How to choose the right sewer backflow valve – diameter, type and placement

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How to choose the right backflow valve for the sewer – diameter, type and location

The backflow sewer valve is one of those devices that most people remember only when they don't have one – and water from the sewer is just flowing through the floor drain into the basement. From my experience, property damage caused by backflow from the public sewer belongs to the most frequent and at the same time easiest to prevent accidents. Despite this, the choice of the right valve is surprisingly confusing – customers ask about the diameter, can't distinguish between inline and terminal, or don't know exactly where in the installation to place the valve. This article addresses the issue comprehensively, from theory to specific dimensions and installation scenarios from everyday practice.

What a backflow sewer valve actually does and why you need it

The principle of operation of a backflow valve is mechanically simple: it is a flap (usually a plastic plate or disk on an axis), which opens during normal drainage of wastewater in the direction of flow – the water simply pushes it. In the moment when the pressure would reverse and the water would try to flow back (from the sewer into the building), the flap closes automatically and blocks the passage.

The problem of backflow most often occurs during heavy rainfall, when the public sewer is overloaded and its level rises above the level of your connection. The water then naturally looks for an escape route – and these are precisely the floor drains, toilet bowls or siphons of equipment located below the level of the sewer. Without a backflow valve, you have an open gate for sewage from the sewer.

Another, less discussed scenario is a situation when a neighbor's sewer manhole or a common inspection manhole is clogged and the water is looking for a way back. In this case, the backflow valve is the only mechanical protection that will save you.

Basic types of backflow valves – what is available on the market

Before we get to choosing the diameter, it is important to know what type of valve we are talking about. On the Slovak market, we commonly encounter several construction types:

Inline backflow valve

This is by far the most common type for home sewer systems. The inline valve is built directly into the pipeline – that is, "into the line" of the sewer line. The pipe is cut at the installation site and the valve is inserted between two pipe sections. Its body has nozzles on both ends compatible with standard PE or PVC pipe. These are precisely the types such as Sewer backflow valve inline DN 110, inline DN 125 or inline DN 200.

The advantage of an inline valve is its universal applicability and ease of installation on existing piping. The disadvantage? It requires access to the installation site for regular cleaning and inspection, which must be considered when planning the location.

Terminal (end) backflow valve

The terminal valve is installed at the end of the pipe, for example, at the end of a drainage pipe or at the outlet into a drainage manhole. It does not have an outlet nozzle in the standard sense – it functions as a closable end. It is used less in home sewer systems, but has its place in drainage systems and manholes. More about the differences between the two types can be found in the article Inline vs. terminal sewer valve: what is the difference and where to use which in this Knowledge Center.

Valves with manual closure

Some inline valves are also equipped with a manual closure mechanism – a lever or a threaded rod, with which you can block the sewer manually, for example, when you are on vacation and a flood is threatening. I recommend this type everywhere where the building is unoccupied for a long time and where the risk of flooding is increased (near a river, in a valley, in a historically flooded area).

How to determine the correct diameter (DN) of the sewer valve

This is the most common question when choosing. The valve must have the same clear diameter (DN) as the pipe into which it is installed – otherwise it simply does not fit, or if you use a reducing insert, you will reduce the flow and the system will be prone to clogging.

In practice, we most commonly encounter the following pipe sizes:

DN 50 – waste from sink, shower, bidet; separate flap only exceptionally (more often part of the trap)
DN 75 – waste from bathtub or toilet with special construction; less common
DN 110 – standard diameter for WC waste, vertical rising pipe in an apartment or single-family house, most common in smaller building connections; this is exactly the size for which the intermediate flap DN 110 is intended
DN 125 – collection waste for a larger single-family house, connections for houses with 5–8 people or with a basement; it is covered by the intermediate flap DN 125
DN 150 – larger single-family houses, recreational buildings, small businesses
DN 200 – main connections, apartment buildings, industrial buildings, inspection chambers; the correct choice here is the intermediate flap DN 200

Practical rule: If you are unsure between two sizes, always measure the outer diameter of the existing pipe and derive the DN from it. For PE pipes, the outer diameter is greater than the DN – for example, PE DN 110 has an actual outer diameter of 110 mm, but the wall thickness depends on the SDR class. A similar logic applies to PVC. Precise tables can be found in the article What DN flap size do I need for my sewer system?

Where exactly to install the check valve in the sewer system

The placement of the flap is just as important as its correct selection. Incorrect placement can effectively render the flap useless, or conversely, cause it to protect only part of the system.

Main principle: the flap must be installed before the connection to the public network

The check valve is always installed on the domestic part of the sewer system – between the house and the inspection chamber, or at the point of connection to the public sewer. Never behind the inspection chamber (towards the sewer network), because it would be unprotected and inaccessible for cleaning.

Typical placements in a single-family house

Scenario 1 – Equipment in the basement below the sewer level: This is the most critical case. If you have a WC, shower, washing machine, or laundry tray in the basement, these devices are typically below the level of the public sewer. The flap should be installed as close as possible to the point where the house connection leaves the basement and heads into the ground. Ideally, it should be placed in an inspection chamber on the property, or in an access point in the basement floor.

Scenario 2 – House without a basement, connection from the ground floor: Even here, a problem can arise if the surrounding terrain is lower than the sewer level during a flood. The flap is installed in the pipe before the exit from the house (or in an inspection chamber on the property).

Scenario 3 – Drainage around the foundation: In drainage systems, the flap is installed on the outlet of the drainage pipe into the chamber or collector, to prevent flooding of the drainage in case of backflow. A smaller diameter corresponding to the drainage pipe is used here (typically DN 100–150).

Inspection chamber or direct installation into the ground?

This is a question I address on every second job. There are two philosophies:

Flap in the inspection chamber: The flap is installed in the chamber, which you can access via a manhole cover. The chamber (for example, with a cover size of 315 or 400 mm) allows direct access for cleaning and inspecting the flap without excavation work. This solution is absolutely preferred wherever conditions allow – more on choosing the right manhole cover can be found in the articles Manhole cover 315 or 400: how to choose the right size for a drainage chamber, with the products – Manhole cover 315 and Manhole cover 400 – available directly in the category.
Flap directly in the pipe in the ground: The flap is installed directly in the pipe route and buried. This solution is cheaper to implement, but every time you need to inspect or clean it, you have to dig – which is expensive and inconvenient. I recommend it only as a temporary solution or where it is physically impossible to install a chamber.

Depth of installation and pipe slope when installing the flap

The check valve must be installed in a horizontal position or with a slight slope in the direction of the outflow (the same slope as the entire sewer pipe, i.e., 1.5–2 % for DN 110, or 1–1.5 % for larger diameters). It must not be installed in a vertical upright pipe – there the flap would function unreliably or not at all.

From the point of view of depth, the flap must be below the frost level (in Slovak conditions at least 80–100 cm below ground level, more in colder areas). The plastic body of the flap will crack when frozen. If the flap is installed in a maintenance shaft, the shaft must be sufficiently deep and covered with a thermal insulation cover or otherwise protected against frost.

Material and construction details – what to look for when choosing a flap

Modern backflow flaps for household drainage are mostly made of PP (polypropylene) or PVC. Both materials are resistant to common flushes, disinfectants, and drainage water. In the case of more aggressive waste water (for example, from photochemicals, factory kitchen or industrial production), it is necessary to verify the chemical resistance of the specific flap material.

The flap disc is usually made of PP or stainless steel. A stainless steel disc is more resistant to mechanical damage and has a longer service life, while a plastic one is lighter and more affordable. The flap sealing is rubber – and this is the most common place of failure. Rubber ages, hardens, or is damaged by solid objects flowing with the waste water. Therefore, access for inspection and replacement of the sealing is crucial – more in the article Common backflow flap failures: why the flap does not seal or does not close.

The flap hinge must be self-cleaning or at least easily accessible. During long-term inactivity (for example, during holidays when the flap is not used), the disc can stick to the rubber sealing – the result is that during the first flow, the flap does not close properly, or on the contrary, it remains glued in the closed position.

Flap installation – what you need to know before digging into the ground

A detailed installation procedure is the subject of a separate article Installation of a drainage backflow flap step by step, but a few key points are repeated here as well, because these mistakes are repeated in every third job:

Installation direction: Each flap has an arrow indicating the flow direction on its body. If you install it backwards, the flap will be permanently closed and the drainage will be non-functional. It sounds trivial, but I have seen it happen more than once in practice.
Connection sealing: The flap socket is connected to the pipe via a rubber sealing (gasket). The sealing must not be twisted or bent – otherwise it will not seal properly and you will have leaks.
Cleanliness before installation: The pipe before the flap must be perfectly clean. Stone deposits, gravel or a piece of plastic film under the flap sealing means that the flap will never seal properly.
Inspection opening: Continuous flaps have an inspection opening on top (usually a screw closure). This opening must remain accessible – it must not be covered by concrete or soil. When installed in a shaft, this is automatically fulfilled, but when directly embedded, an access opening must be created.
Flap support: The flap body must not hang in the air at the junction of two pipes. It must have a support or base from the bottom, otherwise the joints are stressed by bending and crack.

Special situations – when and where you need more than one flap

In a simple family house, one backflow flap on the main waste pipe before connecting to the public sewer is usually sufficient. However, there are situations where it is appropriate to install multiple flaps:

Multiple independent drainage branches: If the house has two separate drainage connections (for example, one for the basement and another for the ground floor with a different connection), each branch should have its own flap.

Drainage + flush drainage: Drainage pipes around the house must not be directly connected to the flush waste before the flap – this could flood the drainage with flush water during backflow. Each system has its own flap, or both systems are separated.

Pumping pit (shaft) in the basement: If the basement is deeper than the connection level to the sewer, all waste from the basement must be pumped – a septic pumping station with an integrated backflow flap is used. A gravity flap at this location is not sufficient, because the pump creates overpressure and the flap must be designed for pressure drainage.

Maintenance of the backflow flap – how to take care of it

A backflow flap is not a "install and forget" device. It requires regular inspection and cleaning, otherwise it becomes a false sense of security – it is present on the outside, but not functional. Detailed instructions can be found in the article Maintenance and cleaning of a drainage backflow flap – how often and what to check, here is a brief overview:

Inspection frequency: At least once a year (ideally in autumn before the rainy season), every 6 months in case of intensive load or an older system.
What to inspect: Mobility of the flap disc (must open and close easily), condition of the rubber gasket (cracks, hardening), cleanliness of the flap shaft (sediments, hair, toilet paper), tightness of the inspection opening.
Cleaning: Open the inspection cover and remove dirt from the area around the flap disc and the gasket using pressure water or manually. Never use sharp tools that could damage the gasket.
Gasket replacement: The rubber gasket has a lifespan of 5–10 years depending on load and water quality. Replacement can be done by an informed layperson, but you must know the type of gasket and have a spare part.

Legislation and standards – what Slovak law prescribes

In Slovakia, the obligation to install backflow valves is regulated by several legal acts. Decree No. 532/2002 Coll. (building regulations) and standard STN EN 13564 (backflow preventers in buildings) together stipulate that drainage devices located below the level of the sewer (so-called "flood-critical level") must be protected against backflow. Therefore, the installation of a backflow valve is not just a precaution – in new constructions, it is a legal requirement for basement areas.

From the perspective of insurance claims, it is also important that insurance companies are increasingly examining whether the policyholder has installed a backflow valve. If not, the payout may be reduced or denied. From this point of view, the investment in a valve and its proper installation is negligible compared to the risk of an uninsured damage.

Common mistakes in selection and installation – from practical experience

Over the years of practice, I have encountered these most common mistakes that regularly repeat:

Wrong diameter: The customer measured the outer diameter of the pipe and ordered a flap with the same DN number – while DN refers to the inner (clear) diameter. Result: the flap does not fit into the existing connections.
Installation without access: A flap embedded directly into a concrete floor without an inspection opening. After two years, the flap was leaking – access to the flap required breaking the floor at a cost of €800.
Flap behind the manhole (towards the sewer network): The customer mixed up the sides. The flap protected only the sewer manhole, not the house – flushes still flowed into the basement through the manhole opening.
Ignoring the flap axis: Installation in a vertical position on a rising pipe. The flap either did not close (the disc fell down = remained open) or did not open (the disc lay on the seat). Every manufacturer explicitly prohibits installation on vertical piping.
Purchase of a low-quality product without an inspection opening: A cheap flap without an inspection cover – at the first inspection, the customer had to physically dismantle the pipe section.
Flap without support – mechanical load on the connections: Due to the weight of the flap body and soil movement, both connection joints cracked after three years.

Most frequently asked questions (FAQ)

Must I have a backflow valve, or is it just a recommendation?

For septic tank connections in single-family homes with ground-level floors above the sewer level, it is generally a recommendation (good practice), not a strict legal obligation. However, for equipment in basements below the level of the sewer backflow, the installation of a backflow valve is required by the standard STN EN 13564 and according to building regulations. In new constructions, the designer must include it in the documentation. In renovations, the building authority may require it when issuing a permit. Regardless of legislation, insurance companies increasingly condition the insurance payout on the fact that a valve was installed – so the question "must I" quickly turns into "is it worth it for me".

Can I install the valve myself, or do I need a professional?

The installation of a through-flow valve into an existing pipe is technically feasible for a skilled homeowner – no welding or special tools are needed. You need to know how to modify the drainage pipe (cut, treat edges, install sockets with gaskets). However, the correct choice of location, access for maintenance, and verification that the installation meets the standards are critical. If you are unsure, it is better to consult an installer – the cost of a wrongly placed valve is much higher than the cost of professional work.

Which valve to choose for DN 110 on a toilet waste in a single-family home?

For a standard toilet waste in a single-family home, the correct choice is a through-flow backflow valve DN 110. This valve is installed into a horizontal or slightly sloped DN 110 pipe, which is the standard size for a toilet and rising pipe in apartment construction. We specifically recommend the Through-flow sewer backflow valve DN 110 – the through-flow design allows direct installation into the pipe route or into an inspection manhole.

What if I have different pipe diameters in my home drainage system?

In this case, install the valve at the point where all branches join into one main pipe – that is, as close as possible to the house exit, where there is already a common diameter. If the branches join at different locations and diameters, and each branch could be a source of backflow, consider a valve for each branch separately. Most common case: branches DN 50 and DN 75 join into DN 110 still inside the house – a DN 110 valve on the main outlet is sufficient.

Can a backflow valve cause a clogged sewer?

Well-designed and properly installed valves have practically no effect on sewer flow – during normal drainage, the disc is fully open and does not create resistance. Problems arise if the valve is clogged (disc clogged with sediments, hair, or solid waste), or if the gasket is swollen. Regular annual inspection and cleaning prevent such situations. Valves with smooth inner body surfaces (PP, PVC) are less prone to dirt accumulation.

What is the difference between a DN 125 and DN 110 valve? Can I use a larger valve "just in case"?

No – the valve must exactly match the diameter of the pipe into which it is installed. A DN 125 valve cannot be installed on a DN 110 pipe without a reduction. If you use a reduction (DN 110 → DN 125 → DN 110), you create a constriction that is prone to clogging, and the valve will not function in such a location. A larger diameter "just in case" is not justified – a DN 125 valve is for DN 125 pipes. If you have doubts about the actual diameter, measure the outer diameter of the pipe and compare it with the manufacturer's dimensional tables.

Conclusion – summary and recommendations

The selection of the correct sewer backflow valve can be simplified into a few key steps: determine the correct DN diameter according to the existing pipe, choose a through-flow design for home drainage (unless you have a specific reason for an end-flow), plan the location to ensure access for regular maintenance – ideally via an inspection manhole with a cover – and during installation, pay attention to the correct direction, quality sealing, and stable support of the valve body.

The investment in a properly selected and professionally installed valve is in the tens of euros. Damage from a single basement flood with sewage is in the thousands. The math is clear. If you still have questions about a specific type or size for your system, also check out other articles in this Knowledge Center – for example, Protection against flooding from the sewer: when and why to install a backflow valve or Common questions about PE fittings, drainage, and sewer valves. You can find the product range directly in the category PE fittings, drainage, valves.

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