Frequently Asked Questions about the HT Waste System

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Frequently asked questions about the HT waste system – a comprehensive guide for anyone planning or solving plumbing in a building

The HT waste system is one of the most widespread solutions for internal plumbing in apartment buildings, single-family homes, and commercial buildings in Slovakia. Despite being a relatively simple system of plastic pipes and fittings, there are still questions that trouble customers, installers, and investors over and over again. Some arise during the initial planning of the installation, others during a bathroom renovation, further during troubleshooting or building commissioning. In this article, we will try to cover as wide a range as possible of common and less common questions – just like those we hear every day from people who are just getting started in this field, as well as from experienced plumbers looking for confirmation of the correct procedure.

If you are looking for specific topics such as Slope of HT waste pipe: what angle is correct and why it matters, or Installation of HT waste pipe step by step, you will find them in the Knowledge Centre as separate, more detailed articles. This text focuses on the question and answer format so you can quickly find what is troubling you.

What is the HT system and what is it made of?

The HT system (from the German Hochtemperatur) is a system of plastic pipes, elbows, tees, reducers, and other fittings intended for the removal of greywater waste within the interior of a building. It is manufactured from polypropylene (PP) or related polymers, which gives it several key properties: resistance to hot waste water up to about 95 °C for short-term exposure, light weight, easy mechanical workability, and a reliable rubber-sealed connection that does not require any glue or special tools.

A typical HT system installation includes the following components:

Pipes – straight sections in various diameters and lengths; the basis of the entire route
Elbows – for changing direction (15°, 30°, 45°, 67°, 87°/88°)
Tees and cross fittings – for connecting multiple appliances to one collector
Reducers – for transitioning between different diameters
Cleaning pieces – for inspection and possible removal of blockages
Caps – for temporary or permanent sealing of the pipe end
Sound-insulating clamps and brackets – for mounting and noise damping

You will find all these components together in the category Waste system HT.

What is the difference between the HT and KG system? When to use which one?

This is by far the most common question when planning plumbing. The answer is relatively straightforward, but in practice, people often confuse these two systems or try to connect pipes that simply do not fit together.

HT system is intended for internal plumbing – that is, for waste pipes running inside the building, in vertical shafts, in suspended ceilings, in false walls, or under the floor in a screed layer. It does not belong in the ground or outside the building. The pipes have wall thicknesses optimized for interior conditions, cannot withstand long-term loading from soil, and their rubber seals are not designed for contact with aggressive underground environments.

KG system (from the German Kunststoff-Grundleitungssystem) is, on the other hand, intended for external and underground plumbing – from the building outlet to the inspection chamber and further to the municipal sewer. It has thicker walls, a higher ring stiffness class (SN4, SN8, SN16), and a seal resistant to soil conditions.

Where exactly do they meet? At the point where the sewer pipe passes through the foundation wall or the floor of the ground floor and exits the building. At this point, a transition from HT to KG is typically made, using a reducer or a special fitting. More about this can be found in the topic HT versus KG system: which waste system to choose and why in the Knowledge Centre.

What diameters of HT pipe are used for individual appliances?

The pipe diameter is a critical parameter – if it is too small, the pipe will clog easily; if it is too large, the waste will flow too slowly and sediment will settle. The golden rule therefore applies: use the diameter prescribed by the standards or the project, not just what you happen to have on hand.

Basic recommendations according to common practice and standards (EN 12056):

DN 32 – washbasin (drain trap), bidet, kitchen sink (short section to the trap)
DN 40 – bathroom washbasin, kitchen sink, bathtub, shower (depends on the configuration)
DN 50 – washing machine, dishwasher, group drain from multiple small appliances
DN 75 – WC connection in some older solutions, air vent stack for lower load
DN 110 – WC (toilet bowl), collector stack for an apartment or floor, ventilation
DN 125 / 160 – main stack for a larger number of floors, transition to the foundation

More detailed tables with calculations of drainage units can be found in the article What diameters of HT pipe do I need for WC, washbasin, and shower.

How does the rubber seal of an HT joint work? Is it reliable?

The rubber seal is the heart of the entire HT system. It is located inside the socket (enlarged end) of each pipe or fitting. When you insert the smooth end of another piece into the socket, the rubber elastically deforms and creates a watertight and airtight joint – without glue, without sealing tape, without silicone.

To ensure the joint functions properly, the following rules apply:

Mark the smooth end of the pipe with a marker at the insertion depth minus 10 mm – this is the so-called compensation gap, which allows for thermal expansion
Apply a little lubricant (usually supplied by the manufacturer or common soap) to the smooth end
Before inserting, check that the rubber is seated in the groove and is not twisted or damaged
Insert the pipe straight, not at an angle
After insertion, check that the mark is still visible – if it disappears, the joint is too deep and the pipe will not have space for expansion

The reliability of the rubber seal is well proven when installed correctly – seals made of high-quality EPDM rubber can last 50 years or more without problems. Issues occur only in the case of mechanical damage (too forceful insertion, foreign objects in the socket), when incompatible chemicals are used, or when temperatures significantly exceed 95 °C (which practically never happens in a regular household). Read more about connecting in the article How to properly connect HT pipes, elbows and tees without leaks.

What is the purpose of an HT cleaning piece and where should it be placed?

This is a question where most "DIY plumbers" fail. HT cleaning piece is a fitting with a side or front closable opening (cleaning inlet), through which a cleaning snake or a high-pressure cleaner hose can be inserted directly into the pipe without the need to disassemble the entire section.

Where the cleaning piece definitely belongs:

At the beginning of each horizontal branch longer than 3–4 meters, ideally right after the last elbow
At the bottom of each vertical riser, just before the branch into the horizontal pipe
At every place where the route turns more than 45° and is not directly accessible (covered by walls, ceiling)
In apartment buildings on each floor for long horizontal sections

What happens if the cleaning piece is missing? When the pipe becomes clogged (and it will sooner or later – usually with fat, hair or foreign objects), you have no other option but to cut the pipe, remove the clog and re-join the entire section. This is a significantly more expensive and time-consuming intervention. Therefore, the installation of cleaning pieces is an "extra" cost of just a few euros, but absolutely essential in the long run.

What is an HT reducer and when do I really need it?

HT reducer is a fitting that smoothly transitions from one diameter to another – for example from DN 50 to DN 40, or from DN 110 to DN 75. It looks simple, but its correct use (and especially its correct orientation) is crucial.

Basic rule: the reducer must be installed so that the larger diameter is oriented towards the outlet (i.e., we always reduce towards the smaller diameter when connecting a smaller device to a larger pipe). So: DN 110 riser → DN 50 connection for a washing machine → reducer 50/40 → trap. Never the other way around – increasing the diameter towards the outlet causes flow slowdown and sedimentation.

Typical situations where a reducer belongs:

Connecting a sink (DN 40) to the main branch DN 110 – via a branch with a reducer
Transition from a kitchen line (DN 40 or 50) to a floor drain (DN 75 or 110)
Connecting a shower drain (DN 50) to a riser DN 110
Outlet from a riser DN 110 to a KG system DN 125 (here it increases – that is acceptable, as we are going into a larger underground pipe)

Can I use HT pipes in the ground or in concrete?

This is one of those questions where we see the biggest mistakes in practice. The technical answer is: not in direct contact with soil. The HT system is not certified for burial in the ground, where it would be constantly exposed to mechanical loading from the soil and groundwater. The walls of HT pipes are thinner than those of KG pipes of the same diameter, and they do not have sufficient ring stiffness.

However – and this is an important difference – HT piping can be poured into concrete in the floor, provided that:

The pipe is not exposed to direct loading from the soil from the outside (it is in the floor structure, not in an open trench)
The concrete is evenly poured around the pipe without point pressure
The pipe is wrapped in soft insulation (acoustic foam, old cardboard) for expansion and acoustics
The slope of the pipe is maintained before the concrete sets (checked with a spirit level)

In practice, HT piping is commonly installed in floor layers of new buildings, where it is not in direct contact with the soil, but is embedded in screed. It works excellently and without problems. In the case of running under the floor of the ground floor, where there is gravel or soil under the concrete, we recommend consulting with the designer and considering a combination with a KG profile.

How to properly design the slope of a horizontal HT pipe?

Slope is a topic where two extreme mistakes often meet: either the pipe is laid almost flat (without slope), or it is sloped at too steep an angle. Both extreme variants lead to problems.

The correct slope for horizontal HT piping is 1–3 % (10–30 mm per meter). In practice, 1.5–2 % (15–20 mm/m) is most commonly used. With a small slope (less than 1 %), the fluid flows slowly, and solid particles settle during the flow, clogging the pipe. With an excessively steep slope (over 3–4 %), the liquid drains quickly, but the solid particles remain behind – the effect is the same: sedimentation and clogging.

This rule applies to horizontal and diagonally laid branches. Vertical risers, of course, run straight down, and slope is not relevant here. For more details, see the article Slope of HT drainage pipe: what angle is correct and why it matters.

What are HT elbows and what angles are available?

HT elbow is used to change the direction of the pipe. They are produced in several angular variations: 15°, 30°, 45°, 67°, and 87°/88° (which is practically a right angle). Choosing the correct angle is not only a matter of space but also of hydraulics – each elbow represents a hydraulic loss and increases the risk of sedimentation.

Practical tips for selecting an elbow:

87°/88° – used mainly for risers (transition from vertical to horizontal pipe), not in horizontal branches – risk of clogging
67° – a compromise for transitioning to a horizontal pipe from a riser, hydraulically better than 87°
45° – used in horizontal branches when a direction change is necessary in the plan view
15° and 30° – fine corrections of the route, when laying pipes in suspended ceilings or behind walls

Golden rule: avoid 87° elbows in horizontal branches. If you need to change direction by 90° in a horizontal plane, use two 45° elbows with a short straight section between them – the hydraulics are significantly better and the risk of clogging is much lower.

When is an HT plug needed and can it be used as a permanent solution?

HT plug is a simple component that seals the end of a pipe or fitting. It is used in two situations:

Temporary sealing – during phased construction, when the piping is installed but the equipment (e.g., a toilet or sink) is not yet installed. The plug prevents dust, dirt, and construction materials from entering the pipe. This is an absolutely common procedure on every construction site.

Permanent sealing of unused openings – for example, when you leave a reserve for future connection of additional equipment (a second toilet, laundry room) during installation, but you don't need it at the moment. The plug serves as a permanent closure of the opening. It is important that the plug is properly inserted and secured – in the case of pressure issues in the piping (e.g., a clogged riser during backflow), an insufficiently secured plug may pop out. In larger diameter risers, it is better to add a mechanical safety device for permanent installation.

What is the lifespan of an HT system and when should it be replaced?

Polypropylene HT pipes have a service life of at least 50 years with proper use, and in reality, even significantly longer. Unlike old cast iron drains or asbestos cement pipes, they do not corrode, no scale forms on them, and the inner surface of the PP material has no places to catch organic deposits – the surface is smoother than any metal pipe.

Replacement of the HT system is justified only in these cases:

Mechanical damage to the pipe or fitting (crack, break during construction work)
Material fatigue in systems from the 70s and 80s (at that time, less quality plastic blends were used)
Change of room layout or new equipment that cannot be connected to the existing route
Recurring clogs at the same location despite regular cleaning (a sign of incorrect slope or defective fitting)

If you are deciding on a complete renovation of the bathroom core or bathroom, it usually makes sense to replace the entire system at once, even if the old system is still working – you will avoid problems with diameter incompatibility and have a clear overview of the entire route.

What is the noise level of an HT system and how to reduce it?

Standard HT pipes made of gray polypropylene belong to relatively noisy systems – the draining water creates significant noise, which is clearly audible through wall structures. In practice, this is a problem mainly with risers running through living rooms, bedrooms, or offices.

Solutions that work:

Noise-reducing HT pipes (black, three-layered with mineral filler) – reduce noise by 15–20 dB compared to standard ones; available on the market, for example, under the names HT-Silent, HT-Plus, or Phonostar depending on the manufacturer
Insulation of pipes with foam sleeves – a cheaper option, less effective, but better than nothing
Installation in separate drywall partitions with mineral wool inside – a double drywall wall significantly improves the situation
Use of noise-reducing hangers instead of rigid metal clamps – every rigid connection transfers vibrations to the structure
Reducing water pressure at inlets – higher water speed = more noise; pressure regulation in the system can help

Where and how to correctly fix HT piping?

Piping fixation serves two functions: it keeps the route at the correct slope and eliminates vibrations and noise. The spacing of clamps depends on the diameter and orientation of the pipe:

DN 32–50, horizontal: every 40–50 cm
DN 75–110, horizontal: every 60–80 cm
DN 110–160, vertical: every meter, for long risers every floor + fixed point clamp

Important: clamps must allow axial movement (expansion) – this means that the pipe must be freely slidable within the clamp along its axis. Only one fixed clamp per branch should be truly fixed; the rest are guiding. If you fix the pipe too rigidly at multiple points, thermal changes will cause it to bend or crack at the joints.

Can I connect HT pipes from different manufacturers together?

Theoretically yes – the HT system is standardized (EN 1451-1) and diameters are standardized, so pipes from manufacturer A should be compatible with fittings from manufacturer B. In practice, however, we encounter minor differences in tolerances of the outer diameter, rubber hardness, and neck geometry, which can cause leaks.

Our experience shows that the most reliable solution is to use products from one manufacturer (or one product line) within a single installation. If you need to combine brands, always test the connection by filling it with water and wait at least 10–15 minutes before final installation. When transitioning between different systems (e.g., an older system from the 90s and new fittings), transition couplings may be necessary.

What to do if the waste pipe smells?

Odors from a waste pipe are not a problem of the HT piping itself, but usually of the trap or the venting of the riser. Despite this, it is a question often associated with the HT system, because people address it when solving drainage issues.

The most common causes of odor:

Dry trap – if the appliance is not used for a long time, water in the trap evaporates and the seal against odor stops working; solution: simply run water
Missing or damaged venting of the riser – negative pressure in the riser can push odor into the space; solution: check the venting cap on the roof
Clogged trap – organic deposits in the trap ferment; solution: clean or replace the trap
Leaky connection in the HT pipe – rare, but possible in old systems or due to improper installation; solution: visual inspection and possibly relocation of connections

Read more about preventive maintenance in the article Cleaning and maintenance of HT piping: how to prevent clogging.

Most frequently asked questions (FAQ)

Can I use HT pipe for waste from a sauna or swimming pool?

Special conditions apply to waste from a sauna or swimming pool – temperature peaks in a sauna can briefly exceed 80–90 °C, which is at the limit of HT PP pipe specifications. HT systems are commonly used for saunas, but when dimensioning, sufficient cooling before entering the regular waste system must be considered. For swimming pools, resistance to chlorine is key – standard PP pipes are resistant to chlorine at normal concentrations, but more aggressive chemistry (e.g., pool acids in direct contact) can damage the surface. In such cases, consult the system manufacturer.

How many 45° elbows can I place in sequence on a horizontal branch?

Hydraulically, each elbow increases resistance and the risk of sedimentation. On a horizontal branch shorter than 5 m, ideally no more than two directional changes (e.g., two 45° elbows). For long branches, ensure that the total equivalent length of elbows does not exceed hydraulic capacity limits. In practice: if in doubt, design the route with fewer elbows or shorten the branch by adding a riser.

What is "wet" and "dry" installation of HT piping and when is each used?

These terms refer to leak testing after installation. A wet test means filling the pipe with water and checking the joints for leaks under water pressure. A dry test uses pressurized air. In practice, for HT waste systems, a simple wet test with water flow is most commonly performed – formal pressure testing is not required as strictly for internal waste piping as it is for water supply piping, according to Slovak standards. Always verify the requirements of the local building inspector and the designer.

What is the minimum slope for an HT system vent pipe?

The vent pipe (vent riser above the roof) must be installed vertically or with a maximum deviation from vertical to ensure proper venting. Avoid horizontal routing of the vent branch – condensate and dirt can accumulate in it. If it is necessary to route the vent pipe at an angle, maintain at least a 10° deviation from horizontal (80° from vertical) and ensure that condensate drains back into the riser, not toward the roof opening.

Can the HT system be adapted during renovation without tearing up the entire route?

Yes – one of the advantages of the HT system is its modularity. If you need to add a branch, you can cut the pipe at any point and insert a fitting using short spacers. If space is limited, extended necks and adapters are available. In practice, during renovations of old apartments, we only replace sections that are either damaged or do not match the new layout – the rest of the route is left intact if functional. In some cases, a transition coupling or individual reduction is necessary for connections between old and new piping.

Where to buy HT pipes and fittings reliably with technical support?

When choosing a supplier of the HT system, ensure that they have a complete range – pipes, elbows, branches, reductions, cleaning pieces and plugs from one manufacturer. Mixing different brands increases the risk of incompatibility. On atria.sk, you will find a comprehensive range of the Hepworth HT system, where all fittings are mutually compatible and technical parameters are available for each product.

Conclusion: HT system as the basis of reliable internal sewage

The HT waste system is exceptionally reliable and durable when properly designed, dimensioned, and installed. Most problems we encounter in practice – clogs, odors, noise, leaks – stem not from the material quality, but from installation or design errors: incorrect slope, missing cleaning pieces, inappropriate diameter, unsecured expansion, or mixing systems without verifying compatibility.

If you are planning an installation or renovation, we recommend also reading other articles in the Knowledge Center – especially How to choose the right HT pipes and fittings for the waste system, Installation of HT waste piping step by step, and Common faults in HT waste systems and how to fix them. Together, they form a comprehensive guide from planning to operation.

And if you encounter a specific problem that you cannot solve – whether it is choosing the right reduction, placing a cleaning piece, or dimensioning a riser – please describe the situation and we will gladly advise you based on real experience from field installations.

Do you have a question on this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we will be happy to help.