How to choose the right HT pipes and fittings for the drainage system
How to choose the right HT pipes and fittings for the drainage system
The waste pipe is one of those parts of the home installation that is rarely considered — unless it works, no one notices it. However, when it stops working, the problem appears quickly and fully: bad odors, leaking joints, stagnant water, or even a real disaster with flooding. Most of these problems arise during the selection and installation — either the wrong diameter is chosen, the wrong type of fitting is used, or the basic rules of slope and laying are ignored. This article will help you navigate the HT system from the basics to the specific details, so that for every job — whether it's a new build, a bathroom renovation, or an extension of a waste branch — you know what and why to choose.
What is the HT system and why it is the standard for internal waste pipes
HT (from the German Haus-Technik) is a system of plastic waste pipes intended for internal installations — that is, for pipes running in walls, ceilings, installation shafts, and behind plaster within the building. It is made of polypropylene material (PP), which is resistant to common waste waters including slightly aggressive chemicals from household cleaning products, and can withstand waste water temperatures up to 95 °C for short-term exposure.
Compared to the older cast iron piping, the HT system is lightweight, quick to install, does not require special tools, and joints are solved simply by inserting into a socket joint with a rubber seal — without gluing, welding, or soldering. This is also the biggest difference compared to the KG system (Grundrohr sewerage), which is intended for external placement in the ground. If you are interested in when to use HT and when to use KG, I recommend looking at the topic HT versus KG system: which waste system to choose and why, where this comparison is detailed.
HT pipes and fittings are produced in several standard diameters: DN 32, DN 40, DN 50, DN 70, DN 75, DN 90, DN 100, and DN 110. Each diameter has its typical application, and in practice it is important not to mix them without a reducer.
How to correctly determine the required pipe diameter — practical guide
The most common mistake made by beginning installers and DIY plumbers is choosing the diameter "by eye" or based on what is currently in stock. The diameter of the waste pipe must be based on the fixtures that will be connected to it and on the overall hydraulic load of the branch.
The basic rule is simple: each fixture has an assigned so-called drainage device (OZ) in the design standard STN EN 12056, according to which the overall capacity of the branch is assessed. For a standard apartment installation, however, it is sufficient to know the following practical values:
- DN 32 — basin (one), bidet. This is the minimum diameter for any fixture. Never use a smaller profile regardless of apparent space savings.
- DN 40 — shower tray, bath, kitchen sink (including a waste hose from the washing machine if it is led into the sink). In practice, DN 40 is the most universal diameter for horizontal branches.
- DN 50 — floor drain in the bathroom, overflow, washing machine (if it has its own branch and is not connected to the sink), dishwasher. Some plumbers use DN 50 for a shower tray in the case of a walk-in shower with a higher flow rate.
- DN 75 — auxiliary vertical waste pipe (shaft) for one floor with multiple fixtures excluding WC, or a branch from the washing machine and sink for a longer run.
- DN 110 — toilet bowl (WC) is mandatory, main waste pipe collecting waste from the entire bathroom including WC, vertical run for 2 or more floors.
More about specific diameters for individual fixtures can be found in the topic Which HT pipe diameters do I need for WC, basin and shower, where these values are also supplemented by length limits for connecting branches.
Types of HT fittings and when to use which
The HT pipes themselves form the backbone of the system, but without the correct fittings, you would not be able to route the waste pipe or connect it. Each fitting has a specific purpose, and its substitution can lead to unnecessary pressure on the joints or hydraulic problems.
HT elbow — change of flow direction
HT elbow is used to change the direction of the pipe. It is produced in various angles: 15°, 30°, 45°, 67°, and 87° (which corresponds to a nominal 90°, since in real hydraulics it is referred to as 87°). This distinction is very important in practice:
- 87° (nominal 90°) — used exclusively for vertical sections, for example when transitioning from a horizontal ceiling pipe to a vertical waste shaft, or when connecting a WC. On horizontal sections, this angle causes swirling flow and a risk of clogging.
- 45° — the most practical choice for most bends in horizontal runs. A pair of 45° elbows replaces one 90° elbow with much lower hydraulic losses.
- 30° and 15° — gentle correction of direction, used for compensating slope or bypassing construction obstacles.
From practice, I know that during renovations in panel buildings, people often find a 90° elbow in the existing pipe in a horizontal section behind the WC. This is a remnant of the old cast iron installation, where the flow speed was different. In a new HT system, never do this – always use a 45° elbow or two 45° elbows in a row with a gap in between.
HT reducer – transition between primers
HT reducer is used to connect a smaller diameter to a larger one. In practice, you encounter two situations:
Eccentric reducer – the bottom of the pipe remains at the same height level. This is the correct choice for horizontal sections, as it prevents the formation of a puddle at the narrowing point, where solid waste could settle.
Concentric reducer – the axis is centered. This is used exclusively in vertical sections.
For example: if you connect a branch from a DN 40 sink to a main DN 110 waste pipe, you don't necessarily need a direct 40/110 reducer – it's sufficient to use an HT branch with the appropriate necks. Use a reducer when changing the diameter on a straight section – typically when transitioning from DN 50 to DN 75 before connecting to a vertical pipe, or when adapting an existing pipe to a new device.
HT cleaning piece – a revision element that must not be missing
HT cleaning piece (clog cleaning fitting) is a fitting with a closable opening through which a cleaning spiral or a water jet can be inserted for mechanical removal of clogs. It is an element that is most often forgotten during planning and installation – and then, during the first problem, wall or ceiling demolition follows to gain access to the pipe.
The cleaning piece is mandatory according to STN 73 6760 on every straight horizontal pipe section longer than 10–15 meters, on every 87° elbow in a horizontal section (which you should otherwise not use, but which appear in existing installations), and ideally also at every embedded pipe in concrete or behind drywall. More about its correct use can be found in the topic How to use an HT cleaning piece and clog cleaning fitting during pipe revision.
HT plug – closures and temporary securing
HT plug is used for temporary or permanent closure of an unused pipe neck. Typical use: during installation, a branch is made for a future device (e.g., a washing machine) that is not yet installed – the plug keeps the neck closed until the time of connection comes. Another use is during pressure testing of the entire system, where all free ends are sealed with plugs. Also important is the permanent closure of pipe necks on branches for devices that have been decided to skip – for example, the project originally planned for a washing machine in the bathroom, but it ended up in the utility room.
Material, wall thickness and stiffness classes – what to know before purchasing
All HT pipes and fittings are made of SN 2 polypropylene polymer according to EN 1451. The difference between cheaper and higher quality products is not only in price – it is reflected in dimensional accuracy, quality of seals, and especially in wall tolerances. Thin-walled pipes from unverified sources have a greater diameter variation, and the socket joint then does not seal reliably, or the seal deforms when inserted.
Wall thickness is standardized for the HT system and depends on the diameter:
| Diameter DN | Outer diameter (mm) | Min. wall thickness (mm) |
|---|---|---|
| DN 32 | 32 | 1.8 |
| DN 40 | 40 | 1.8 |
| DN 50 | 50 | 1.8 |
| DN 75 | 75 | 1.9 |
| DN 110 | 110 | 2.7 |
| DN 125 | 125 | 3.1 |
It is important to note that the HT system is not intended for burial in the ground. For this purpose, the KG system with a thicker wall and greater resistance to soil loading is used. Therefore, if your pipe runs from the interior into the ground, you must transition the system at the point of crossing through the foundation slab or wall to KG – typically via an HT 45° elbow and an HT/KG reducer.
Slope of the waste pipe – a critical parameter often underestimated
A gravity-fed waste pipe functions properly only if it has a way for water to drain – and this is precisely controlled by the slope. Too small a slope causes stagnant water and sedimentation of solid matter. Too large a slope may seem better at first, but water drains too quickly and solid waste remains in the pipe – the so-called "self-cleaning effect" of the pipe only works within the correct slope range.
Recommended slopes according to the standard STN EN 12056-2:
- DN 32 – DN 50: slope 2–3 % (2–3 cm per meter of length)
- DN 70 – DN 100: slope 1.5–2 %
- DN 110 and above: slope 1–2 %
From practice: during renovations in panel buildings, I often encounter cases where the previous "plumber" installed the sink slope at only 0.5 % – the pipe was almost flat. After half a year, the customer had chronic issues with bad odors and slow drainage. It was enough to extend the pipe by 5 cm and the problem was solved without replacing the entire system. The topic of slope is discussed in more detail in the article Slope of HT waste pipe: what angle is correct and why it matters.
Installation of the HT system – basic principles before starting the work
HT pipes are connected by insertion – the smooth end of the pipe or fitting (the smooth part without a socket) is inserted into the socket of the adjacent component, where a rubber gasket is located. Before inserting, it is necessary to apply installation lubricant to the smooth end of the pipe – never use oil, vaseline or other unsuitable lubricants that attack rubber. Manufacturers supply certified lubricant directly with the system.
Important rule: the smooth end of the pipe is cut straight (not at an angle) and the plastic connector is chamfered so that the edge is straight and does not interfere with the gasket during insertion. When cutting the pipe with a pipe cutter, always remove sharp edges with fine sandpaper or a plastic trimmer.
Another critical detail: the smooth end of the pipe is not inserted all the way to the bottom of the socket – a thermal expansion gap of 10–15 mm must be left (the temperature of the waste water from the washing machine can be 90 °C, which causes linear thermal expansion). Without this gap, hot waste water can pull the joint apart or bend the pipe. A detailed installation procedure is available in the topic Installation of HT waste pipe step by step.
Pipes running in walls and ceilings must be supported by clamps at specified intervals:
- DN 32 – 50: every 0.5 m
- DN 75 – 100: every 0.75 – 1.0 m
- DN 110 and above: every 1.0 – 1.25 m
For vertical pipes, fixed clamps are recommended on each floor and sliding (free) clamps in between, so the pipe can freely expand or contract along its length.
Acoustic properties and sound insulation of HT pipe
A standard HT system is cost-effective, but it has one property that is often underestimated in standard design documents: it is relatively noisy. The flow of water through a thin-walled polypropylene pipe is audible in adjacent rooms, especially during nighttime toilet flushing above a bedroom or when the waste pipe runs through a living room.
Solutions include the following:
- Acoustic HT pipes – some manufacturers offer a special version with a mineral layer inside the pipe wall (typically labeled as HT-safe or similar). These are significantly heavier, more expensive, but acoustically much better.
- Mineral wool or acoustic bands – wrapping standard HT pipe with mineral wool and securing it with adhesive acoustic film is a cheaper alternative, but it requires consistent work for every meter of pipe.
- Installation shaft with acoustic insulation – if the pipe runs through an installation shaft, filling the shaft with acoustic drywall (e.g., Knauf SILENT or Rigips Activ'Air) and mineral wool significantly reduces sound transmission into adjacent rooms.
From practice, I know that in renovations of row houses or apartment buildings, where a bathroom of a neighbor is located above a bedroom, the acoustics of the pipe can sometimes be a bigger issue than the tightness of the joints. Don’t address it as an afterthought – it is worth considering the acoustics when selecting the pipe even at the stage of material purchase.
How to combine HT with existing installation – renovation vs. new construction
In new construction, you have complete freedom – design the pipe route ideally, with a minimum of elbows, the correct slope, and a sufficient number of inspection fittings. In renovations, it is not that simple.
Connecting HT to existing cast iron pipe is a common situation in older buildings. You need an adapter – a transition piece with one side socket for the HT pipe and the other side a mechanical connection (usually threaded or with a rubber gasket) for the cast iron collector. These parts are not part of the standard HT range, but are sold as accessories.
Connecting to existing PVC pipe is easier – the outer diameter of PP pipes and old PVC pipes is usually the same, so fittings are often compatible, but always check the actual outer diameter and type of sealing. Some older PVC pipes have slightly different outer diameters, which complicates the connection.
Adjusting the length to the existing route – HT pipes are available in lengths of 150 mm, 250 mm, 500 mm, 1000 mm, 1500 mm, 2000 mm, 3000 mm, and sometimes 5000 mm. Shorter pieces are suitable for tight installations (behind drywall, in installation cores of panel apartments). When purchasing, always consider not only the total length of the route, but also how you will cut the pipes and what actual lengths you really need – unnecessary joints unnecessarily complicate the installation and increase the number of potential leak points.
Most common mistakes when selecting and installing the HT system
Over the years working in this field, I have seen dozens of projects where problems did not arise from poor quality materials, but from bad decisions during selection or installation. Here are the most common ones:
- Connecting a WC to DN 75 or DN 50 – a toilet bowl must always be connected to DN 110, without exception. DN 75 is not sufficient to carry away solid waste and the risk of clogging is very high.
- Omitting the venting of the waste pipe – without a vent pipe (an extension of the vertical waste pipe above the roof, or an air admittance valve), a vacuum occurs during flushing, which sucks water from the traps and allows foul sewer smells to enter the space.
- Horizontal pipe without slope – described above. Without slope, water stands still and waste accumulates.
- Too long connecting pipe without slope – a sink located 3 m from the vertical stack with DN 32 without a 2 % slope is a problem. The limit for a connecting pipe DN 32 without special modifications is 1.0–1.5 m, for DN 40 up to 3 m.
- Gross mistakes in selecting the elbow – a 90° elbow on a horizontal section instead of two 45° elbows. This is a classic example that leads to clogging within 1–2 years.
- No inspection fittings (cleaning pieces) – in case of the first clogging, you will have to break the wall. Always plan access to each longer section and to each more complex route.
- Forgetting the expansion gap – with hot water (washing machine, dishwasher), the pipe experiences significant elongation. Without a gap, the joints deform or tear apart.
You will read more about repairs caused by these mistakes in the topic Common faults in HT waste systems and how to fix them.
Practical examples – real customer projects and their solutions
Bathroom renovation in a panel apartment (3rd floor): The customer wanted to move the shower area 1.5 m further from the original location, which extended the horizontal branch DN 50. The original core of cast iron pipes remained. Solution: new HT route DN 50 routed under the ceiling with a constant slope of 2 %, connected via an adapter to the existing cast iron branch in the shaft, cleaning piece installed in the ceiling behind access doors. The customer initially did not want the doors, but after explanation, he eventually approved them – which was the correct decision.
New single-family house – waste distribution in the technical room: The designer planned all waste branches DN 110, even for the sink in the laundry room. During consultation, we reduced the DN to 40 for the sink and DN 50 for the floor drain, which saved on materials and, more importantly, smaller diameters can be routed much easier between ceiling joists without the need to cut into concrete. The main collection pipe DN 110 remained and everything was connected to it via proper branches.
Garden house – part of the waste pipe outside: The customer wanted to route the HT pipe along the outside wall of the garage (not underground). This is not ideal – HT is not designed for UV exposure and for freezing below –10 °C. Solution: the pipe routed in a protective plastic channel with insulation, transition through the wall to a KG system before entering the ground. Alternatively, it would have been better to route the pipe inside, which was eventually implemented.
How to buy HT material efficiently – what to prepare before ordering
Before placing an order, prepare a simple sketch of the route (even a hand-drawn one) with marked fittings, lengths of sections and places of direction change. From this sketch you can calculate:
- Total length of pipes for each diameter (round up and add 10 % as a reserve for cutting errors)
- Number and angles of elbows at each direction change
- Number of branches, their diameters and angles
- Number of reductions when transitioning between diameters
- Number of cleaning pieces (at least one for each branch longer than 5 m)
- Number of caps for unused outlets
- Clamps – for each diameter at the appropriate intervals
A common mistake is buying pipes and forgetting the fittings, or vice versa. A good approach is to make a complete list of materials, where each piece of the system is listed. For larger projects, it pays off to consult the list with a specialist or salesperson before ordering.
Most frequently asked questions (FAQ)
What is the difference between grey and black HT pipe?
The color of HT pipes and fittings is not technically significant. Grey color is traditional for waste HT pipes, black is common for sound-insulated variants (so-called silent/ acoustic pipes), but some manufacturers also produce black pipes as standard. Always follow the technical specification and not the color – the decisive factors are the material (PP), nominal diameter, wall thickness and compliance with the EN 1451 standard.
Can I connect a washing machine directly to HT DN 40 or must I use DN 50?
A washing machine can be connected to DN 40 if it is connected via a waste hose to the kitchen sink (i.e., not via a separate branch). If the washing machine has its own branch and its own waste branch, DN 50 is recommended, because when the washing machine is draining, a large volume of water is released at once and DN 40 may be at the limit of capacity for longer runs. For connecting pipes longer than 1.5 m, always choose DN 50.
Can I route HT pipes in a concrete floor screed?
HT piping is not primarily intended for embedding in concrete or in the ground, but short sections (e.g., wall or floor through-passage) are acceptable if the pipe is in a protective sleeve or insulated so that it can freely expand. For longer installation in the floor or in the ground, the correct choice is a KG system made of PVC with a thicker wall and greater resistance to mechanical load.
How long does HT piping last and when is it time to replace it?
Polypropylene HT piping has a service life of 50 years or more with proper installation and normal use. Problems arise more often from incorrect installation (too aggressive chemicals, mechanical damage, UV exposure when routed outside) or from neglected maintenance. Replacement is justified if cracks appear, chronic leaks occur in the joints that cannot be repaired, or if the pipe is physically deformed. More on preventive care can be found in the topic Cleaning and maintenance of HT piping: how to avoid clogging.
Do I have to replace the entire HT system during reconstruction, or is it enough to replace only the damaged section?
During reconstruction, it is not necessary to replace the entire piping if the rest is in good technical condition. You can replace only the damaged section with new HT piping and connect it to the existing piping using adapter fittings. It is important to check the diameters of the existing piping, the type of material (PP, PVC, cast iron) and use the correct transitions. If the existing system is more than 30–40 years old and has multiple problem areas, it usually pays off to replace the entire route to avoid repeated interventions.
What to do if the gasket pulls out of the socket during installation?
The rubber gasket is inserted – it usually sits in a groove inside the socket and can be removed and reinserted. If the gasket is damaged or missing, do not try to seal the joint with silicone or other sealant – this is only a temporary patch that will fail with hot water or pipe movement. The correct procedure is to obtain a replacement gasket for the corresponding diameter and socket, or to replace the entire fitting. Proper joining of HT pipes, elbows and branches without leakage is also covered in the topic How to properly connect HT pipes, elbows and branches without leakage.
Conclusion – good preparation saves time, money and nerves
Selecting the right HT pipes and fittings is not complicated if you know what to focus on. Three key things are important: the correct diameter according to the fixtures, the correct geometry of the route (slope, elbow angles) and a complete set of fittings including inspection elements. Underestimating any of these points leads to problems that will appear either immediately after commissioning or during the first few years.
The product range in the category waste system HT covers all common diameters and types of fittings – from basic HT pipes through elbows and reductions to cleaning pieces and plugs. If you are unsure about the selection, the simplest way is to make a sketch of the installation with specific dimensions and consult it before ordering – this will save you unnecessary reorders and returns of unsuitable materials.
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