difference between a check valve and a non-return valve
Introduction: Why Distinguishing Check Valves and Non-Return Valves is Critical for Heating System Safety
In the field of plumbing and heating technology, we often encounter situations where a customer or even an inexperienced technician says: "I need a check valve for water," or conversely, "Give me a non-return valve for the boiler." At first glance, it may seem like the same thing. Both devices have one goal: to prevent backflow of the medium. However, if we imagine two different solutions for the same problem, it would be like choosing between a heavy truck and a sports motorcycle — both are vehicles, both drive on the road, but their use is completely different and interchangeability can lead to catastrophic consequences.
Within the category of ball valves and sanitary fittings on our e-shop atria.sk, these two types of devices are found next to each other, but their functional principles, construction, maintenance, and especially their placement in the system differ significantly. The difference is not only in the name, but in what happens inside the body of the valve, what pressure it can withstand, what flow rate (Kv) it allows, and what happens if a phenomenon called hydraulic shock occurs.
The goal of this article is not only to define terms, but to provide you with a deeper technical perspective based on real-life experience from installation sites. We will focus not only on theoretical differences, but also on practical cases where the wrong choice between a non-return valve and a check valve led to pipe rupture, pump damage, or disruption of the system's thermal insulation. You will understand why different types are used in small domestic systems than in industrial networks, why the direction of flow is important, what material to use, and how to properly size the valve so that it does not function as a flow restrictor, which would reduce the efficiency of the entire heating system.
This text serves as a comprehensive guide for those who want to understand what they are installing. From the average homeowner to the certified plumber, everyone should know whether they need a light spring-loaded non-return valve or a robust two-way check valve with a counterweight. The right choice means a long system lifespan, quiet operation, and prevention of costly repairs.
Functional Principle: How the Anti-Backflow Mechanism Works
To understand the essential difference, we must look inside these valves. Both share the function of one-way flow, but the way they realize this function is diametrically different. The difference lies in the closing mechanism and the force that activates it.
Non-Return Valve: The Precision of a Spring and Small Movement
The non-return valve (often also called a plug valve or check valve) operates on the principle of spring closure. Its core is a conical or spherical element (the plug), which is constantly pressed against the seat by a spring. When water begins to flow in the correct direction, the pressure of the medium exceeds the spring force and pushes the plug into the open position. As soon as the flow stops or the direction changes, the spring immediately and quickly pushes the plug back onto the seat.
The key property of a non-return valve is its sensitivity. Since the spring is designed to resist only minimal pressure, the valve closes almost instantly after the flow stops. This is critical in systems where it is necessary to prevent even the smallest backflow, for example in drinking water, where reverse flow could cause contamination, or in systems with a high risk of hydraulic shock.
Most modern non-return valves for domestic and commercial heating systems have what is called a "light" design. This means they use fine springs and light plugs that react to very small pressure differences. For example, models we offer in our range, such as non-return valve EURA light - 1/2"FF; Kv 4,27, are designed to function in low-pressure systems, where fast response and minimal pressure loss are important. These valves are ideal for installation after pumps, where they protect the pump from reverse rotation when turned off, thus preventing mechanical damage to the bearings and shaft.
Check Valve: Utilizing Gravity, Weight, and Moment of Force
The check valve (or flap check valve) operates on a completely different principle. Its main component is a flap (disk), which is mounted on an axis and can rotate. The flap is either self-balanced or equipped with a counterweight or spring. When the flow is in the correct direction, the flap tilts away from the axis and opens the passage. When the flow stops or the direction changes, the flap returns to the closed position.
The difference from the valve is that the closing moment is not generated only by the spring, but often also by gravitational force (if the valve is installed horizontally and the flap is heavy) or a combination of gravity and spring. Check valves are designed for larger diameters and higher flows. Their construction allows the flap to open and close more smoothly, which reduces shocks in the system if properly dimensioned.
In practice, check valves are often used in industrial networks, in sewage systems, or in large heating systems, where a small spring of a non-return valve would not be sufficient to overcome the flow resistance or would be too prone to wear. A large flap valve has lower hydraulic resistance in the fully open position, which means less pressure loss at high flows. However, its response time is slower than that of a spring valve, which can be a disadvantage in systems sensitive to water hammer.
Figure 1: Schematic comparison of the construction of a check valve (left) and check flap (right). Note the difference in the closing mechanism – spring and plug vs. a wing on an axis.
Hydraulic parameters and pressure loss: Which type is suitable for your system?
One of the most common mistakes when selecting a valve is ignoring the parameters Kv (volumetric flow) and pressure loss. These parameters determine how much water will flow through the valve at a given pressure difference and what impact the valve will have on the overall efficiency of the pump.
Pressure loss in check valves
Check valves tend to create higher pressure loss compared to check flaps of the same size. The reason is the shape of the passage. In order to function, the valve must have a narrowed space through which water flows, and a plug element that moves perpendicular to the flow direction. This movement and change in flow direction create turbulence and resistance.
In practice, this means that if you install a check valve at a great distance from the pump or in a system with low pressure, you may notice that the pump has to work harder. This is why small check valves with a low Kv value are often used in small systems, such as individual radiator circuits. For example, the EURA light check valve - 3/4"FF; Kv 7,11 is designed to provide sufficient flow at a 3/4 inch diameter while maintaining high spring sensitivity. A Kv value of 7.11 means that at a pressure difference of 1 bar, 7.11 m³ of water will flow through the valve per hour. This is sufficient for a typical family home, but in larger systems, this value could be limiting.
Pressure loss in check flaps
Check flaps, especially those with a wing, have a direct passage in the fully open position, which is often similar to the pipe diameter. The wing swings to the side and creates minimal resistance. This means that check flaps are preferred in high-flow systems, where it is important to minimize pressure loss. Therefore, flaps are often selected in industrial applications or in large heating systems with multiple branches.
However, there is one critical aspect: if the check flap is oversized for the given flow, the wing may not open fully or may sway freely, which can lead to vibrations and noise. Conversely, if it is too small, it will create high resistance. Therefore, precise calculation and sizing are essential for flaps.
Comparative table of hydraulic parameters
| Parameter | Check valve (Spring-loaded) | Check flap (Wing type) |
|---|---|---|
| Response time | Instant (milliseconds) | Relatively slow (seconds) |
| Pressure loss | Moderate to high (depends on type) | Low (with proper sizing) |
| Suitable diameters | Small to medium (up to DN 50) | Medium to large (DN 50 and above) |
| Noise when closing | Low (if the spring is of good quality) | High (risk of "drumming") |
| Installation length | Shorter | Longer (depends on type) |
Material aspects and resistance to the environment
Material selection is another key factor that differentiates these two types of valves. The material affects not only corrosion resistance but also thermal resistance and compatibility with other parts of the system.
Check valves: Commonly used materials
For check valves, brass, stainless steel (inox), and plastic (PP, PVC) are commonly used in standard practice. Brass valves are the standard for domestic installations due to their good mechanical properties and corrosion resistance. Inox valves are used in extreme conditions, for example in the chemical industry or in systems with distilled water, where maximum purity is required.
In our products, you will find ball valves with drain, which are often installed near check valves. These valves are made of brass and have a lever, which indicates that the valves in this category are often made of brass. Brass is ideal for temperatures up to 120 °C and pressures up to 16 bar, which covers most standard heating systems.
Check flaps: Variety of materials
Check flaps are available in an even wider range of materials. In addition to brass and inox, cast iron, plastic, and even special polymers for aggressive media are often used. Cast iron flaps are typical for industrial applications and sewage. Plastic flaps are used in coolant water or in systems with low pressure.
An important aspect is also the material of the wing. It can be metallic (brass, inox) or plastic. A metallic wing is more resistant to high temperatures, but it can be noisier when closing. A plastic wing is quieter, but it can deform at high temperatures.
Installation requirements and flow direction
One of the most common installation errors is incorrect flow orientation. Both types of valves have a specified flow direction, which is usually indicated by an arrow on the body. If the valve is installed backwards, no protection will occur and it may damage the system.
Installation of check valves
Check valves are usually compact and their installation is simple. They should be installed in a horizontal position with the spring facing upwards, if possible, to prevent dirt from settling on the spring. In some cases, it is also possible to install them vertically, but always according to the manufacturer's instructions.
In practice, we often encounter the situation where check valves are installed directly after the pump. This is a critical location, because if water starts to flow back, it could cause the pump to rotate backwards, which could damage the bearings and shaft. Therefore, it is important that the valve reacts immediately.
Installation of check flaps
Check flaps are more sensitive to orientation. In horizontal installation, the flap must be installed so that the wing falls under the influence of gravity into the closed position. If the flap is installed horizontally without a counterweight, it may happen that at low flow the wing does not close completely. Therefore, for horizontal installations, flaps with a spring or counterweight are often used.
In vertical installation, it is important that the flow is from bottom to top so that the wing can open. If the flow is from top to bottom, the wing will close automatically and block the flow.
Figure 2: Schematic of correct check valve installation. Note that the spring must point upward even in vertical installation to prevent settling of impurities. Incorrect installation (flow from top to bottom) leads to blocking.
Practical scenarios from practice: When to use what?
Theory is nice, but practice shows the truth. Let's look at a few real cases from the installation practice, where the decision was made whether to use a check valve or a check flap.
Case 1: Home boiler room with a gas boiler
The customer had a new gas boiler and the plumber installed a check flap after the pump. After a few weeks, noise and vibrations started to appear. The cause was that the check flap was not sensitive enough and, when the boiler was turned off, the flap closed with a delay, which caused a hydraulic shock. The solution was to replace the flap with check valve EURA light - 1/2"FF; Kv 4,27. The valve reacted immediately, the noise stopped and the system operated quietly.
Lesson: In small systems with sensitive components (such as gas boilers), it is better to use a spring-loaded check valve.
Case 2: Large apartment building with radiator heating
In this case, it was a system with high flow and large distribution. The plumber used small check valves, which created high pressure loss. As a result, the pump did not have enough power to achieve the required flow to the distant radiators. The solution was to replace the valves with larger check flaps with low resistance, which allowed the required flow without increasing the pump power.
Lesson: In large systems with high flow, check flaps are more advantageous due to lower pressure losses.
Case 3: Drinking water and disinfection
In a drinking water system, it is critical to prevent the backflow of contaminated water. Check flaps are not suitable here because they may have gaps or be less hermetic. A pair of check valves with a gap (so-called double check valve) was used, which ensured maximum safety.
Lesson: For drinking water, check valves are always suitable, not flaps.
Maintenance and fault diagnosis
Both valves require certain maintenance, but the frequency and type of intervention differ.
Maintenance of check valves
Check valves are prone to the settling of impurities on the spring or seat. If the valve starts to leak back or gets stuck, it is often sufficient to clean the spring and the plug. In some cases, it is necessary to replace the spring if it is corroded or has lost its elasticity.
A common fault is also "chattering", which can be caused by excessive stiffness of the spring or incorrect pressure in the system.
Maintenance of check flaps
Check flaps are prone to the settling of impurities on the flap axis. If the flap gets stuck, the system may be blocked. Maintenance involves cleaning the axis and lubrication. In some cases, it is necessary to replace the entire flap if it is damaged.
Another common fault is "chattering" of the flap, which can be caused by excessive weight of the flap or incorrect spring setting.
Connection with other valves
It is important to remember that check valves and flaps are often installed together with other valves, such as ball valves. For example, ball valve water - with drain - 3/4"FF; lever is often installed before the check valve to allow the system to be isolated for maintenance. Without such a valve, the entire system would have to be shut down, which is impractical.
For more information on maintenance and replacement of threaded filter in the system, read our article in the Knowledge Center section. We also recommend reading about common faults and leaks in sanitary valves.
Hydraulic shock and dynamic load: Why a flap in a small system "boycotts"
In technical practice, one often encounters a problem that is not visible immediately after installation, but manifests itself only during the first sudden event - so-called hydraulic shock (water hammer). This phenomenon occurs when the flow of medium in the pipe suddenly stops or changes direction, causing a pressure wave that spreads through the entire system at the speed of sound in water. The difference between a check valve and a check flap is decisive for the safety of the entire system.
A check valve with a spring is designed to react to a change in the direction of flow in milliseconds. When the pump is turned off and the pressure drops below the critical value, the spring immediately pushes the plug onto the seat. This closes the passage before a significant reverse flow of water can occur. This quick reaction eliminates the formation of a pressure wave or at least significantly dampens it. In a system with a gas boiler or a modern pump with variable speed, this property is essential, as these devices are very sensitive to reverse rotation and pressure peaks.
On the other hand, a check flap has an inherent mechanical inertia. The flap, which is often heavy and mounted on an axis, needs time to swing into the open position and then return to the closed position. If the pump suddenly stops, water can still flow back for a while until the flap has moved sufficiently to resist the flow. During this time, a huge pressure difference is created, which hits the flap. The result is often a characteristic "chattering" sound, vibrations of the entire piping system, and in extreme cases, cracking of connections or damage to the flap itself. Therefore, in systems with a high risk of water hammer (e.g., long pipe runs, large pumps), flaps without special dampers are not recommended.
Figure 3: Comparison of reaction to sudden flow stop. Check valve (top) reacts immediately and maintains stable pressure. Check flap (bottom) reacts with a delay due to mechanical inertia, creating a dangerous pressure wave (water hammer).
Critical parameter Kv and its influence on pump sizing
One of the most common technical mistakes is the assumption that any valve with the same thread (e.g. 1 inch) will have a similar flow. This is not the case with check valves and check flaps. The key parameter is the volumetric flow rate marked as Kv, which expresses the amount of water in m³/h that flows through the valve at a pressure drop of 1 bar.
Check valves often have lower Kv values compared to check flaps of the same size. The reason is their internal construction. The flow passage diameter is often smaller than the pipe's outer diameter and the vertical movement of the plug against the flow direction creates additional hydraulic resistance. If you install a check valve with a low Kv in a system where a flap with a high Kv should be used, you must expect that your pump will have to work with higher performance to overcome this resistance. This leads to increased electricity consumption, higher pump temperatures and reduced service life.
For example, the model check valve EURA light - 1/2"FF; Kv 4,27 has a Kv value of 4.27. This means that at a maximum allowable pressure drop of 1 bar, 4.27 m³ of water will flow through it per hour. If your system requires a flow of 5 m³/h, this valve will form a significant "bottleneck" in the system. On the other hand, a check flap of the same size may have a Kv value above 6 or 7, which means it will have minimal impact on the pump's performance.
When selecting a valve, always check the manufacturer's technical data sheet and compare the Kv value with the required flow of your system. If the difference is too large, it may be necessary to use a valve of a larger size, which will again affect the installation space and cost. The correct choice is therefore not only about whether the valve works, but about how it affects the energy efficiency of the entire heating system.
Specific applications in heat distribution: The practical difference in installation and operation
To fully understand the practical significance of these differences, we must look at specific locations in the system where the choice is critical. Not every valve is suitable for every part of the heating system. The correct choice depends on whether it is the boiler's primary circuit, a floor heating distributor, or a system with multiple heat sources.
Installation behind the pump and against hydraulic shock
In standard home boiler rooms, it is common to install a check valve directly behind the circulation pump. Here, the largest pressure difference and the highest risk of impact occur when the pump is turned off. If you were to install a flap with a heavy wing here, the water would "bounce" off the wing, which would start to vibrate and generate vibrations. These vibrations are transmitted throughout the entire pipe system and can damage the pump itself or the connections. A spring-loaded check valve functions as an "electronic brake" here – it reacts immediately, without mechanical delay, thus eliminating the occurrence of a hydraulic shock.
Hydraulic balancing and distributors
In systems with multiple circuits (e.g. floor heating with multiple loops), check valves are used not only as protection but also as part of control heads. In such cases, it is important that the valve has a sufficiently precise and stable opening characteristic. A spring-loaded check valve allows for better setting of the pre-pressure, which is key to correct functional balancing of the system. Check flaps in these locations are usually not suitable, as their closing torque depends on gravity and flow velocity, which complicates the precise setting of pressure balance between individual circuits.
Common questions and answers (FAQ)
1. Can I use a check flap instead of a check valve and vice versa?
In a theoretical sense, yes, both valves prevent reverse flow. In practice, however, it depends on the specific system. A check flap may be too slow for systems with a high risk of hydraulic shock, while a check valve may create too high a pressure loss in large systems. Always follow the manufacturer's instructions for the boiler or pump.
2. Why is my check valve rattling or humming?
Noise can be caused by several factors: excessive spring stiffness, dirt on the seat, or hydraulic shock. Check whether the valve is correctly installed and whether the spring is in good condition. If the problem persists, consider replacing it with another type of valve.
3. Is the flow direction important during installation?
Yes, absolutely. Both valves have a specified flow direction, which is indicated by an arrow on the body. Installing them backwards means the valve will not function properly and may cause damage to the system.
4. How often should check-return valves and check valves be inspected?
It is recommended to perform regular inspections at least once a year, especially if the system is older or if the water is hard. The inspection should include checking whether the valve closes properly, whether there is visible corrosion, and whether there is any noise.
5. Why is a drainable check valve used?
Drainable check valves are designed to allow water to be drained from the system without having to open the entire valve. This is useful for maintenance or when draining the system.
6. What is the difference between a check valve and a ball valve?
A ball valve is used to fully open or close the flow, while a check valve automatically prevents backflow. These are two different functions, although they can be installed next to each other. For more information, read our article on how to choose the right ball valve for heating systems.
Conclusion: The right choice for long-term system performance
The difference between a check valve and a check flap is not only in the name, but also in their construction, operating principle, and suitability for specific applications. Check valves are ideal for small systems where fast response and minimal pressure loss at low flow rates are important. Check flaps, on the other hand, are preferred in large systems with high flow rates, where it is important to minimize pressure loss and ensure smooth operation.
Do not forget that choosing the right valve is key to the long-term functionality and safety of your heating system. Always check the system parameters, pressure, flow, and temperature before installation. If you are unsure, do not hesitate to contact professionals or read our other articles in the Knowledge Centre section, such as regular inspection and maintenance of sanitary valves or frequently asked questions about ball valves and check valves.
Remember that investing in high-quality valves, such as drainable ball valves or EURA check valves, always pays off in the long run. Avoid cheap alternatives that may lead to costly repairs and damage to the entire system.
Your system deserves the best protection. Choose the right valve and enjoy warmth and comfort without worries.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.