HT versus KG system: which waste system to choose and why
HT versus KG system: which waste system to choose and why
When dealing with waste installation in a house or apartment, you will sooner or later come across two basic families of plastic pipe systems: HT and KG. At first glance, they look similar – these are plastic pipes and fittings in shades of grey – but their use, construction and technical parameters are fundamentally different. Confusing them is one of the most common mistakes in DIY renovations, and correcting such a mistake can be quite expensive. In this article, we will examine both systems in depth: where they are used, why they differ, how to distinguish them, and in which situations you should choose HT and in which KG.
What is the HT system and where can you find it
HT is an abbreviation from the German Hochtemperatur – meaning a high-temperature system. It is a plastic waste pipe system primarily intended for internal vertical and horizontal horizontal runs in buildings: risers, waste lines from toilets, sinks, showers, bathtubs, kitchen sinks and washing machines. The pipes and fittings are made of PP (polypropylene) or ABS plastic, typically in grey or anthracite colour with characteristic socket joints with a sealing ring.
The key property of the HT system is its resistance to hot water up to 95 °C in short-term peaks, and it can handle temperatures around 60–70 °C in the long term. This makes it suitable for waste from dishwashers, washing machines or direct hot water discharge connections. The material also absorbs noise due to the wall thickness – there are even special thick-walled HT pipes with a double-layer wall marked as HT silent or HT-B, intended for apartment cores, where acoustics are important.
In practice, the HT system is the "home player": the entire vertical core of the bathroom, branches to the sink, connection to the toilet bowl, horizontal run along the basement ceiling before exiting the building – all of this is HT. The connections are solved with a socket and a rubber sealing ring without gluing, which simplifies the installation and also allows for thermal expansion.
What is the KG system and where can you find it
KG is an abbreviation from the German Kunststoff-Grundleitung – plastic ground (underground) pipe. It is primarily intended for installation in the ground: horizontal gravity waste lines under the foundation slab, sewer connections from the house to the public network, manholes, inspection pieces in the garden, connections of septic tanks and pumping stations. The material is PVC (polyvinyl chloride), the colour is typically terracotta red-brown (marked RAL 8023 or "terracotta red"), which optically immediately distinguishes it from the grey HT.
The KG system is designed for static loading from the soil and possible vehicle traffic (load classes SN4, SN8, sometimes SN12). It has a thicker wall, different tolerances and connections – also a socket with a rubber sealing ring, but shaped and dimensionally incompatible with HT. The temperature resistance is lower: KG is certified for continuous temperatures up to 45 °C and short-term peaks up to 60 °C, which is sufficient for underground lines, as the waste cools during flow.
It is also important that KG pipes are designed with larger tolerances for installation in a sand bed and for position changes due to ground settlement. They are stiffer and heavier – a KG pipe DN 110, length 3 m, weighs significantly more than an equivalent HT pipe, which is intentional: greater stiffness means resistance to deformation under soil load.
Key technical differences HT vs. KG – summary table
| Property | HT system | KG system |
|---|---|---|
| Material | PP (polypropylene) or ABS | PVC (polyvinyl chloride) |
| Colour | Grey / anthracite | Terracotta red / red |
| Temperature resistance | up to 95 °C (short-term), 60–70 °C continuous | up to 60 °C (short-term), 45 °C continuous |
| Use | Inside buildings – vertical and horizontal | In the ground, outside buildings, connections |
| Static load capacity | Not intended for burial in soil | SN4 / SN8 (also under road) |
| Noise | Lower (thicker wall, PP dampens) | Higher (PVC resonates more) |
| UV resistance | Limited | Limited (underground = irrelevant) |
| Connection method | Socket + rubber sealing ring | Socket + rubber sealing ring |
| Compatibility with each other | NO – systems are mutually incompatible without transition pieces | |
| Typical diameters | DN 32, 40, 50, 75, 90, 110, 125, 160 | DN 110, 125, 160, 200, 250, 315 |
How to distinguish HT from KG on a construction site – a practical view
When you arrive at the reconstruction of an older house, the first thing you do is visually assess the existing piping. Grey color = HT (or older black ABS). Brick-red color = KG. This rule applies in 95% of cases with modern plastic piping. Old cast iron or clay (grey, heavy, brittle) pipes are a different category – these are usually replaced with new HT or KG according to their location.
The second identifier is location: if the pipe is in a wall, in a shaft, goes through the ceiling or hangs under the basement ceiling, it is almost certainly HT. If the pipe is buried in the garden, in a concrete floor, or runs from the building to a public sewer manhole, it is KG. A transition point through the foundation or basement wall is a key transition, where one system ends and the other begins – this is precisely where a transition piece HT/KG is installed.
The third identifying feature is dimensional series: KG usually starts from DN 110 (only occasionally DN 100 in older standards) and goes to larger diameters. HT, on the other hand, offers small diameters starting from DN 32 for sink traps, DN 40 for showers and baths, DN 50 for kitchen sinks and washing machines, up to DN 110 and DN 160 for vertical stacks and horizontal collectors.
Why you cannot simply replace HT with KG and vice versa
This is the point where beginners – and sometimes even experienced DIYers – make mistakes. Both systems have socket joints with rubber sealing rings, but the dimensions and geometry of the sockets are not compatible. The external diameter of HT pipe DN 110 differs from the external diameter of KG pipe DN 110: KG is dimensionally standardized according to EN 1401 (external diameter exactly 110 mm), while HT is standardized according to EN 1451 (external diameter 110 mm, but wall thickness and tolerances are different). In practice, this means that the sealing ring of a KG socket does not fit reliably on an HT pipe – the connection may seem tight, but under pressure, thermal expansion or movement, it can loosen.
Moreover, there is chemical resistance. PP (HT) behaves differently from PVC (KG) when in contact with certain aggressive substances. For household waste, this is usually not a problem, but in industrial use or in a chemical lab, it is important. Also, when gluing – KG PVC can be glued with PVC adhesive, while PP is not commonly glued – effectively not at all (only welding or mechanical joints).
At the transition point from the HT system to the KG system (usually at the exit from the building through the foundation or basement wall), special transition fittings are used. These have a socket on one side with tolerances for the HT pipe and on the other side a socket or smooth end compatible with KG. These transition pieces are readily available and their use is simple, as long as you know you need them.
When to definitely choose the HT system
The answer is simple: always when it comes to waste piping inside the building. This includes:
- Vertical stacks – from the toilet bowl, through the WC connection up to the vertical pipe in the basement or on the building's basement level
- Horizontal runs within the layout – horizontal pipes under the basement ceiling, in the ceiling of the bathroom or embedded in the interior floor
- Connection pipes from fixtures – sink, shower, bathtub, bidet, kitchen sink, washing machine, dishwasher
- Apartment cores and bathroom shafts in apartment buildings
- Floor waste lines within the floor structure (as long as they are not under the foundation slab in the ground)
For the HT system on Atria.sk, you will find a complete range of fittings. HT elbow is a basic fitting for changing direction – it comes in angles of 15°, 30°, 45° and 87° (also marked as 88.5° or "right angles") and in diameters from DN 32 to DN 160. HT pipe is the load-bearing element of the system and is supplied in lengths of 0.25 m; 0.5 m; 1 m; 1.5 m; 2 m and 3 m, allowing for precise cutting without unnecessary material waste.
When to definitely choose the KG system
The KG system is your choice whenever the pipe leaves the building, goes into the ground, or serves as a sewer connection:
- Sewer connection from the building to the public sewer network or to a septic tank
- Horizontal underground lines under the garden, driveway, terrace or under the foundation slab
- Drainage and rainwater systems outside the building (runoff from roof gutters to infiltration or rainwater drainage)
- Inspection and cleaning shafts in the garden
- Gravity waste networks between multiple buildings on the property (garage, utility building)
Special case: horizontal line under the foundation slab – HT or KG?
This question is addressed in practice in every new build or in deep reconstruction with floor demolition. If the horizontal waste pipe is embedded in a concrete floor or laid in a gravel bed under the foundation slab of a house, a simple rule applies: if the pipe is in contact with the soil or is enclosed in sand/concrete without free movement, use KG. This ensures the necessary static stiffness and resistance to point loads during concreting and later floor loading.
An HT pipe freely placed in a protective sleeve or in a covered channel is fine – but if you directly embed it in concrete, you risk its deformation or cracking due to settlement of the structure. Therefore, in practice it is recommended: KG under the slab, HT above the slab. The transition is realized with a transition piece directly at the slab penetration point.
HT reducers, plugs, and cleaning fittings – indispensable during installation and service
Along with pipes and elbows, it is important to know smaller but critically important fittings when designing an internal sewer system. HT reducer is a piece that connects two different diameters of HT pipe – for example, DN 50 to DN 40, or DN 75 to DN 50. It is used wherever the diameter changes: when connecting a smaller appliance to a larger vertical pipe, when collecting multiple waste flows into a common line. Without reducers, you would have to choose one diameter for the entire system, which would be uneconomical and structurally problematic.
HT plug is a simple but important element. Plugs are used to temporarily close pipe ends during installation (to prevent dirt or rodents from entering the system), as well as permanently to seal unused branches and T-fittings. They are very useful during renovations – if you have an old branch that you do not want to use, a plug will reliably close it.
HT cleaning fitting is a fitting that everyone who has ever dealt with a clogged sewer system will appreciate. It is a special piece with a removable lid or plug, installed at a strategic point in the pipe (usually at a change of direction, at the base of a vertical pipe, or at the end of a horizontal line), through which a high-pressure cleaning hose, a spiral cable, or an endoscopic camera can be inserted. Proper placement of cleaning fittings is the foundation of any well-designed waste system – it is something that is often overlooked during the first installation, but is quickly remembered when the first clog occurs.
Practical examples from customer projects
Example 1 – Bathroom renovation in a panel building: The customer was replacing the entire core in a 3-bedroom apartment on the 6th floor. The old sewer system was made of asbestos cement and cast iron vertical pipe. The entire new system – from the WC, sink, shower, and washing machine – was implemented exclusively in the HT system in DN 110, DN 50, and DN 40. The cast iron vertical pipe remained, and the connection to it was made using a cast iron-to-HT transition adapter. KG was not used at all, as the system had no underground or external piping. The focus was on the correct slope (2 % for connecting lines), the use of cleaning fittings at the base of each vertical pipe, and acoustic support for HT silent pipes at the ceiling penetration.
Example 2 – New construction of a family house: The project was clearly divided. Under the foundation slab and in the garden: exclusively KG DN 160 and DN 110, laid in a sand bed with 15 cm cover above the top of the pipe, slope 1 %. At the penetration of the foundation slab: a transition piece KG/HT. From the transition piece upwards and throughout the entire interior: HT system DN 110 for the vertical pipe, DN 75 for the horizontal collection line in the basement, DN 50 for the kitchen and washing machine, DN 40 for the sinks and shower. A HT cleaning fitting with a removable lid was installed at the base of the vertical pipe in the basement, accessible without demolition. The project complied with the STN EN 12056 standard for gravity-based waste systems.
Example 3 – A mistaken swap and its correction: These are cases where the technician acts more like a detective than a plumber. The customer bought an older house and after the first heavy shower, a leak occurred at the base of the pipe. Cause: the previous owner used a DN 110 KG pipe inside the house (he found it cheaper, and confused the color). The KG pipe was not structurally designed for ceiling suspension, and the support failed under tension, causing the joint to loosen. Solution: complete replacement with HT DN 110 with elbows, proper clamps every 100 cm for vertical lines and every 50–60 cm for horizontal lines. Lesson: the color distinction between the systems is not random, it is a protection against exactly such mistakes.
Dimensioning – which HT diameter to use for which appliance
When designing an HT system, the following basic rules apply (a more detailed discussion of this topic is available in the article Which HT pipe diameters do I need for the WC, sink, and shower in the Knowledge Center):
- DN 32 – connecting line from the sink, bidet (maximum length 1.5 m without ventilation)
- DN 40 – shower, bathtub, bidet, some sinks (length up to 3 m)
- DN 50 – kitchen sink, washing machine, dishwasher, connecting line for a group of appliances
- DN 75 – horizontal collection line for a group of appliances, piping in smaller buildings
- DN 110 – WC (mandatory!), vertical pipes, horizontal collection lines for entire floors
- DN 125 / 160 – base of the vertical pipe in multi-story buildings, collection lines for larger numbers of WCs
The basic principle is that the pipe diameter must never be smaller in the direction of flow – a reduction to a smaller diameter would cause a rise in the water level and backflow. Therefore, HT reducers are always installed to increase the diameter in the direction of flow (expansion), not to reduce it.
Standards and regulations – what does the legislation say
In Slovakia, the design and implementation of internal sewer systems are governed by the standards STN EN 12056-1 to -5 (gravity-based waste systems inside buildings) and for external piping by STN EN 1610 (installation and testing of sewer connections) together with STN EN 1401 (plastic piping systems for non-pressurized underground sewerage – PVC, i.e., KG). The HT system is standardized in STN EN 1451 (plastic piping systems – polypropylene for waste systems inside buildings).
For a typical family house, these standards in practice mean: correct slope (the article Slope of HT waste pipe: what angle is correct and why it matters discusses this in detail), correct diameters, ventilation of vertical pipes, accessible cleaning fittings, and durability of connections. Inspection authorities check these basic parameters during the final inspection.
Price: is one system more expensive than the other?
In absolute terms for standard residential installations, the price difference between HT and KG is not dramatic, as long as each system is used where it belongs. Problems arise when the customer tries to save money by using cheaper KG material inside the building – where HT is required – or vice versa, by installing more expensive HT material underground – where KG would be sufficient. Both approaches are economically counterproductive: KG inside will cause installation issues (heavier material, different joints, unsuitable fastening), and HT underground will deform and crack under higher loads.
A properly dimensioned and designed system with HT inside and KG outside is a long-term investment. Both systems have a lifespan of decades when used correctly – PP and PVC are extremely resistant to corrosion, chemically aggressive substances from household use, and biological degradation in sewer conditions.
Most frequently asked questions (FAQ)
Can I connect an HT pipe directly to a KG pipe without a transition piece?
No, it does not work reliably without a special transition piece. The socket of a KG pipe has different dimensions and groove shape for the sealing ring than an HT socket. It may seem solid at first glance, but under long-term load, thermal expansion, or pressure testing, it may leak. Always use a certified HT/KG transition piece that has the correct socket geometry on each side.
I have an old cast iron vertical pipe in a panel building – can I connect HT directly to it?
There are special adapters for cast iron–HT transition in various diameters (most commonly DN 100 cast iron to DN 110 HT). These transition couplings are usually rubber with stainless steel clamps (type Fernco or Coupling) and are sealed by friction and pressure of the sealing rubber. It is a common and reliable technique, but you must ensure the correct diameter – cast iron vertical pipes are sometimes DN 100 (outer diameter approx. 113 mm) or DN 150 (outer diameter approx. 170 mm), which differs from HT standards.
Why is my HT waste pipe noisy – I can hear it throughout the apartment?
A standard HT PP system has better acoustic properties than, for example, an old cast iron system. However, if noise loops pass through the ceiling or wall without damping, they will transfer into the structure. The solution is twofold: either use thick-walled HT silent pipe (double-layered), or wrap the standard HT pipe with an acoustic pipe sleeve (mineral wool, acoustic foam) wherever it passes through the structure. The fastening clamp must always have a rubber insert – a direct metal clamp on the pipe is a rigid connection and will transfer vibrations directly into the wall.
What is the maximum slope an HT horizontal pipe can have?
Standard STN EN 12056 recommends a slope of 0.5 % to 3 % for larger diameters (DN 75 and DN 110). At a low slope (less than 0.5 %), sediments settle and the pipe clogs. At an excessive slope (over 15 %), water drains quickly, but solids remain in the pipe and later cause clogging. In practice: for a DN 110 horizontal branch or collection line, the ideal slope is 1–2 %, i.e., a 1–2 cm height difference per 1 meter of length.
Can I run HT pipe through an exterior wall or outside?
HT is not intended for outdoor use. PP degrades when exposed to long-term UV radiation and extreme temperatures (frost at –20 °C or lower can damage the pipe if water is standing in it). If it is necessary to run a waste pipe along the facade, use either HT in a thermal insulation sleeve or another material. A short passage through the wall (up to 0.5 m) is usually fine, but longer outdoor runs are not recommended.
What is HT silent and when is it worth paying extra for it?
HT silent (or HT-B, or "noise-dampening pipe") are pipes with a thicker double-layer wall made of mineral-filled polypropylene. They reduce the noise of flowing waste by 10–15 dB compared to standard HT. It is worth paying extra in multi-family homes where the vertical pipe runs along living rooms or bathrooms, in apartment buildings where the vertical pipe runs next to living rooms or bathrooms of neighbors, and in new constructions where the customer values living comfort. The price difference compared to standard HT is about 30–60 %, which is a relatively small additional cost for a significant comfort benefit in the overall price of a residential installation.
Conclusion: the decision is simple if you know the rules
Choosing between HT and KG systems is actually not too difficult if you understand the basic principle: HT belongs inside the building, KG belongs underground. This rule applies in 95 % of cases and following it will save you problems, unnecessary repairs, and dissatisfied customers. The transition between systems is made at the point where the sewer leaves the building – with an HT/KG transition piece.
For every installation, pay attention to the correct pipe diameter for each fixture, follow the required slopes, don't forget HT cleaning pieces at strategic locations, and ensure proper pipe fastening with clamps with rubber inserts. A complete HT system for internal piping – from HT pipes through HT elbows to HT reducers and HT plugs – can be found in the HT Wastewater System category. Further technical guides on this topic – from dimensioning through installation to troubleshooting – can be found in other articles in the Knowledge Center: Step-by-step installation of HT waste pipe, Common problems with HT waste system and how to fix them, or How to properly connect HT pipes, elbows and tees without leaks.
Do you have a question on this topic?
Not sure what to choose or facing a specific situation at home? Write to us – we are happy to help.