How to determine the correct light and filter length according to pipe diameter
Why the correct pipe diameter and filter length are key when selecting a fitting
When ordering a threaded filter for a heating or water supply system, many customers focus only on one piece of information - "what thread do I have". The reality, however, is more complex. In order for the filter to function reliably, not cause unnecessary pressure loss and at the same time be easily installable and later cleanable or replaceable, it is necessary to correctly determine two quantities simultaneously: the diameter (nominal diameter, denoted by DN) and the construction length of the filter (L). These two parameters are closely related and are often confused or underestimated in practice - for example, a plumber orders a filter according to the thread he remembers from the original fitting, but does not consider that in the new assembly there is less installation space available, or that the flow in the system requires a larger diameter than the original fitting had ten years ago.
In this article, we will examine in detail how to determine the pipe diameter, how to measure it directly at the installation site, why the overall length of the filter is important and how these two parameters are interrelated with the performance of the system, the flow of the medium and pressure loss. We will also focus on specific dimensional recommendations, which you will find in the category stainless steel and flanged fittings, specifically on the series of threaded filters BRA.10.000.
What does the pipe diameter (DN) mean and how is it related to the filter thread
The pipe diameter, denoted by the abbreviation DN (from the French "diamètre nominal"), is the nominal (not actual) internal diameter of the pipe or fitting expressed in millimeters. It is important to understand that DN is a standardized value that does not exactly equal the actual internal diameter - it is rather a category into which the diameter is classified for compatibility of fittings, seals and connecting elements from different manufacturers.
In everyday heating and plumbing practice in Slovakia, however, we encounter the designation of the thread in inches much more often than DN (for example 1/2", 3/4", 1"). These two systems are interrelated and when ordering a filter, it is necessary to know which data corresponds to which.
Conversion table DN to inch thread
| Pipe diameter DN | Inch thread (BSP) | Approximate internal diameter | Typical use |
|---|---|---|---|
| DN8 | 1/4" | approx. 8-9 mm | measuring circuits, small connections, pressure gauges |
| DN10 | 3/8" | approx. 10-12 mm | connections of thermostatic heads, small circuits |
| DN15 | 1/2" | approx. 15-16 mm | radiator branches, apartments, small family houses |
| DN20 | 3/4" | approx. 20-21 mm | main circuits of family houses, boilers up to approx. 30 kW |
| DN25 | 1" | approx. 25-27 mm | larger houses, boiler cascades, larger flows |
| DN32 | 1 1/4" | approx. 32-35 mm | apartment buildings, technologies, industrial circuits |
| DN40 | 1 1/2" | approx. 40-42 mm | boiler rooms, heat exchanger stations |
| DN50 | 2" | approx. 50-53 mm | larger technological circuits |
This assignment applies to common pipe threads according to the ISO 228 (BSP) standard, which are used almost exclusively in heating. When ordering a filter, it is therefore worthwhile to first identify whether it is an external or internal thread and what its actual diameter is - only then assign the correct DN and look for a specific product.
How to correctly measure the pipe diameter before selecting a filter
In practice, it is worthwhile not to rely only on documentation (projects often change during implementation) and to verify the diameter directly at the site. There are two basic cases that need to be distinguished - measuring on a smooth pipe (before welding a transition or before installing a threaded insert) and measuring directly on an existing thread.
Measuring the outer diameter of a smooth pipe
Using a sliding caliper or a tape measure, measure the outer diameter of the pipe. In the case of steel and copper pipes, the internal diameter differs from the outer diameter by the wall thickness on both sides, so in the case of thicker-walled pipes (for example, older steel pipelines), the actual diameter may be smaller than would correspond to the outer diameter of a thinner-walled pipe of the same outer diameter. When designing a new connection for a filter, it is therefore always based on what thread will be on the end (insert, reducer, T-piece), not directly on the diameter of the pipe itself.
Measuring an existing thread
When measuring an existing thread, it is most reliable to measure the outer diameter of the thread (in the case of an external thread) or the inner diameter of the hole (in the case of an internal thread) and compare it with the tabular values of pipe threads. As a rough guide:
- 1/4" - outer diameter of the thread approx. 13.2 mm
- 3/8" - outer diameter of the thread approx. 16.7 mm
- 1/2" - outer diameter of the thread approx. 20.9 mm
- 3/4" - outer diameter of the thread approx. 26.4 mm
- 1" - outer diameter of the thread approx. 33.2 mm
In case of doubts, it is better to place the old fitting directly against the new one or to find out the exact type of connecting thread in advance from the technical documentation of the boiler, pump or distributor. This topic is also dealt with in more detail in a separate article "What thread and filter size do you need (1/4" to 2")" in the Knowledge Centre, where you will also find the procedure for combined and reduced connections.
Why the construction length of the filter (L) is also important
While the diameter determines the permeability and flow capacity, the construction length (L) determines how much space the filter will occupy in the axial direction between two connecting elements of the pipe. This data is given in millimeters and is always listed in the catalog sheets for a specific size. Why is this important?
Relationship between diameter and length
In general, the larger the diameter, the longer the construction length of the filter - a larger thread requires a larger body of the fitting, a larger filter insert and therefore a longer threaded part with a bottom closure. In the case of the BRA.10.000 series, which you will find in the category of stainless steel and flanged fittings, this dependency is clearly visible:
| Thread | DN | Construction length L |
|---|---|---|
| 1/4" | DN8 | 65 mm |
| 3/8" | DN10 | 65 mm |
| 1/2" | DN15 | 65 mm |
| 3/4" | DN20 | 80 mm |
| 1" | DN25 | 90 mm |
Note that at smaller sizes (1/4", 3/8", 1/2") the length remains the same - 65 mm - because the filter body is structurally dimensioned for the same type of insert and shut-off mechanism. Only from 3/4" upwards does the length increase, as both the diameter and the volume of the filter chamber also change. This is a common phenomenon among other valve manufacturers as well, and is not a characteristic of a single series.
Why the length matters during installation
Construction length is critical in three situations:
- Renovations and replacements - if you are replacing an old filter or valve with a new one in an existing pipe section, the length must match, otherwise it will either be too short or, conversely, protrude and you will have to adjust the fittings or cut the thread again.
- Tight spaces - in technical rooms, niches near the boiler, or floor distributors, space is limited and a longer filter may simply not fit between existing components (ball valves, check valves, pump).
- Service space for removing the insert - the filter requires the bottom closure to be removed and the filter mesh pulled down or sideways during cleaning. If the valve is mounted too close to the wall or another component, servicing will be complicated or even impossible without disassembling the entire assembly.
How system performance and flow rate influence the choice of size
The size of the filter should always match the size of the pipe into which it is installed - in principle, the filter is never reduced to a smaller diameter than the surrounding pipe, because it would become a local bottleneck and increase pressure loss and flow speed at the point of constriction (which, in the case of a filter, also worsens its cleaning function, as the increased speed "blows through" impurities over the mesh). Nevertheless, it is useful to know how the pipe size is derived from the heat source's output and the required flow rate, because when designing a new branch or replacing an undersized pipe, these calculations are done in exactly this order - first the output, then the flow rate, then the size, and finally the valve including the filter is selected.
Flow rate calculation from output
The basic relationship between thermal output, temperature difference and water mass flow rate is:
Q [l/h] = P [kW] / (1.163 × ΔT [K])
where P is the thermal output in kilowatts, ΔT is the temperature difference between the supply and return (typically 10-20 K for low-temperature systems, 15-20 K for classic radiator systems), and 1.163 is the specific heat capacity of water converted to Wh/(kg·K).
Example 1: A boiler with an output of 24 kW, temperature difference of 15 K. Flow rate Q = 24 / (1.163 × 15) = 24 / 17.45 = 1.38 m³/h, which is approximately 1,380 l/h. At the recommended flow speed of around 0.8-1.2 m/s in the supply pipe, the size DN20, i.e., thread 3/4", is suitable. This corresponds, for example, to Threaded stainless steel filter 3/4"; L=80mm; PTFE.
Example 2: A smaller boiler with an output of 12 kW, the same temperature difference of 15 K. Flow rate Q = 12 / 17.45 = 0.69 m³/h, i.e., approximately 690 l/h. In a typical family house with DN15 (1/2") piping, the flow speed is around 0.9-1 m/s, which is within the typical recommended range. In this case, Threaded stainless steel filter 1/2"; L=65mm; PTFE is suitable.
Example 3: A measuring or impulse line, or a small circuit for a filling system or manometer, where only minimal flows in the range of 50-100 l/h are flowing. In this case, the smallest size DN8, i.e., 1/4", is commonly used, for example Threaded stainless steel filter 1/4"; L=65mm; PTFE.
Recommended flow speeds as a control indicator
If you are unsure whether the selected size corresponds to the actual flow rate, the following approximate ranges of water flow speeds in heating circuits are valid:
- 0.3-0.7 m/s - underfloor heating and low-temperature circuits (emphasis on low noise)
- 0.5-1.0 m/s - main radiator circuits in family homes
- 0.8-1.5 m/s - main circuits in apartment buildings and boiler rooms
- over 1.5-2 m/s - generally not recommended due to noise and erosion, or due to increased pressure loss and faster wear of the filter insert
If during the calculation you find that the selected size results in a speed exceeding these values, it is a signal that you should choose a larger pipe and filter size - not, on the contrary, to fit a smaller filter into a larger pipe using reductions, because this would create exactly the mentioned bottleneck with increased pressure loss.
Step-by-step procedure for selecting the correct size and length of the filter
In practice, we recommend proceeding systematically to avoid additional adjustments at the installation site:
- Determine or calculate the medium flow rate in the given section (from the heat source output or from the project documentation).
- Verify the actual pipe size by direct measurement at the site, ideally by placing it against an existing valve or fitting.
- Assign the size to the inch thread according to the conversion table (DN8=1/4", DN15=1/2", DN20=3/4", DN25=1" etc.).
- Check the available installation space - measure the distance between adjacent components (valve, pump, T-piece), subtracting the reserve for seals and possible fittings.
- Compare this distance with the construction length L of the specific filter in the catalog.
- Verify that there is enough space under the filter to remove the bottom closure and pull out the filter insert during future cleaning (at least as much as the length of the insert itself, usually 1.5-2x the filter diameter).
- Select the specific product with the correct thread, length and material finish (stainless steel, PTFE seal).
Specific dimensional advice from the BRA.10.000 series
The BRA.10.000 threaded filter series from the range of stainless steel and flanged fittings covers the common range of nominal diameters you encounter when designing and renovating household and smaller technological pipe systems. It features a stainless steel body with PTFE sealing, which ensures good corrosion resistance and long-term joint tightness even after repeated disassembly for cleaning (more on this topic can be found in the article "What does PTFE sealing mean and why is it important").
| Product | Thread | Nominal diameter DN | Length L |
|---|---|---|---|
| Stainless steel threaded filter 1/4" | 1/4" | DN8 | 65 mm |
| Stainless steel threaded filter 3/8" | 3/8" | DN10 | 65 mm |
| Stainless steel threaded filter 1/2" | 1/2" | DN15 | 65 mm |
| Stainless steel threaded filter 3/4" | 3/4" | DN20 | 80 mm |
| Stainless steel threaded filter 1" | 1" | DN25 | 90 mm |
When choosing between 1/4" and 3/8" (both with the same length of 65 mm), the decision is based solely on the connection diameter, not on spatial limitations – the length is identical, only the flow capacity differs. On the other hand, when moving from 1/2" to 3/4", an increase in length by 15 mm must be considered, and when moving to 1", an additional 10 mm. This is important to consider when ordering, especially in tight pipe systems in technical shafts or near walls, including connecting materials (inserts, reducers).
Common mistakes when selecting the nominal diameter and length of a filter in practice
Downsizing the filter due to "what was in stock"
A very common situation – a 1/2" filter is available in stock or in the store, but the pipe system is 3/4". It is installed with reducers to "make it fit". The result is a reduced flow cross-section, increased flow velocity at the filter location, higher pressure loss in the entire branch, and faster clogging of the mesh. In the case of repeated complaints about "frequent filter clogging", this is one of the first things to check on site.
Insufficient space for maintenance
A second typical mistake is installing the filter directly under the ceiling of a technical shaft or very close to the wall, so that during future cleaning, it is not possible to unscrew the bottom plug and remove the insert. The service technician then has to disassemble adjacent components as well, which prolongs the maintenance time and increases the risk of damaging seals during repeated disassembly of surrounding connections.
Mixing DN in a series connection of multiple circuits
When using distributors with multiple branches of different capacities, it sometimes happens that a single universal filter size is used for all branches "for the simplicity of ordering". However, with a larger performance spread between the branches (e.g., a main branch to the boiler 3/4" and a secondary branch to a TÜV tank only 1/2"), this leads either to unnecessarily oversized filters on weaker branches (higher costs, larger footprint), or conversely to undersized filters on the main branch.
Installation recommendations related to nominal diameter and length
When installing the filter into the pipe, we recommend following several principles that are directly related to the selected nominal diameter and length:
- Install the filter so that the flow direction corresponds to the arrow on the body of the fitting – in the wrong direction, the mesh does not effectively capture impurities and the bottom closure may be stressed by pressure in the opposite direction to what is structurally intended.
- Leave free space below the filter at least in the range of its length L in addition to the length needed to remove the insert (for DN20-25, allow a reserve of 100-150 mm downward or sideways according to the plug orientation).
- When transitioning between different nominal diameters (e.g., main pipe 1" and branch 1/2"), use standardized reducers before the filter, not after it, so that the filter always operates under flow parameters corresponding to its nominal diameter.
- The BRA.10.000 filter is installed in a horizontal position in both vertical and horizontal pipes with the plug pointing downward, which is also the orientation that simplifies future cleaning without the need to drain the entire system.
A detailed step-by-step installation procedure, including the recommended tightening torque and the use of sealing tapes, can be found in the separate article "Installation of the BRA.10.000 threaded filter into the pipe."
Frequently asked questions about selecting the nominal diameter and length of the filter
Must the filter have exactly the same nominal diameter as the pipe?
Yes, in the vast majority of cases. The filter should match the nominal diameter of the surrounding pipe to avoid local flow restriction with increased pressure loss and faster clogging. An exception are specific measuring or impulse lines, where a smaller diameter is intentionally used due to the low required flow.
What if I am missing a few centimeters for the filter's construction length on site?
A solution is either to choose a filter with a shorter construction length within the same nominal diameter (if the manufacturer offers multiple variants), or to adjust the length of the connecting inserts on both sides, or to move the nearest T-piece or elbow by the required distance. Shortening the filter body itself is not possible.
Can a smaller filter be used on a larger pipe with the help of reducers?
Technically yes, but it is not recommended in practice for main pipe systems, as the reducer creates a local flow restriction with increased velocity and pressure loss. In branches with significantly lower flow (e.g., to a separate measuring point), this is a common and acceptable practice.
How do I determine the correct nominal diameter if I do not have access to the project documentation?
The most reliable method is direct on-site measurement – measuring the external diameter of the existing thread or comparing it with an existing fitting of the same type. In case of greater uncertainty, an approximate flow calculation based on the heat source power according to the formula Q = P / (1.163 × ΔT) can also help.
Does the filter length vary by manufacturer for the same nominal diameter?
Yes, construction lengths are not universally standardized as strictly as threads, so when replacing a product from another manufacturer, it is always worth comparing the exact length L in the catalog sheet, not just relying on thread compatibility.
Does the filter's nominal diameter also affect its filtration efficiency?
Indirectly yes – when the correct nominal diameter is chosen according to the flow, the flow velocity through the mesh is in the optimal range and impurities are captured efficiently. In the case of an undersized nominal diameter (too high velocity), it may lead to the blowing through of finer particles through the mesh and a faster increase in pressure loss between cleanings.
Summary
Selecting the correct nominal diameter and length of the filter is not just a matter of "what thread was there before", but requires a short yet systematic approach – verifying the actual pipe diameter by direct measurement, assigning it to the standardized DN and inch thread, checking the flow in relation to the system's capacity, and finally verifying the available installation and maintenance space in relation to the construction length of the specific product. With the BRA.10.000 series, this process is simplified due to the clear range of dimensions from 1/4" to 1" with precisely defined lengths of 65, 80, and 90 mm, which cover most common and professional applications in heating practice. In case of any doubts about the selection of a specific fitting material, it is worth also reading the article "Stainless steel vs brass fittings – differences and material selection", which complements this topic from a material perspective.
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.