Running vs. End Sewer Trap: What's the Difference and Where to Use Each
Through vs. End-of-line backflow valve: what is the difference and where to use each
When a customer decides to protect their home from sewer backflow, they usually focus on what diameter of flap they need. This is an important question – but before that, there is another one that determines the whole installation concept: should you use a through or end-of-line (blind) flap? It matters much more than it might seem at first glance. The wrong choice can mean either non-functional protection, unnecessarily complicated installation, or a flap that you can never properly inspect or clean.
In this article, I will examine both types in depth – what they are structurally, where they are used, what their strengths and weaknesses are, and most importantly: which specific scenario requires which type. I will also include a few examples from real customer projects, because it is precisely in practice that people make mistakes.
What is a backflow flap – the basics before comparison
In order for the comparison to make sense, we need to clarify the basic principle. A backflow flap is a one-way valve inserted into a pipe or into a manhole. It allows wastewater to flow in one direction – out of the building into the sewer – but automatically closes when the water pressure starts acting in the opposite direction (i.e., when the sewer is overloaded and water starts flowing back into the building).
The mechanism is simple: the flap (a wing) is hinged on a pivot in the upper or middle part of the pipe cross-section. Under normal flow, the water current opens it. When the flow stops or reverses, the flap drops by its own weight (or with the help of a spring) and seals the cross-section. No electricity, no electronics – a purely mechanical device.
Flaps differ in diameter (DN 50, DN 75, DN 110, DN 125, DN 150, DN 200 and more), material (stainless steel, PP, PE, cast iron) and precisely the type of installation: through or end-of-line.
Through sewer flap – what it is and how it works
A through flap (also called a "flow-through" or "inline") is a fitting that is inserted into the flow of the pipe. It therefore has two openings – an inlet and an outlet – and is connected to both sides of the pipe of the same diameter. Wastewater flows through it continuously, the flap is located in the middle of the route and under normal circumstances does not restrict the flow in any way.
Structurally, the body of the flap is made of PE or PP (for plastic types), or of cast iron or stainless steel for larger diameters. Inside is the flap itself – usually made of stainless steel or hardened rubber – hinged on a pivot. The sealing surface is usually made of soft rubber, which ensures tightness even with minor impurities in the wastewater.
Through flaps have a height-adjusting or inspection opening – either an open opening with a lid, or an inspection cover, through which the condition of the flap can be checked without dismantling the entire device. This is a major practical advantage compared to the situation where the flap is buried deep in concrete without any access.
Where a through flap is typically installed
A through flap is intended for installation in the flow of a horizontal wastewater pipe, i.e., on a horizontal section of the route between waste risers and an inspection manhole or connection to the public sewer network. You will most often find it:
- In the foundation slab or under the floor of the ground floor – inserted into concrete so that the inspection lid points upward and is accessible through a floor cover
- In a manhole or sewer pit – where there is enough space for installation and later maintenance
- In basements – if the pipe runs freely or in a concrete bed, the through flap is easily accessible
- In drainage manholes – where it is combined with other elements of the sewer system
A typical example of available products is the through backflow flap DN 110, which fits the most common diameter of household sewerage. For toilets and main collection pipes, there is the through backflow flap DN 125, and for larger buildings or collection pipes, the through backflow flap DN 200.
End-of-line backwater valve – what it is and how it works
An end-of-line valve (also called a "blind" or "end-of-pipe" valve) is structurally different: it has no outlet neck for connecting further piping. It is installed at the end of a pipe that discharges into a manhole, inspection chamber, retention tank, or directly into the open environment. The valve forms the end of the pipe – water flows out through the valve and continues to drain by gravity. In the case of reverse flow, the valve closes.
End-of-line valves are commonly made of stainless steel, cast iron, or plastic. A typical construction is simple: a flanged or socketed connection, onto which a valve flap is mounted. The flap is hinged at the top and opens forward. When the outflow stops, the flap drops down and seals the end of the pipe.
Where end-of-line valves are typically installed
An end-of-line valve is used in situations where the pipe discharges into an open space and the valve itself forms the final closing element. Typical locations include:
- Outflow structures into a water body (stream, drainage ditch, lake) – drainage or relief piping, where the valve prevents backflow when the water level in the receiving body rises
- The end of a pipe discharging into a manhole or chamber – where there is enough space for the flap to open
- Drainage outlets – when the drainage system discharges into a drainage channel or inspection chamber
- Vacuum drainage systems – where the transition from the vacuum section to the gravity section is solved by a valve
Important construction detail: an end-of-line valve requires sufficient free space in front of the pipe end for the flap to open – typically at least the same dimension as the DN diameter. This must be considered when designing the manhole or outflow structure.
Key structural differences – overview
Practical examples: where to use which type
Theory is one thing, real-life applications are another. Over the years of practice, I have encountered several typical situations where people hesitate or make the wrong choice. Here are a few concrete scenarios:
Scenario 1: Residential house in a flood-prone area – protection of ground-level sewerage
Common situation: a residential house with ground-level facilities, a toilet and a sink connected to a common DN 110 pipe running through the foundation to the sewer system. During heavy rain, the sewer becomes overloaded and water flows back. The customer wants protection.
The correct choice is clear: a continuous valve DN 110, installed in the horizontal pipe in the foundation, with an inspection lid accessible via a laminate cover in the floor. The valve is part of the pipe run, and the pipe continues beyond it – this is exactly the definition of a continuous valve. A product such as continuous sewer backwater valve DN 110 is specifically designed for this application.
A common mistake I see: the customer sometimes buys an end-of-line valve because it is cheaper. However, an end-of-line valve has no outlet neck – it cannot be easily integrated into a continuous pipe run. Either special fittings must be made around it, or (which often happens in practice) the valve is installed incorrectly and leaks.
Scenario 2: Drainage outlet into an inspection chamber
The customer has a perimeter drainage system around the foundation, collecting subsurface water and discharging it into an inspection chamber, which is connected to a stream or sewer. They want to prevent backflow from the stream into the drainage system during high water levels.
In this case, an end-of-line valve is suitable at the end of the drainage pipe, where the pipe discharges into the chamber or directly into the water body. The valve forms the end of the pipe, and the flap opens into the free space of the chamber. When the water level in the chamber rises, the flap closes and prevents water from flowing back into the drainage system.
If a through-flow check valve were used here, it would have to be installed somewhere before the manhole – which makes less sense, because the pipe would have to continue further behind the valve, which is not the case for an outfall structure.
Scenario 3: Basement Toilet (WC in the basement)
A basement facility (toilet, shower or washing machine) located in a basement below the level of the sewer connection – the situation is more complicated here, because the wastewater must first be pumped upwards and then drained by gravity. The check valve is part of the pressure pipe behind the pump in this case.
In this case, a through-flow check valve (or a special pressure check valve) is usually installed behind the discharge pump on the discharge pipe to ensure that when the pump is stopped, water does not flow back into the basement collection tank. This is typically a small diameter – DN 50 or DN 75 – but the principle of through-flow installation applies equally.
Scenario 4: Drainage manhole with multiple inlets
A larger plot of land, several drainage branches discharging into a central inspection manhole. The customer wants to ensure that in the case of an overload of one branch, there is no backflow into other drainage branches.
Here, end check valves would be suitable at each inlet collar of the manhole – that is, at the point where each drainage pipe enters the manhole. The valves protect the individual branches from each other. The manhole can be, for example, 315 mm or 400 mm in diameter. For these manholes, suitable manhole covers 315 or manhole covers 400 can also be used, which ensure access for maintenance.
Scenario 5: Apartment building, common waste pipe DN 200
A larger apartment building, main collection pipe DN 200, running through the basement and connected to the public sewer. The operator wants to protect the entire building from reverse surging.
Solution: a through-flow check valve DN 200 inserted into the horizontal pipe before connecting to the public network, ideally in a manhole where there is access for regular maintenance. The sewage check valve through-flow DN 200 is specifically designed for this size. Note: for DN 200 and larger, it is important to ensure easy access, because cleaning and inspection are more demanding and must be done regularly.
Hydraulic aspects: flow losses and nominal flow
Check valves are not hydraulically neutral devices. Each valve introduces a certain local resistance into the system – it depends on the design, the angle of the flap opening and the flow velocity.
A through-flow valve typically has a higher flow resistance than an end valve, because the water has to pass through the entire body of the valve including the mechanism itself. This is usually negligible for normal household drainage with gravity flow, but in long pipe runs or with a low pipe slope, it can cause clogging or insufficient drainage. Therefore, it is recommended to install through-flow valves with sufficient pipe slope – at least 1–2 % (1–2 cm per meter of length).
An end valve at the outlet into free space has a lower flow resistance, because there is no further pressure column behind the valve. The flap can fully open (typically to 90° or more) and water flows freely.
When designing a system, it is always necessary to check whether the valve at maximum expected flow (e.g. during a flood situation, at maximum pump operating condition) does not create such resistance that it does not stop the backflow, but only slows it down. The valve must always be dimensioned for the same or higher DN than the corresponding pipe.
Materials and durability: what determines performance in real conditions
The sewer environment is aggressive: water with faeces, fats, cleaning agents, occasionally also chemicals from the household. The valve must be resistant to all of this over a long period – ideally 20 or more years without replacing the sealing.
PE and PP valve body is resistant to most chemicals of normal household waste, light and easy to handle. The disadvantage is lower mechanical strength under soil loading or improper installation (e.g. without a concrete base).
Stainless steel (AISI 304 or 316) for the flap of the valve is today the de facto standard for any higher quality valve. Resistant to corrosion, mechanically strong, easy to clean.
EPDM rubber sealing is the most common choice – resistant to a wide range of temperatures (from -30 °C to +120 °C), chemically resistant and flexible even after years of operation. Nitrile rubber (NBR) is used where contact with oils or hydrocarbons is likely.
The most common reason for valve failure is not the material of the body, but deposition on the valve seat – fat deposits, fibres or other impurities prevent the proper seating of the flap and the valve stops sealing. This is why through-flow valves with inspection openings have such a big advantage – they can be cleaned without excavation. More about this issue can be read in the article Common check valve failures: why the valve does not seal or close.
Legislation and standards: what does STN and European practice say
The installation of backflow valves in the sewer system is not only a matter of technical choice, but also of standards. In the Slovak environment, the following mainly apply:
- STN EN 13564-1: Non-return closures for buildings – Requirements. The standard defines categories of closures according to the level of protection (type A, B, C, D) and the conditions under which each type is required.
- STN EN 1253: Floor drains and inspection openings – relevant when installing valves in floors.
- Decree No. 532/2002 Coll. (Building Act, technical requirements for buildings): Speaks about the obligation to protect sewerage facilities from backflow in areas with an increased risk of flooding.
The standard STN EN 13564 divides non-return closures into classes according to function: from simple valves (class I) through combined closures with manual closure (class II, III) to systems with electric actuators (class IV). For standard home protection, class I or II is sufficient – that is, a gravity backflow valve, possibly with an additional manual closure.
In reconstructions in flood zones (Q100 or Q500), insurers and designers may require a higher level of protection – in practice, this means a combined valve with a manual closure and inspection access.
Installation: what must be correct from the beginning
Technical selection of the valve is half the success. The other half is correct installation. A few points that are in practice the most common source of problems:
Slope of the pipe at the through valve
The through valve must be installed in a pipe with the correct slope – at least 1 %, ideally 2 %. If the slope is less, water will stop before the valve and stagnant water causes sedimentation. If the pipe is without slope or even in reverse slope, the valve will still work, but it will clog much faster.
Installation direction
The body of the valve is always marked with the flow direction (by an arrow). Installing the valve backwards is one of the most common mistakes in DIY installation. A valve installed in reverse hinders normal flow and opens even more in the case of backflow. A more detailed procedure can be found in the article Installation of a sewer backflow valve step by step.
Access for maintenance
If a through valve is embedded in concrete without any access, it becomes a time bomb. After five years, fat and deposits on the valve seat are accumulated so much that the valve no longer seals – and you will find out only during a flood. An inspection cover must always be accessible: either via a floor inspection hatch or via a manhole cover. Here, it is also useful to have the right combination of valve and manhole with a suitable cover.
Depth of laying and cover
A through valve embedded in concrete in the foundation must have protection against frost – at a depth of laying below 0.8 m (depending on the area), frost is not a problem, but in a small garden house with shallow sewerage, insulation of the pipe must be considered.
Combined protection: when one valve is not enough
In higher risk situations (low-lying building, historically flooded area, large slope of the terrain towards the sewer network), one valve may not be enough. Professional practice speaks of so-called double protection: two valves in series, or a valve combined with an electromechanical closure.
A double through valve (two valves embedded in the pipe in series at a distance of approx. 0.5–1 m) multiplies reliability – if the seat of one is contaminated and does not seal, the other takes over the function. This solution is common in basement facilities or in buildings in Q100 flood zones.
For a more comprehensive view of where and why to install a valve, I recommend the article Protection against flooding from the sewer: when and why to install a backflow valve.
Correct choice of diameter DN: the basis before choosing the type
Before deciding on the type of valve, you need to have clarity on the diameter. Sewer pipes in family homes have these typical dimensions:
- DN 50: sinks, basins, showers (drain without a toilet)
- DN 75: washing machines, bidets, floor drains
- DN 110: toilets, lift stations, collection pipes in smaller houses
- DN 125: collection pipes in larger houses or apartment units
- DN 150: larger objects, garage courtyards, parking lots
- DN 200: apartment buildings, larger commercial buildings
The valve must always have the same or higher DN than the pipe – it must never be narrowed! A more detailed calculation and rules are in the article What DN size of valve do I need for my sewer system.
Most frequently asked questions (FAQ)
Can I install a through valve vertically (in a vertical pipe)?
Most through gravity valves are designed for horizontal (flat) installation. In vertical piping, the flap of the valve may not properly seat by its own weight. If you need a valve in vertical piping (e.g. behind a pressure pump), look for a special type with a spring closure, which is intended for vertical installation. Always check the technical sheet of the specific product.
What is the difference between a backflow valve and a check valve?
A backflow valve (flap valve) has a flap rotating on a hinge – it opens by outflow. A check valve (check valve or ball valve) has a different closing mechanism: a ball, a disc or a membrane. In sewerage, valves are used because they are more resistant to contamination and large cross-sections. Check valves are more common in pressure pipes (water supply, pump discharge pipe). Both perform the same function – one-way flow – but for different applications.
Is a DN 110 valve compatible with a DN 110 pipe from any manufacturer?
In principle yes, because DN (Diamètre Nominal) is a European standard and the outer/inner diameter of pipes must correspond to the standard. In practice, however, minor deviations may occur with cheap pipes outside the standard, or with pipes from old systems (e.g. clay pipes have different tolerances). Always check before purchase what type of pipe (PE, PP, PVC, clay) the valve is intended for and what type of connection is used (smooth end, rubber gasket, welded joint).
How often should a valve be cleaned and inspected?
The recommended frequency for a standard home through valve is at least once a year, ideally in autumn before the winter season, when the risk of storm events and sewer overloading is highest. A visual inspection (view through the inspection opening, checking the free movement of the flap), or cleaning the seat of fat deposits. More details can be found in the article Maintenance and cleaning of a sewer backflow valve – how often and what to check.
When is the installation of a backflow valve mandatory by law?
Slovak legislation does not directly require the installation for every owner of a family house. The obligation may arise from: (a) project documentation for a new building in a flood zone, (b) conditions of the insurer (many insurers refuse to cover damages from backflow without a valve), (c) a decision by the sewer network operator. In practice, a valve is almost always required in a flood zone when a building permit is issued.
Can I place the valve outside the house, not under the floor?
Yes, the valve can also be installed in a manhole or inspection chamber behind the house. From a hydraulic point of view, it is better to place the valve as close to the building as possible – this shortens the pipe section where backflow is a risk. If the valve is placed far from the house (e.g. in an entrance chamber at the property boundary), the pipe section between the valve and the house remains unprotected. From a maintenance access point of view, however, a manhole is more advantageous than embedding in concrete under the floor.
Conclusion: how to decide in two steps
The choice between a through valve and an end valve is not complicated if you proceed methodically. First step: determine where the valve is physically located in the system. Is it in the middle of the pipe, where the route continues further? Then it is a through valve. Is it at the end of the pipe, where the pipe ends in free space, a chamber or a recipient? Then it is an end valve.
Second step: check the pipe DN diameter and choose the correct size. For household sewage systems, the most common diameter is DN 110 – ideally covered with a DN 110 flush gate. For larger collector pipes or multi-storey buildings, choose a DN 125 gate or a DN 200 gate.
Do not invest only in the gate – also think about access to it. A suitable inspection chamber with a manhole 315 or manhole 400 will ensure that you are actually able to maintain and inspect the gate – and that the protection of your home will function not only in the first year, but throughout the lifetime of the installation.
If you are unsure about the correct choice for your specific case, also read the article How to choose the right sewage backflow gate – diameter, type and location or the overview of answers to Frequently asked questions about PE profiles, drainage and sewage gates.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we will be happy to help.