Backflow protection: when do I need a check valve in the shaft?

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Backflow protection: when do I need a check valve in a manhole?

A check valve in a manhole is a topic that at first glance seems technically trivial – after all, it is just a simple mechanical component that allows water to flow in one direction and closes in the opposite direction. Yet precisely this apparent simplicity leads to it being overlooked in practice, its importance underestimated, and in many cases it is installed incorrectly or not installed at all. The consequences are often much more expensive than the valve itself: flooded basement, damaged equipment, sewage overflow in the basement or failure of the pumping system. This article will thoroughly examine the topic – from the physical principle of backflow through specific practical situations to technical parameters for selecting the right valve for a specific type of manhole.

What is backflow and why does it occur?

Backflow is a phenomenon in which water (or wastewater) flows in the opposite direction in a piping system than intended. In the case of sewerage, this means that sewage or rainwater flows back from the public sewer network into the connection pipe, and from there into the house, basement or manhole. In the case of potable water supply, backflow may mean that water from the system "flows back" into the well or into the public water supply, causing problems with contamination or pressure loss.

The main causes of backflow are:

Overloading of the sewer network – during heavy rain, the sewer becomes filled beyond capacity and the pressure in the pipe reverses. This is by far the most common scenario in practice.
Pump failure in the pumping station – when the pump stops working, the column of water in the pressure pipe returns back into the manhole or tank by gravity.
Water hammer (hydraulic shock) – sudden pump stop creates a pressure wave that can cause backflow even during normal operation.
Incorrect connection of gravity drainage – if gravity drainage discharges into a sewer that is under pressure during a flood, backflow is inevitable without protection.
Siphon effect – under negative pressure in the pipe (e.g. during rapid emptying of a large amount of water), a vacuum can be created that draws water back in.

Where in water supply and sewerage practice does backflow actually pose a real threat?

From my own experience, I can say that customers greatly underestimate the places where backflow actually poses a threat. It is not only large floods – a large part of failures occur during a regular 30–60 minute rain, if the sewer is old or designed for smaller flows. Let's look at specific scenarios:

1. Inspection manhole with gravity discharge into the sewer

This is a classic case. An inspection manhole is used to connect the house sewer to the public sewer network. If the bottom of the manhole is lower or at the same level as the water level in the sewer during a flood, water will flow back. Practice shows that most older built-up areas have inspection manholes without any protection, and homeowners discover this only during the first major storm.

The solution is bottom of the manhole with check valve 400/160 – inspection manhole for sewerage, which integrates the valve mechanism directly into the bottom of the manhole. This solution is structurally cleaner than an additional valve installed in the pipe, as it eliminates the risk of being overlooked during installation and at the same time allows visual inspection and possible manual closure of the valve.

2. Pumping sewer manhole – pump failure

In the case of pumping manholes, the situation is different. Backflow does not threaten from the public network here, but from the discharge pipe of the pump itself. When the pump stops (planned stop, power outage, failure), the column of water in the pressure pipe returns back into the manhole by gravity. With a longer discharge pipe (e.g. 5–10 meters or more), this volume can represent dozens of liters, unnecessarily overloading the manhole and triggering the pump unnecessarily. Worse is when water hammer occurs – a sudden backflow can damage the pipe or the pump itself.

Pumping manholes such as pumping sewer manhole MIDI 1600x1000 or pumping sewer manhole MIDI 1300x1000 are typically equipped with a discharge pipe with an integrated check valve or with a flanged neck for its installation. This is a standard part of a complete assembly – without a check valve, the pumping station could not function reliably.

3. Water meter manhole and protection of potable water supply

This situation is specific. Water meter manholes primarily do not serve to protect against sewage backflow – they are objects for measuring potable water consumption from the supply network. Despite this, a check valve can also be relevant here in two cases:

Protection against backflow into the public water supply – the standard STN EN 1717 requires that every potable water installation be protected against backflow of contaminated water into the public network. A check valve behind the water meter is mandatory or strongly recommended in these cases.
Protection of the pump in a home water treatment system – if a home water treatment system with a pressure tank is connected to the water supply or well, a check valve on the suction pipe prevents the siphon effect and loss of pre-pressure in the system.

For water meter manholes, such as circular PP 1300x1000 plastic self-supporting water meter manhole, the valve is usually installed on the outlet neck of the manhole behind the water meter. For a larger diameter, such as circular PP 1600x1000 plastic self-supporting water meter manhole, there is enough space to install a more extensive valve group including a valve, filter and pressure regulator.

Technical types of check valves: what exists and what to choose?

Check valves are not just one thing – there are several structural types, each suitable for different conditions. Their substitution can lead to malfunction or premature wear.

Ball valve (ball check valve)

The closing element is a ball, which rolls over and seals the seat when reverse flow occurs. A very simple design, minimal maintenance. Suitable for flush water with solid particles, as the ball is not sensitive to small impurities. Disadvantage: requires a certain flow to open (higher head loss). Used mainly in sewer systems.

Swing flap valve (flap check valve)

A classic design with a rotating plate on an axis. It opens under the pressure of flowing water, and when the flow stops, it closes by gravity or a spring. Suitable for larger diameters (DN 100, DN 150, DN 200 and more). More sensitive to impurities; if the wastewater contains coarse sludge, the seal can become clogged and the valve may not close tightly. This type is most commonly used in pumping units.

Spring-loaded valve (quiet check valve)

Closes faster than a gravity valve, thus reducing water hammer. Ideal for systems with frequent pump starts or in discharge pipes with a higher elevation (over 5 meters). The spring increases resistance to flow, which must be considered in pump sizing.

Integrated valve in the bottom of the manhole

A special category – the valve is built directly into the plastic bottom of the inspection manhole. The advantage is that it forms one compact element without the risk of incorrect installation. An example is the manhole bottom with check valve 400/160, where the valve is part of the inlet-outlet neck of the manhole with a diameter of DN 160. This solution is particularly practical in new constructions or renovations, where the manhole is completely replaced.

When is a check valve mandatory by law and when is it only recommended?

In Slovakia, protection against backflow in sewer systems is regulated by several laws. The most important is Act No. 442/2002 Coll. on public water supply and public sewerage, which imposes an obligation to prevent the public network from being endangered by backflow. Technical requirements are specified by STN EN 13564 (protective devices against backflow for flush water) and STN EN 1717 (protection of potable water from contamination by backflow).

In practice, the following applies:

Mandatory: A check valve (or other certified protective device) is mandatory for each connection of a house sewer to the public sewer network if there is a real risk of backflow (typically for basement drainage devices located below the level of the sewer base).
Mandatory: Protection against back-siphonage in potable water (STN EN 1717) – every device where contamination of the public network by backflow is a risk must have protection.
Strongly recommended: A check valve on the discharge pipe of a pumping station – not always explicitly legal, but it is part of a correct technical solution and pump manufacturers require it to maintain warranty.
Recommended: Inspection manholes in areas with historically overloaded sewer systems or in flood-prone areas.

In all cases, if you are unsure, the law requires you to consult the connection with the operator of the public sewer system. They have the capacity assessment of the sewer and can determine whether backflow is a real risk in a specific location.

Backflow valve sizing: what diameter and pressure class?

The check valve must be correctly sized for the pipe diameter and expected flow. Under-sizing causes excessive pressure losses (the pump works unnecessarily harder, increasing consumption and wear), while over-sizing causes the valve to open only partially at low flows and the sealing surface is unevenly loaded.

Basic sizing rules:

Valve DN = Pipe DN. This is the basis – never reduce the valve diameter compared to the pipe for "savings".
For residential house sewer connections, typical diameters are DN 100 or DN 150. For industrial buildings and larger apartment blocks, DN 200 and above.
For discharge pipes of pumping stations, work with DN 50 to DN 100 depending on the pump performance.
Pressure class: For gravity sewer systems, PN 4 (pressure up to 4 bar) is sufficient. For pressure systems (pump discharge), choose at least PN 10, and for deep wells and strong pumps, PN 16.
Material: For blackwater, use PP or PVC valves with rubber seals. For potable water, use valves certified for contact with drinking water (marked KTW or ACS).

A special issue is the placement of the valve in vertical or horizontal piping. Not every type of valve works in both positions – ball valves are generally position-independent, while flap valves with gravity closure only work in horizontal piping or with a special orientation of the axis. Always read the manufacturer's technical sheet!

Most common installation and operation errors with check valves

Over the years of working with customers, I have seen dozens of cases where the check valve was installed but did not work because something was neglected. Here are the most common errors:

Installation in an inaccessible location

The check valve must be accessible for inspection and cleaning. Customers sometimes install it directly into a concrete foundation or into an inaccessible section of pipe, justifying it by saying "nothing will ever happen there". After a year of operation, the seat gets clogged with sludge, the valve no longer seals, and the flood occurs. A manhole, such as a maintenance one, is precisely designed for this purpose – the valve is accessible from above, through a cover.

Installation "backwards" (wrong orientation)

Each valve has an indicated flow direction (arrow on the body). Carelessness or haste during installation causes the valve to block normal drainage and water to rise in the system. This seemingly trivial error occurs far more often than you might think during the stress of installation.

No inspection opening for valves in a row

If the valve is installed in a long section of pipe without an inspection T-piece or manhole, its cleaning requires disassembling the entire section. Always design the system so that the valve is accessible without breaking anything.

Forgetting manual closure for valves in a manhole

Some valves with built-in manual closure allow manual closing before an expected flood (e.g., before a storm). If the customer is unaware of this feature or the valve is not accessible, this option is useless.

Combining the valve with an incorrect pipe slope

Gravity sewer systems must have the correct slope (typically 1–3 % depending on the diameter). If the slope is too small, water stagnates in the pipe and sludge settles just before the valve, causing it to jam in the open position.

Regular maintenance and inspection of the check valve

The check valve is not a device to forget about. Its proper function depends on whether the seat is clean, the seal is undamaged, and the moving parts are free. Recommended inspection intervals:

Once a year – visual inspection (open the manhole cover, check the condition of the valve, possibly manually rotate or lift the closure).
Every 2–3 years – physical disassembly, clean the seat and body of the valve, check the seal.
After each major flood – unconditionally check whether the valve functioned (was not clogged and sealed properly). Floodwater carries fats, fibers, and coarse particles that can damage the valve.
Before winter – for manholes where freezing of the mechanism is a risk (surface or shallow manholes), check the temperature in the manhole and possibly add thermal insulation to the cover.

For topics on the correct placement of manholes and their maintenance, please refer to our Knowledge Center articles Manhole burial depth: how to correctly place the manhole below the frost level and Common manhole and pumping unit failures: causes and solutions.

Combining the check valve with other protective elements

The check valve is strong, but it is not always sufficient on its own. In practice, it is combined with other measures:

Check valve + pumping station

In basements where gravity drainage is not possible (the basement floor is lower than the sewer), the primary protection becomes the pumping station – all drainage devices are connected to the pumping manhole, from which the pump discharges the wastewater above the sewer level. The check valve on the discharge then prevents backflow from the pressure pipe. This is the safest and most reliable solution for deep basements where flooding is a recurring risk. More about this solution can be found in the article Installation of a pumping sewer manhole: what to know before installation.

Check Valve + Odors (Trap)

A check valve does not prevent the entry of sewer gases (hydrogen sulfide, methane) – this is the purpose of traps with a water seal. In inspection chambers with a check valve, it is always necessary to check whether all inlet pipes are equipped with traps. Otherwise, during long-term operation of the valve without flow, odors may penetrate through the valve seat into the house.

Backup Valve

In very sensitive buildings (archives, server rooms, valuable cellars), two valves are installed in series. The probability of simultaneous failure of both is significantly lower. Such a solution requires an appropriately sized chamber and inspection access to each valve separately.

Practical Examples and Experience from Practice

Example No. 1: A customer in Bratislava, an older single-family house, cellar 1.2 m below ground level. The sewerage was drained directly by gravity into the sewer through an inspection chamber without a check valve. During a summer storm (110 mm of rain in 40 minutes), the sewer could not drain the water fast enough, the pressure reversed, and about 15 cm of wastewater entered the cellar through the connection. Damage: water heaters, electrical installation, and stored furniture were ruined. The solution we proposed: replacement of the chamber base with a base with a check valve and addition of a sump with a submersible pump as a backup. The cost of this measure was a fraction of the resulting damage.

Example No. 2: New construction, single-family house with a pumping sewer chamber. The pump was installed, but the discharge pipe did not have a check valve. Result: after each pump shutdown, the water column (4.5 m of discharge pipe, DN 50 diameter) returned by gravity. The pump therefore immediately started again, briefly operated, and then stopped – so-called pendulum operation. The pump bearings lasted 8 months instead of the usual 5–7 years. Installation of a simple spring-loaded check valve DN 50 completely solved the problem.

Example No. 3: Water meter chamber at a garden water supply with its own well. The customer did not install a check valve on the pump suction pipe. Each time there was a pressure drop in the water supply, the pump lost its prime and, upon restart, had to start up "dry" – pumping air instead of water. Result: pump overheating and repeated failures. A simple self-closing check valve on the suction pipe completely eliminated the problem.

We also thoroughly cover the topic of selecting the correct type of chamber in the article How to Choose a Chamber: Water Meter, Inspection or Pumping?, where you will find a comprehensive overview of the entire category of chambers including their functions and differences.

Where to Buy and What to Check When Choosing?

When selecting a check valve and chamber, we recommend always:

Verifying that the valve is certified according to the relevant European standard (EN 13564 for sewage, EN 1717 for potable water).
Checking material compatibility with the medium (PP and PVC for sewage, brass/stainless steel for potable water).
Finding out the availability of replacement seals and balls/plates – some cheaper products do not have available spare parts and must be completely replaced after 3–5 years.
For integrated solutions (chamber base with a valve), verifying the dimensional standard of the necks to match the used pipe (SN 8, SN 10, DN 100, DN 160, etc.).

Frequently Asked Questions (FAQ)

Do I need a check valve if my house is on flat ground and the sewer is deep under the house?

If the outlet of your sewer connection (the point of connection to the sewer) is significantly higher than the maximum water level in the sewer during a flood, the risk of backflow is low and a valve is not mandatory. In most cases, this can only be confirmed by consulting with the public sewer operator, who has hydrological data. If you have any doubts, a check valve is a cheap insurance – its installation will cost you significantly less than cleaning up after a flood.

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

It depends on the specific solution. An integrated chamber base with a check valve, such as chamber base with a check valve 400/160, is structurally simple and a skilled DIY enthusiast can handle it. More complex cases (cutting an existing concrete pipe, connecting multiple systems, valves in pressure pipes) require a professional, ideally certified for sewer work. Improper installation of a valve is worse than no valve at all – it can create a false sense of security, while the valve is actually not functioning.

What is the lifespan of a check valve and when should it be replaced?

High-quality PP and PVC valves for gravity sewerage have a lifespan of 15–20 years with regular maintenance. Rubber seals (seat seal, gasket) require replacement every 5–8 years depending on the frequency of use and the aggressiveness of the medium. Brass or stainless steel valves for potable water last 20–30 years. Valves in pumping systems are stressed more (repeated shocks when the pump is started) and should be inspected annually.

The check valve is complicating the drainage – water is stagnating in front of the valve. What am I doing wrong?

The most common reason is an insufficient slope of the sewer pipe before the valve (less than 1%), or a valve with too large an opening (pressure loss when opening). Check the slope of the pipe and increase it to at least 1.5–2% if necessary. Another possibility is sediment buildup on the valve seat, which prevents it from opening fully – the valve needs to be cleaned. If the problems persist, consider replacing it with a type of valve with a lower pressure loss (e.g., ball instead of flap).

Can a check valve freeze in winter?

Yes, if the chamber is not sufficiently buried or poorly thermally insulated. A frozen valve may remain in the open position (losing its protective function) or in the closed position (blocking drainage). Therefore, it is crucial to properly bury the chamber below the frost line – in Slovakia, this ranges from 80 cm in the south to 120 cm in the north and at higher altitudes. Details on burial depth can be found in the article Chamber burial depth: how to correctly place the chamber below the frost level.

Do I need a check valve for the rainwater sewer?

If the rainwater sewer is connected to a combined (separate) sewer, the rules are the same as for the foul sewer – a valve is needed if the inlet or outlet is below the risk water level in the sewer. If it is a separate rainwater sewer or connected to a watercourse (stream, river), the situation is assessed individually. For garden pond-like reservoirs connected to a rainwater sewer, a check valve on the waste pipe protects the reservoir from dirty water from the sewer during a flood.

Conclusion: A check valve is a cheap insurance with great value

A check valve is a component that reliably protects your home from one of the most unpleasant failures – a flood of wastewater – when the correct type is chosen, installed in the right place, and maintained regularly. The value of this protection is much greater than the cost of the product itself. On the contrary, its absence or incorrect installation can lead to damages in the thousands to tens of thousands of euros.

If you are dealing with a new inspection chamber, adding protection to an existing system, or a complete pumping solution for a basement sewer, all relevant products can be found in the category Chambers and Tanks. If you need help with your selection, do not hesitate to contact us – each situation is different and the correct choice depends on the specific conditions of your connection, sewer depth, and type of construction.

Do you have a question about this topic?

Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.