How to choose the right sewer check valve – through-flow vs. end type
Backflow valve in drainage – through or end type? A comprehensive guide to selection
The backflow valve in the drainage system is one of those devices that most people don't even think about – until the moment when they don't have one and the sewage starts flowing back through the toilet or floor drain into the basement during a heavy rain. At that point, it is usually too late and the property damage is often painful. A correctly selected and installed valve is, however, a relatively simple and inexpensive protection that can prevent thousands of euros in damage.
This article focuses on one specific but crucial decision point: whether to use a through or end type backflow valve. This is a question that in practice causes the most confusion – people often confuse these types, install the wrong variant and then wonder why the valve doesn't seal, why it gets stuck or why the installation takes twice as long as it should. We will go through both types from the ground up, show real-life situations from practice and by the end you will know exactly what to buy.
What a backflow valve actually does – basic principle
Before we get into comparing the types, it is good to refresh the basic physics. A backflow valve is a passive valve – that is, it works without any external power or control. Inside the body of the valve is a rotating or pivoting plate (the valve in the narrower sense of the word), which is held in the open position by the normal flow of waste water towards the public drainage system. If the flow direction is reversed – that is, if the water starts flowing back into the house – this plate closes by gravity or water pressure and stops the reverse flow.
The simplicity of this principle is its strength. There is nothing to break down electronically, no maintenance is required except occasional cleaning. The weakness is that the mechanical plate can be blocked by dirt, fat or fibers, which are common in the drainage system – but this is a topic we cover in the article Common backflow valve failures – why the valve doesn't seal or gets stuck.
Two basic types: through and end type valve
There are dozens of different backflow valve constructions on the market, but for domestic drainage connections in practice we almost exclusively talk about two basic types according to the method of connection and position in the system:
Through backflow valve
A through valve is installed inside the pipe, into its running line. In other words, the pipe is interrupted at a given point, the valve is inserted into this gap and connected to both parts of the pipe. The flow goes through the entire body of the valve, the valve is part of the pipe like any other fitting – a coupling, elbow or T-joint.
Through valves are intended for installation into inspection chambers, where access to them is from above through the chamber opening. Typically, they are placed into the domestic drainage connection just before the entrance to the public network, or at the point where we want to protect a specific branch of the domestic drainage. The inspection chamber (usually DN 315 or DN 400) also serves as an access point for maintenance and inspection of the valve.
Typical representatives of this type include, for example, drainage backflow valve through type DN 110, drainage backflow valve through type DN 125 or for larger connections drainage backflow valve through type DN 200.
End type backflow valve
An end type valve is installed at the end of the discharge pipe – that is, at the free end of the pipe that leads into a chamber, pit, watercourse or reservoir. It does not have a double flanged connection; from one side it is connected to the pipe, the other side is free and the valve functions here as a one-way closure of the entire opening. Water normally flows out, but nothing from outside gets inside through this opening.
End type valves are typically used in connection with pumps – for example, in drainage systems, where the pump pumps water out of a sump and the valve at the end of the discharge pipe prevents backflow after the pump is turned off. Another typical use is at outlets of stormwater drainage or drainage into drainage ditches.
When to use a through valve – real-life scenarios from practice
A through valve is the right choice in most situations that involve protecting domestic drainage from backflow from the public network. Here are the specific situations that are most commonly addressed in practice:
Detached house below the level of the sewer collector: This is a classic scenario. The house is located in a hollow or on a slope, where the floor level of the basement is lower than the top edge of the main sewer collector in the street. During heavy rain or during a blockage in the network, the pressure in the collector is higher than the water column height in the domestic connection – and water starts to flow back. A through valve installed in the inspection chamber on the connection (typically DN 110 or DN 125) stops this backflow.
Basement areas with floor drains or toilets: Every layout below the level of the finished ground is risky. If you have a toilet, shower or washing machine in the basement, the waste water from these appliances flows down by gravity – but in the case of a blockage or overpressure in the drainage system, the backflow is directed exactly where it will cause the most damage. A through valve on the basement branch or on the entire domestic connection before the outlet into the network is the solution.
Garden houses and additions connected to the domestic drainage: If you have a garden house, garage or other building connected to the domestic drainage and located lower than the main building, it is advisable to protect its branch with a through valve directly on that branch. Then, in the event of problems, it will not threaten the entire system, only the specific building.
New buildings in risk areas: In areas with documented history of flooding or problematic drainage, the installation of a through backflow valve is practically standard. In some municipalities and cities, it is even a condition for the issuance of a certificate of compliance – more about this legal aspect can be found in the article Protection of the house against backflow of sewage – when is the valve mandatory?
When to use an end check valve – real-life scenarios from practice
The end check valve has a precisely defined use and where it is suitable, there is no substitute for it. At the same time, it makes no sense to install it into a running pipe – the shape of the body does not allow for two-sided connection.
Drainage pump discharge pipe: A classic situation – a pump in a catch basin or water meter chamber pumps collected water out. The pipe leads to the garden, a ditch or to the sewer. After the pump is turned off, there is a risk that the water remaining in the discharge pipe (and at the outlet location, if it is higher) will flow back into the pit. An end check valve at the outlet prevents this and also protects the pump from unnecessary starts.
Roof drainage outlet into a watercourse or ditch: Drainage from the garden, roof gutters or garden drainage system discharges into a ditch or stream. In the case of high water levels in the stream, backflow into the garden drainage system is a risk. An end check valve at the outlet prevents water from the stream from filling the drainage system and potentially flooding the garden or basement.
Reservoir and irrigation technology: In gardens with ponds and pump systems, end check valves are used at the outlets of filtration circuits, where it is necessary to prevent backflow when the pump is turned off.
Key structural and installation differences – what you need to know before purchasing
Understanding the technical differences correctly will save you time, money and unnecessary returns. Here are the most important differences:
Method of connecting to the pipe
A running check valve has sockets (ferrules) on both sides – that is, cavities into which the ends of the pipes from both sides are inserted and sealed with rubber gaskets. The sizing must correspond to the outer diameter of the pipe, not the inner one. For standard household sewage, the following applies: DN 110 corresponds to a pipe with an outer diameter of 110 mm, DN 125 to an outer diameter of 125 mm and DN 200 to an outer diameter of 200 mm.
An end check valve has a socket only on one side. The other side is closed with a cover or a direct outlet with a flap. Installation is therefore simpler in terms of access, but the valve must be placed at the actual end of the pipe – it cannot be "extended" with a pipe from the other side.
Position in the system and depth of installation
Running check valves are designed for installation in inspection chambers, where they are protected from frost and mechanical damage. The minimum depth depends on the climatic region, but in most of Slovakia we are talking about a minimum depth of 80–100 cm below ground level, ideally more. For the correct selection of the chamber size through which you access the running check valve, see the article Veko šachty 315 alebo 400 – ako vybrať správnu veľkosť? – and for the chamber covers themselves, for example, veko šachty 315 or veko šachty 400 are available.
End check valves can also be placed above ground (at outlets into ditches) or at a minimum depth – it depends on the specific application. If the outlet is in a frost-prone area, it is necessary to ensure that the valve itself does not freeze.
Direction of the flap and installation position
This is a technical detail that people often underestimate and then the flap either does not seal or gets stuck. The flap of a running check valve must move upwards during normal flow. This means that the valve must be installed so that the axis of rotation of the flap is horizontal and the flap opens into the body of the valve (opened by the pressure of water from above). Most running check valves have an arrow on the body indicating the direction of flow – this must be followed.
Some types of running check valves can also be installed in vertical pipes (risers) – but different rules apply here and this is more of an exception. A standard household connection is horizontal or slightly sloped, which is ideal for a running check valve.
Choosing the correct diameter DN – it shouldn’t just be guessing
The type of flap (intermediate vs. end) is the first decision. The second, equally important, is the correct diameter. An incorrect DN flap either does not fit physically or fits with a gap and does not seal – in both cases, it results in unnecessary expenses.
Basic rule: the diameter of the flap must match the diameter of the pipe into which it is installed. For domestic sewer connections in Slovakia, the following is almost universal:
- DN 110 – standard domestic connection for single-family homes, regular sewerage from the kitchen, bathroom and WC; corresponds to pipes with an outer diameter of 110 mm (usually PVC, PP or PE)
- DN 125 – less common diameter, but occurs in older installations or when connecting a larger number of devices; sometimes used in stone or concrete pipes, where the dimensions differ from plastics
- DN 160 – larger single-family homes, multi-generational homes, smaller apartment buildings, connections with multiple apartment units
- DN 200 – apartment buildings, commercial buildings, larger industrial connections; sewage check valve intermediate DN 200 is suitable precisely for these larger dimensions
If you are unsure of the diameter of the existing pipe, measure the outer diameter of the pipe with a caliper (not by estimation!). Note that "DN" for plastic sewer pipes usually corresponds to the outer diameter, whereas for older stone or PVC-U pipes of other standards it may be the inner diameter – in that case, compare the actual dimensions with the catalog values. A more detailed comparison of diameters can also be found in the article Backflow flap DN 110 vs. DN 160 vs. DN 200 – comparison of use.
Material design and durability – what to look for in long-term use
Modern sewage check valves for domestic connections are mostly made of polyethylene (PE) or polypropylene (PP), with the flap plate being made of the same material, stainless steel, or in combination with rubber sealing. PE flaps have the advantage of being chemically resistant to common household wastewater, do not corrode and are easy to clean.
Rubber seals on the flap plate (EPDM or NBR) ensure sufficient flexibility when closing, even if the plate is not perfectly clean. High-quality flaps have seals designed to maintain sufficient pressure to close even with slight contamination (fibers, fat). When we talk about a "leaking flap" in practice, it is usually due to heavy contamination, physical damage to the plate or the use of a cheap product with thin seals – not a systematic flaw in this solution.
Installation of an intermediate flap – what to pay attention to
An intermediate flap is not complicated to install, but a few errors occur regularly:
Reversed installation direction: The most common mistake. The flap has an indicated arrow indicating the flow direction – from the house towards the sewer. If you install it with the arrow reversed, the plate will open during backflow and close during normal outflow – exactly the opposite of what you want. Always check the arrow before backfilling the shaft.
Insufficient sealing of the collars: Rubber seals must be properly seated in the grooves and the pipe inserted to the required depth. Incomplete insertion causes leakage at the joints – not only during backflow, but also during normal operation, leading to seepage into the ground.
Installation outside the shaft in an inaccessible place: An intermediate flap requires regular inspection and possible cleaning. If you embed it in the foundation or bury it without access for inspection, you will have a problem at the first malfunction. Always install only in an inspection shaft with a cover.
Incorrect pipe slope: The sewer pipe must have the correct slope (minimum 1–2 % towards the network, ideally 2–3 %). A flap installed in a place where the pipe does not have sufficient slope may not open properly under normal conditions or remain partially closed. A detailed installation procedure can be found in the article Installation of a sewage check valve step by step.
Combination of both types – when it is the right choice
In practice, we also encounter situations where it makes sense to use both types of flaps at the same time within one system. For example: a single-family house has a basement with a pump for pumping rainwater from a catch basin. The pump has a discharge pipe with an end flap at the outlet (protection against backflow after the pump is turned off). At the same time, the entire house connection is protected by an intermediate flap in an inspection shaft near the fence, so that backflow from the sewer does not threaten the basement or the catch basin itself.
Such double protection is not excessive – on the contrary, each flap protects against a different type of backflow and together they form a comprehensive protection. The cost of both flaps is a fraction of the possible damage from a basement flood.
Most Common Mistakes in Selection – Summary from Practice
After years of working with these systems, it is possible to summarize the recurring mistakes in selection:
- Confusing through-flow and end-of-line flap: The customer buys an end-of-line flap intending to connect it into a through-flow pipe – it physically does not fit, or an improvised solution is used that does not seal properly.
- Wrong DN diameter: Ordering DN 110 for a DN 125 pipe or vice versa. The pipe either does not fit, or it fits loosely and the sealing does not last long-term.
- Installation without access for inspection: A flap installed without a manhole and cover. In the case of the first blockage by debris, there is a problem – you will have to dig.
- Installation in a frost-prone area without thermal protection: A flap installed in shallow depth without insulation will freeze and not seal, or will be physically damaged.
- Neglecting maintenance: A flap installed and then forgotten. Once a year (ideally before the autumn flood season), you should visually inspect and clean the flap from sediment. The procedure can be found in the article Maintenance and Cleaning of a Sewer Backflow Flap.
Legal and Regulatory Aspects – When is a Flap Mandatory?
Although we dedicate a separate article to this topic (Protection of the House Against Backflow of Sewage – When is a Flap Mandatory?), briefly: STN EN 13564 regulates flood protection devices for buildings and in many cases their installation follows directly from the project documentation or from the conditions of the building permit. In risk areas (low-lying buildings, historically flooded locations), the installation of a backflow flap is essentially mandatory for newly issued building permits. Even during the reconstruction of older buildings, it is advisable to check with the sewerage operator whether your area has specific requirements.
Most Frequently Asked Questions (FAQ)
Can I use a through-flow flap instead of an end-of-line flap at the outlet of a drainage pump?
This is not a correct solution. A through-flow flap requires pipe connection from both sides and must be installed in a closed environment of a manhole. At a free outlet (the end of a pipe that hangs or discharges into a ditch), an end-of-line flap is exclusively intended. If you were to install a through-flow flap there, the second opening would be uncovered and non-functional – water would flow through this point uncontrollably.
How often should a through-flow flap in a manhole be inspected per year?
At least once a year, ideally twice – in spring after the winter season and in autumn before the period of heavy rain. During each inspection, lift the flap plate by hand (or with a rod), check if it moves freely, and inspect the sealing and the surface of the plate. If it is significantly dirty, clean it with a water jet or a soft sponge. A detailed procedure can be found in the article Maintenance and Cleaning of a Sewer Backflow Flap.
What should I do if a DN 110 flap is jammed and does not allow even normal drainage?
This is an emergency situation that needs to be resolved as soon as possible – if the flap does not allow drainage, your home sewer system is practically non-functional. Lift the flap plate manually (with a rod or a gloved hand) through the open manhole cover and try to determine what is blocking it. Most often it is fibers (towels, damp cloths), pieces of fat or solid matter. Remove the debris, move the plate several times, and check the sealing. If the plate is physically damaged, the flap must be replaced. More about the causes can be found in the article Common Flap Failures – Why the Flap Does Not Seal or Gets Stuck.
What is the maximum pressure a through-flow sewer flap can withstand during backflow?
Standard PE through-flow flaps for household connections are dimensioned for a hydrostatic pressure corresponding to a water column height of 3 to 5 meters (0.3–0.5 bar), which fully covers real pressure conditions during backflow from the sewer system even in case of strong clogging. For industrial applications or pressurized sewer systems, there are special types with higher pressure ratings – but these are outside the scope of typical household connections.
Can I install a through-flow flap in a vertical (rising) pipe?
The majority of standard through-flow sewer flaps are designed for horizontal installation. Some products are marked as suitable for vertical installation – in such cases, the manufacturer explicitly states this. Do not install a flap in a vertical pipe without such a marking: the plate may not open or close reliably, as its gravitational behavior depends on the horizontal position of the pivot axis. Always read the installation instructions for the specific product.
Is it better to install the flap as close to the house or to the network as possible?
The recommended position is as close to the public network as possible – ideally directly before the house connection enters the main collector, in a manhole at the property boundary. The reason: if you install the flap too close to the house and the pipe between the flap and the network is filled with backflow, there is a risk of overpressure on the flap from a long pipe section. The closer to the sewer, the shorter the pipe section exposed to backflow before the flap closes.
Conclusion: the decision is not complicated, but it must be correct
The choice between a through-flow and an end-of-line sewer backflow flap is not technically complicated – but it must be well thought out. A through-flow flap belongs in the through-flow pipe and inspection manhole, where it protects the entire home sewer system from backflow from the public network. An end-of-line flap belongs at the end of a pump discharge pipe or at the outlet of a drainage system, where it prevents backflow after the pump is turned off or when the water level in the recipient is high.
If you are dealing with standard protection of a residential house sewer connection, choose a through-flow flap DN 110 or through-flow flap DN 125 according to the diameter of your connection, install it in a manhole, and ensure regular access via a cover of the correct size. It is an investment that pays off during the first heavy rain – and a protection you will be glad you never need, but grateful you have.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.