrong type of thread – can it be returned?

When purchasing via an online shop, you are legally entitled to return the product within 14 days from the date of receipt (if it is unused and in its original condition). In practice, I recommend always physically measuring the existing thread with a caliper and comparing it with the technical specifications of the selected pump before ordering. If you are unsure, write to the store and provide the boiler power, pipe dimensions, and the type of existing pump – we will gladly help you make the correct choice.

Are inch-based BSP threads compatible with NPT threads from the American market?

No, despite the same nominal designation (e.g., 1"), BSP (British Standard Pipe) and NPT (National Pipe Thread) differ in the angle of the thread profile – BSP has 55°, NPT has 60°. These threads cannot reliably engage and are not interchangeable. In European installations, we always use BSP/G threads; NPT appears only exceptionally in imported American appliances.

Does the type of thread also affect the maximum working pressure of the pump?

The maximum working pressure mainly depends on the pump body construction and material, not directly on the size of the thread. Standard circulation pumps for central heating are dimensioned for a maximum working pressure of 6 bar (0.6 MPa), which applies to both types of threads. For higher pressures (e.g., in industrial applications above 6 bar), special versions with flanged connections are used, not threaded ones.

Do the inlet and outlet threads of the pump have to be the same?

Yes, in standard circulation pumps for central heating, the inlet and outlet threads are always of the same type and size. Pumps are not directionally symmetrical (they must be correctly oriented – inlet/suction port on the correct side), but both threads have the same dimensions. This applies to all Avansa models and other common brands.

Conclusion

The conclusion of this article can be summarized in a few sentences worth remembering. The G 1" thread and the G 6/4" thread are not interchangeable or compatible without an adapter. G 1" has an outside diameter of 33.25 mm and is suitable for systems with lower flow – typically boilers up to 25 kW and smaller family homes. G 6/4" has an outside diameter of 41.91 mm, offers a larger flow cross-section, and is suitable for larger systems, higher power, and situations where flow is a limiting factor.

When replacing an existing pump, always measure the actual thread with a caliper, do not copy the designation blindly. When designing a new installation, follow the power of the heat source and the required flow, not trends or price – a correctly dimensioned thread is cheaper than later retrofitting and adaptation. And if you are unsure, the category Avansa pumps on atria.sk offers products with clearly stated thread type directly in the name, making selection much easier than in a physical store with unmarked boxes.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.