DN/OD 125
In the DN/OD 125 house drainage pipe system with 125 mm outer diameter, you will find over 110 HT pipes and fittings for your stack, collecting, and connecting lines. The standardized plastic components are connected via socket joints with rubber lip seals and are specifically designed for impact-resistant, chemical-resistant, and low-noise wastewater drainage inside buildings.
Buy DN/OD 125 — Guide & Tips
DN/OD 125 refers to pipes and fittings with a defined outer diameter of 125 millimeters. This nominal size is a central standard in the domestic drainage pipe system and is manufactured according to DIN EN 1451-1 manufactured. The components are made of high-temperature resistant plastic (HT), which was specifically developed for use in wastewater pipes inside buildings. The material is impact-resistant, chemical-resistant, and due to its insulation properties ensures quiet operation – a decisive advantage in multi-family houses or in the renovation of existing buildings.
The connection of the individual elements is done via a well-thought-out push-fit socket system with integrated rubber lip seal. This technique allows quick, tool-free assembly and at the same time ensures a permanently tight connection. Typical applications for DN/OD 125 are downpipes (vertical stacks) and Collecting pipes (horizontal main branches) in house drainage. Connections for toilets or multiple sanitary objects are also frequently made with this diameter, as it offers a high wastewater volume with a low risk of clogging.
When purchasing, pay attention to the exact designation of the individual parts. In addition to standard pipes in lengths from 250 mm to 3000 mm, there are numerous fittings. These include bends (usually 15°, 30°, 45° or 87°), branches (for diagonal or vertical connections) as well as reducers (transitions to smaller diameters such as DN/OD 110 or DN/OD 75). Before planning, check whether you will use exclusively DN/OD 125 or whether a connection to existing pipes of other nominal diameters is necessary – suitable reducers are available in the house drainage pipe system.
A common mistake is confusing HT pipes (indoor) with KG pipes (for underground). DN/OD 125 in HT version is intended exclusively for building drainage within the thermally insulated building envelope. When joining pipes, ensure clean cut edges: use a fine saw and slightly chamfer the inside of the socket to avoid damaging the seal. For sealing, the factory-installed rubber lip is usually sufficient – additional sealant is not required and can even be harmful.
Popular DN/OD 125 products
These three products cover the basic functions in the DN/OD 125 system. The HT pipe with push-fit socket forms the backbone of any installation – it is available in various lengths and can be extended indefinitely by simple push-fitting. The 67° branch is the preferred choice when you want to realize a lateral connection without significant height loss, for example for connecting a toilet stack to the main riser. The 87° branch with reduced outlet (110/75), on the other hand, is used where you need to connect smaller pipes from bathrooms or kitchens almost horizontally into the main line. Decide based on the arrangement of your sanitary fixtures and the available space in the installation shaft. For transitions between different diameters, you will find suitable reductions and sockets in the entire range.
Frequently asked questions about DN/OD 125
What is the difference between DN/OD 125 and DN 125?
DN/OD indicates the exact outer diameter of the pipe – for DN/OD 125 that is 125 millimeters. In older systems, only "DN" (nominal diameter) was often specified, which referred to the inner diameter. Since HT pipes are now standardized according to DIN EN 1451-1 and the push-fit sockets are matched to the outer diameter, the DN/OD designation is the more precise standard. If you measure an existing pipe and find an outer diameter of approx. 125 mm, you can safely use DN/OD 125 components. Note that no transition pieces are necessary, unless you encounter an older system with a different standard – in that case, please measure before purchasing.
Can I install DN/OD 125 outdoors?
HT pipes in DN/OD 125 are primarily designed for building drainage within the thermally insulated envelope and do not tolerate permanent sub-zero temperatures well. For buried pipelines outside the building, the KG system (according to DIN EN 1401) is the right choice, as it has higher ring stiffness and frost resistance. In exceptional cases, DN/OD 125 can also be used for short, thermally insulated outdoor sections if the pipe is protected from direct sunlight and frost. For the transition from the HT interior area to the KG exterior area, you need a special transition socket.
From what drop height do I need to plan a pipe ventilator?
For vertical downpipes in DN/OD 125, the use of a pipe ventilator (also called a vent valve) is recommended from a drop height of more than 3 meters. This prevents negative pressure that could compromise the tightness of the connections or empty the traps in the sanitary fixtures. For even higher downpipes (from approx. 8 meters), secondary ventilation or a ventilated downpipe system is mandatory. When planning, ensure that the pipe ventilator is installed at the highest point of the pipe and receives a cover to protect against small animals. For the exact requirements in your federal state, check the local drainage regulations.
How many sanitary objects can I connect to a DN/OD 125 pipe?
The maximum number of connections depends on the slope and length of the collecting pipe. Rule of thumb for practice: A horizontal collecting pipe in DN/OD 125 with a slope of 2 cm per meter can easily supply up to four toilets or a mix of six to eight sanitary objects (washbasin, shower, WC). For pure vertical stacks, the capacity is significantly higher – here, ventilation limits the maximum. Plan conservatively: Better one extra branch than an overloaded pipe that leads to odor problems or blockages. If you are unsure, refer to the drainage calculation tables of DIN 1986-100.
Further nominal diameters in the house drainage pipe system
The DN/OD 125 category is part of the overarching house drainage pipe system, which includes a total of nine different nominal diameters. Depending on the application and required drainage capacity, a different size may be the better choice. Here is an overview of the directly comparable diameters:
- DN/OD 110: The smaller standard diameter for connection pipes for washbasins, showers, bathtubs, and individual toilets. If your main pipe is already planned with DN/OD 125, you can connect the smaller pipes via a reducing branch (e.g., 125×110).
- DN/OD 160: The larger sister – often used for main collection lines in multi-family buildings, for long horizontal runs or for downpipes with many connected units. The transition from 125 to 160 is made using conical reducers.
- DN/OD 75: A slim solution for individual washbasins, urinals or as a reserve line for condensate from heating systems.
If you are renovating or building a single-family home with two bathrooms and a guest WC, DN/OD 125 is usually the right dimension for the main stack. DN/OD 110 can become tight if several fixtures flush at the same time. DN/OD 160 is usually oversized and causes higher material costs as well as space problems in the shaft. For smaller secondary connections, use the diameters DN/OD 50 or DN/OD 40When combining, always ensure matching branches and reductions – all parts in the system are compatible with each other as long as the nominal diameters match.
In short
DN/OD 125 is the reliable standard for main wastewater pipes inside buildings. With an outside diameter of 125 mm and manufactured according to DIN EN 1451-1, you connect pipes and fittings without tools via push-fit sockets with rubber lip seal. The HT material is impact-resistant, chemical-resistant and low-noise – ideal for soil stacks and collecting lines in single and multi-family houses. Choose DN/OD 125 for the main line and the smaller diameters DN/OD 110 or DN/OD 75 for secondary connections. Check your gradient and ventilation requirements before purchase to ensure a permanently functioning drainage system. With the wide selection of bends, branches and reductions from the overarching house drain pipe system you have all the components for your sanitary renovation or new construction at hand.