how to choose the correct kvs for a heating valve

We are still preparing products.

But you can look at other categories.

Choosing the correct kvs value for a heating valve is a key factor for the stability of the entire system and the efficiency of temperature regulation. If you choose a valve with too high a flow capacity, the system will be unstable and the so-called "servo motor hunting" will occur more frequently, while an insufficient value will result in underheating of the rooms.

Understanding the essence of the kvs value

The kvs value represents the volume of water in cubic meters per hour that flows through the valve at full opening with a pressure drop of 1 bar (0.1 MPa). This parameter is not just a technical specification on the label, but a direct measure of the valve's ability to deliver the required amount of heat to a specific circuit. In practice, this means that a valve with a lower kvs will not transfer enough heat even when fully open, if the design thermal output of the circuit is higher.

Calculating the required flow according to thermal output

The design begins with calculating the thermal output that the circuit must deliver, taking into account the temperature difference between the supply and return (delta T). For standard radiator systems, a delta T of 20 °C is often used, while for floor heating it is typically 5 °C, which results in significantly higher flows for the same output. An incorrect assumption of this difference is the most common mistake when selecting the valve size.

Calculate the thermal output in kilowatts (kW) for the given circuit.
Choose the relevant delta T (usually 10 to 20 °C for radiators).
Use the formula: Kvs = P / (0.86 * ΔT), where P is the output in kW.
Always choose a valve with a kvs slightly higher than the calculated value to have a reserve.

Consequences of incorrect valve selection

If you choose a valve with an excessively high kvs value, for example 10.6 instead of the required 6, the valve will have to open only by a few percent to deliver the required heat. This means that a small change in the seat position will cause a huge change in flow, which forces the actuator to constantly change position and creates system instability. This condition is called hysteresis or hunting, which significantly shortens the life of the actuator and reduces indoor comfort.

On the contrary, if the kvs value is too low, the valve will never open sufficiently to deliver the required heat, and the room will remain cold even under maximum load. An example could be the installation of a EUROMIX F4 1" valve with kvs=10 into a small circuit, where a model with kvs=6 would be sufficient, which would lead to inefficient regulation and excessive wear of the mechanism.

Connection with pipe size and DN

Although the kvs value is a priority, it must be in line with the nominal diameter DN of the valve and the pipe through which the water flows. There are various combinations where the same DN diameter can have different kvs values depending on the type of seat and the shape of the valve, but generally, a larger diameter allows for a higher flow. For example, a ROTODIVERT DN20 valve has a different capacity than a ROTODIVERT DN25, with the larger diameter being suitable for main lines or long floor heating circuits.

It is important not to confuse the pipe diameter with the kvs value; you can have a large pipe, but if the valve is narrow, it becomes a bottleneck. When selecting, always check the kvs table for the specific model, for example, the EUROMIX F4 5/4" with kvs=10.6, which is ideal for medium-sized circuits, while the 3/4" model with kvs=6 is intended for shorter branches or smaller rooms.

Adjustment of kvs in practice and control characteristics

In real conditions, the kvs value of a valve can vary depending on whether it is a valve with a linear or equal percentage characteristic. A valve with an equal percentage characteristic provides a more stable flow rate in relation to the opening, which makes it easier to set the actuator and reduces the risk of hunting. Therefore, when selecting, it is necessary to consider not only the pure kvs value, but also the quality of manufacturing and the type of seat, which affect the accuracy of regulation.

Technicians often use a valve with a slightly higher kvs value and then limit it with an adjustable seat or a thermostatic head, if possible. However, this method works only to a certain extent, and if the difference between the designed and actual kvs is too large, it is better to replace the valve with the correct model, for example, changing the installation from kvs=10 to kvs=6, to achieve an optimal control curve.

The role of hydraulic balancing and kvs

The majority of installations require setting the flow in each circuit, which is solved by an integrated or separate control valve. Choosing the correct kvs value is a prerequisite for having a sufficient control range (usually 1:50 or 1:100) and avoiding the need to constantly "drill" the valve to its maximum opening.

Frequently asked questions

How does the calculation differ for floor heating?

For floor heating, a smaller delta T (usually 5 °C) is used, which means that for the same thermal output, you will need twice to three times the water flow, and therefore a valve with a higher kvs value.

Can I use a valve with a higher kvs and limit it?

Theoretically yes, but it is advisable to avoid this, as the valve will operate in an extremely small opening range, leading to unstable regulation and faster wear of the actuator.

How can I find out if my valve is jammed?

Regular maintenance and replacement of seals in the EUROMIX valve are key, but if the valve jams, it is often caused by an inappropriate choice of kvs, which forces the valve to operate in extreme positions, or by deposits in the system.

Conclusion

Selecting the correct kvs value is a fundamental step that determines the efficiency and lifespan of the entire heating system. An accurate calculation based on thermal output and delta T will help you avoid instability issues and ensure the long-term functionality of your installation.

Do you have a question on this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.