What DN diameter do I need for a backflow valve?
What DN size do I need for a backflow valve?
This is probably the first question every homeowner asks when deciding to protect their property from reverse sewage flow. And it's the right question – because if you buy a valve with the wrong diameter, you won't be able to simply test it and return it. You'll have to remove it, possibly redo the entire manhole, add adapters, and waste time and money. This article will guide you through the entire decision-making process: from what DN actually means, through the type of piping you have in your home, to specific recommendations based on the type of installation and number of devices.
What does DN mean and how is it related to the physical dimensions of the pipe
The abbreviation DN comes from the French Diamètre Nominal – that is, the nominal diameter. In practice, it is a standardized designation that does not exactly match the internal or external diameter of the pipe, but is a reference number used to design and manufacture fittings, bends, traps, and precisely backflow valves. For standard PVC and PE sewage pipes, the DN marking is numerically close to the outer diameter of the pipe (in millimeters), but is not identical to it.
For example: a pipe marked DN 110 has an outer diameter of usually 110 mm, an internal diameter of about 103–104 mm (depending on the wall thickness and stiffness class). A DN 125 pipe has an outer diameter of 125 mm. A DN 160 pipe has an outer diameter of 160 mm and DN 200 has an outer diameter of 200 mm. When purchasing a backflow valve, you must always base your choice on the DN marking of your pipe, not on a manually measured diameter.
So when you measure the pipe in the manhole or in the basement, always measure the outer diameter and compare it with the tabular values for the respective DN. If your measurement falls between two values, try to identify the manufacturer and series of the pipe – old stone or concrete pipes may have different dimensions than modern PVC ones.
Most commonly used diameters in family homes
In practice, we encounter a few "favorite" diameters in home installations. Here is an overview of where they commonly appear:
- DN 50: waste from the sink, shower, bidet. Almost always connected to a larger drain, and a DN 50 backflow valve is not common in practice – it is usually addressed at a higher level.
- DN 75: waste from the washing machine, dishwasher, sometimes from the shower. Again, usually connected to a larger drain.
- DN 110: the most common diameter in family homes – waste from the WC (toilet), main drain of ground floor bathrooms, connecting drain from the basement. This is the diameter you are interested in in 70 % of cases in a family home.
- DN 125: less common, but occurs – larger drains, connecting pipes for multiple toilets, some older installations. It is important not to confuse it with DN 110 – the 15 mm difference is enough to prevent the valve from fitting tightly.
- DN 160: main house drain, where several toilets, bathrooms, and kitchens are connected. Or the main connection before the house.
- DN 200: large drains, connections for apartment buildings, larger properties, sometimes also the main municipal sewer when connecting to the house.
Before you buy anything, physically determine the diameter of the pipe at the location where you want to install the valve. Do not estimate. Take a tape measure, measure the outer diameter, or look at the house's project documentation (if you have it). In case of doubts, it is worth opening the manhole and looking directly at the pipe – or calling a plumber who can confirm it within 10 minutes.
How to determine the pipe diameter in your home – step by step
In practice, I have repeatedly encountered customers calling to say that "the valve doesn't fit". After a short conversation, it always turned out to be the same – they had measured the diameter only by eye, or had confused DN 110 with DN 125 or old stone pipe with new PVC. Measuring takes literally two minutes and will save you unnecessary complaints and service calls.
DN 110 – the most common choice for single-family homes
The vast majority of single-family homes built after 1980 have a main drainage line from the basement to the sewer with a diameter of DN 110. This pipe can handle waste from a standard WC, bathroom, kitchen, and washing machine, provided they are connected centrally. For such a house, the logical choice is therefore the backwater valve for DN 110.
The through-type valve is installed into the existing pipe by cutting the pipe and inserting the valve between the two sections. It does not change the diameter and operates on the principle of a flap (floating disc), which freely drops and opens the path during normal wastewater flow from the house to the sewer. When water starts flowing back from the network, the flap is lifted by the water and seals the opening. Result: wastewater from the public sewer will not enter your basement, storage room, or bathroom.
Typical scenarios where DN 110 is the correct choice:
- A single-family home with one WC, bathroom, and kitchen – the entire drainage system is DN 110.
- A house where the fixtures are connected to a single shared vertical pipe with a DN 110 diameter.
- A new construction where the designer specified DN 110 as the main drainage line (typically for homes with up to ~6 people).
- Renovation of an older house where the drainage system was upgraded to PVC DN 110.
DN 125 – when you need it and how to recognize it
DN 125 is a somewhat problematic diameter – customers often confuse it with DN 110, as the visual difference is not dramatic. Despite this, it is a different size and a DN 110 valve will not fit properly into a DN 125 pipe, will not seal, and may remain partially open.
Where DN 125 appears most often:
- Older homes from the 1960s to 1980s with stone or PVC piping according to the standards of that time.
- Installations where DN 125 was used as a transition line between DN 110 (vertical pipe) and DN 160 (outlet from the house).
- Certain apartment buildings where the drainage line from the apartment unit has DN 125.
- Garden or industrial buildings with higher wastewater flow.
For these cases, the backwater valve for DN 125 is suitable. If you are unsure whether you have DN 110 or DN 125, measure the outer diameter with a tape measure. If the measured value is around 110 mm, it is DN 110. If it is around 125 mm, it is DN 125. Simple, but precise measurement is essential.
DN 200 – protection for apartment buildings and larger properties
Diameter DN 200 is the standard for main drainage lines in apartment buildings, larger single-family homes (multi-generational, large villa-type), and residential projects. It is a pipe that can handle waste from multiple apartments or multiple bathrooms and WCs simultaneously. In the case of backpressure in the sewer – for example, during heavy rain or a blockage in the public sewer – this is a serious risk: backflow through a DN 200 pipe can quickly flood the entire basement and cause massive damage.
For such installations, the backwater valve for DN 200 is intended. With this diameter, it is essential to pay special attention to the correct placement of the valve and access for maintenance – cleaning a DN 200 valve requires a sufficiently large manhole with an appropriate cover.
If you have DN 200 and are installing the valve into a manhole, also consider the size of the cover. For manholes with larger diameters, a manhole cover 400 is commonly used, which provides sufficient access and handling space. For smaller manholes with DN 110 or DN 125, a manhole cover 315 is usually sufficient.
Comparison table: Which DN for which type of property?
| Diameter DN | Typical property / use | Number of fixtures | Note |
|---|---|---|---|
| DN 110 | Single-family home (1–2 bathrooms, 1–2 WCs) | up to 4–6 fixtures | Most common choice, for new constructions and renovations |
| DN 125 | Older homes, transition lines, some apartments | similar to DN 110 | Accurate measurement is necessary – easily confused with DN 110 |
| DN 160 | Larger single-family home, 3+ bathrooms, main drainage lines | 6–12+ fixtures | Main waste pipe from the house, before entering the sewer |
| DN 200 | Apartment building, multi-functional building, large villa | 12+ fixtures, multiple apartments | Larger manhole, regular service required, cover 400 |
Practical examples from the field: What customers most often address
Case 1: Single-family home with a flooded basement after rain
The customer had a two-story single-family home built in 1995 with a bathroom and washing machine in the basement. After heavy rain, the basement was repeatedly flooded. The main drainage line was DN 110, PVC. Solution: a through-type valve DN 110 installed in the manhole in front of the house, together with a revision cover. Since then, no problems – and the cost of the valve and installation was a fraction of what it would have cost to pump and disinfect the basement.
Case 2: Apartment building, incorrectly identified diameter
The building manager ordered a DN 150 valve (an industrial size, but not common for residential sewage). After consultation, it was found that the main drainage line had DN 160, not DN 150. The manager had only measured the inner diameter and miscalculated the wall thickness. The correct valve for this building was DN 160. Lesson: always measure the outer diameter and compare it with the table of standard DN values.
Case 3: Older home with stone piping
A house from the 1950s, with stone drainage. The outer diameter measured at 130 mm – did not exactly match any standard PVC DN. In this case, a transition from the old diameter to the new standard had to be made, and then a through-type valve DN 125 with corresponding PVC fittings was installed. Important: never force a valve into a pipe for which it is not intended – leakage is guaranteed and backflow is likely to occur exactly when you need it least.
Case 4: New construction, where the designer forgot the valve
It happens. Houses are built, drainage is laid without a valve (not required in all cases), and the owner deals with it only after the first problem. If the house is in an area with a risk of backflow (low-lying construction, old sewer system in the town), the valve is essential. Installing it later into an existing pipe – dig up, cut the pipe, insert the through-type valve, backfill – is feasible, but more expensive than doing it right from the start. The topic of valve obligations is discussed in more detail in the article Protecting your home from sewer backflow – when is a valve required?
Schematic: Where in the system the flap is installed and what diameter is typical there
From the diagram it is clear that the diameter of the flap must match the diameter of the main drain at the point of installation. The flap is typically installed just before the house outlet or in a inspection chamber, maintaining the same pipe diameter. If there is a manhole behind the house, it is an ideal location – it is accessible, can be inspected, and in an emergency can also be manually closed (if the flap is equipped with a manual closure).
What if the diameters differ on different sections?
In older houses or during renovations, it can happen that the pipe diameter changes – for example, the vertical pipe is DN 110, but before entering the public sewer it is DN 160. In such cases, a simple rule applies: install the flap in the pipe with the diameter that corresponds to the pipe at the installation location. Most often, this is the location in the manhole, before entering the public network.
If you have a transition from DN 110 to DN 160 on one section, the flap should be installed on the section with the larger diameter (DN 160), because the risk of reverse flow is more real there, and the flap must be able to seal the entire cross-section. Installing a DN 110 flap before the transition to DN 160 would work, but in the case of reverse flow, the pressure would act on a larger area behind the flap and could stress it in a non-standard way.
About the differences between types of flaps (intermediate vs. end) you will read in the article Intermediate vs. End Drain Flap: What is the difference and where to use each, where it is also discussed which variant is more suitable for which type of installation.
Impact of diameter on flow and proper flap function
A larger pipe diameter logically means lower flow resistance – water flows more easily. On the other hand, a flap with a larger diameter must seal a larger area during reverse flow and must withstand greater pressure. Therefore, flaps for larger diameters (DN 160, DN 200) are usually more robust, with a thicker body and a sturdier lever. When the flap is properly designed and installed, this does not represent a problem – the flap functions reliably under all normal operating conditions. Problems arise when the flap is undersized (too small a diameter) or oversized (too large a diameter) – in both cases, it does not fit the pipe correctly.
What if you need a flap for DN 160?
Flaps for DN 160 are very common for the main drains of larger family houses. Although in the current portfolio of atria.sk, intermediate flaps DN 110, DN 125 and DN 200 are available, the range is continuously expanding. In case you are looking for a specific size, we recommend checking the current offer in the category PE fittings, drainage, flaps or contacting us – we can advise you on an alternative or arrange a special order. A more detailed comparison of DN 110, DN 160 and DN 200 is covered in the article Backflow flap DN 110 vs. DN 160 vs. DN 200 – comparison of use.
Important: Do not confuse DN with the outer diameter of PE pipes for drainage
One nuance that should be noted: in the category of PE fittings and drainage, in addition to sewer pipes, there are also drainage pipes, where the labeling system may be slightly different. A drainage pipe DN 100 has an outer diameter of 110 mm – which can be confusing. If you are dealing with a sewer, always refer exclusively to the labeling of sewer pipes (PVC-U or PE sewer), not to drainage pipes. These two groups are interchangeable only in some parts of the system and you should never install a sewer flap intended for a different type of pipe.
Correct manhole size for the selected flap
When you choose the correct DN, don't forget where you physically place the flap. A through flap is typically installed in a manhole. The manhole must be spacious enough to allow the flap to be inserted and to remain accessible for maintenance later.
For a flap DN 110 or DN 125, a standard manhole with a diameter of 315 mm – i.e., a manhole cover 315 – is used. For flaps DN 160 or DN 200, a manhole with a diameter of 400 mm with manhole cover 400 is more suitable, which provides a sufficient opening for handling a larger flap and allows for comfortable inspection and cleaning. Choosing the correct manhole cover is the subject of a separate article: Manhole cover 315 or 400 – how to choose the correct size?
From experience, I know that underestimating access to the flap is the second most common problem (right after the wrong diameter). The flap is buried, poured with concrete, and no inspection opening is left – and in two years, when inspection or cleaning is needed, they have to dig. A proper manhole with an easily removable cover is an investment that pays off.
FAQ – Frequently asked questions about selecting the diameter of a backwater flap
How do I find out what DN my pipe has if I don't have the project documentation?
The most reliable method is to physically measure the outer diameter of the pipe using a caliper or a tape measure. Compare the measured outer diameter in millimeters with the tabular values: DN 110 = 110 mm, DN 125 = 125 mm, DN 160 = 160 mm, DN 200 = 200 mm. For pipes with non-standard diameters (e.g., old concrete or stone), consult with a plumber.
Can I install a DN 110 flap into a DN 125 pipe?
No. A DN 110 flap has a smaller outer dimension than the inner diameter of a DN 125 pipe. It will not fit properly, will not seal, and the backflow may bypass it. You must always use a flap with the same DN as the pipe.
Is one flap enough for the whole house, or do I need more?
In the vast majority of single-family homes, one flap on the main drain before the connection to the public sewer system is sufficient. An exception is when the house has multiple independent waste branches with separate outlets (e.g., a garage with a separate drain) – in such cases, you may need two flaps. In most cases, one properly placed flap is fully sufficient.
What is a through flap and does it differ in diameter from an end flap?
A through flap is installed within the flow of the pipe – it has a neck on both sides and is inserted between two pipe sections. An end flap is mounted at the end of the pipe, for example, at the outlet of a pump. As far as diameter is concerned, both types are marked the same way according to the DN system and must match the diameter of the pipe into which they are installed. A more detailed comparison can be found in the article How to choose the right backwater flap – diameter, type and placement.
Why does a flap of the correct size not seal? Could it be a problem with the diameter?
If the flap is of the correct DN but does not seal, the problem usually does not lie in the diameter, but in debris (paper remnants, solid particles) that prevent the flap from seating properly, or in a damaged gasket. Occasionally, a flap of the correct DN may have slightly different tolerances from a specific pipe – this is solved by a high-quality gasket on the flap body. If you suspect a leak, read the article Common backwater flap problems – why the flap leaks or gets stuck.
Is DN 110 suitable for connecting a washing machine and a dishwasher?
A washing machine and a dishwasher have their own drain hose, typically with a diameter of DN 50–75, which is connected to a trap or a larger drain. A DN 110 flap is installed on the main drain, not directly on the machine's drain. Since the water from the washing machine flows through the main drain, the DN 110 flap on the main drain protects the entire system, including the waste from the washing machine. It is therefore not necessary to install a flap on each individual hose separately.
Conclusion: DN diameter is the foundation – but not the only factor that matters
The correct diameter of a backwater flap is a necessary condition for the proper protection of your home. Without the correct DN, the flap will not function – it will either not seal or will not fit. But the diameter alone is not enough. You also need to consider the type of flap (through vs. end), the installation location (manhole, basement, interior), the size and accessibility of the manhole, the flap material, and regular maintenance. All these topics are covered in more detail in the articles of the Knowledge Center – for example, Step-by-step installation of a backwater flap or Maintenance and cleaning of a backwater flap.
If you are sure about the diameter and are looking for a specific product, check our selection directly in the category: flap DN 110, flap DN 125 or flap DN 200 – all are available in stock and can be delivered quickly.
Do you have a question about this topic?
Not sure how to proceed or dealing with a specific situation in your home? Write to us – we are happy to help.