UNI 9182 Standard: Design Flow Rate Calculation for Sanitary Plumbing Systems and Domestic Hot Water

While the European standard EN 806 provides general calculation criteria, in Italy UNI 9182 remains the fundamental cornerstone for the design of sanitary plumbing systems, especially in complex contexts with a high density of draw-off points. This standard defines a precise method for determining the design flow rate, an essential parameter for the correct sizing of the sanitary plumbing system and domestic hot water (DHW) under any load condition.

Calculation of the design flow rate for domestic hot water according to UNI 9182

Similarly to other standards, UNI 9182 also uses the Loading Units (LU, Loading Units) method to estimate the flow rate. However, it features a distinctive approach: LUs are not associated with a specific unit of measure, but are dimensionless quantities that represent the impact of each fixture on the network.

To calculate the demand, the designer must:

Identify the fixtures present in each section of the system (washbasins, showers, WCs, etc.).
Assign the tabulated LU values. For example, a shower with mixing valve corresponds to 1.50 LU for hot water (HW), while a bidet corresponds to 0.75 LU.
Sum the LUs for each section of the system to obtain the total load.

How to read the flow-rate chart: why are there two curves?

Once the sum of the LUs has been obtained, the design flow rate (Q_D), expressed in l/s, is derived using an interpolation chart. This is where a technical doubt often arises: why are there two distinct curves?

The difference lies in the type of flushing system used for sanitary bowls, which drastically affects the instantaneous peak flow rate required from the network:

"Bowls with cisterns" curve: Used when WCs are fitted with a flushing cistern (external or concealed). In this case, the cistern refills with a moderate and steady flow rate over time, requiring less instantaneous water from the network.
"Bowls with direct flush or flushometer" curve: Used for systems (often public or industrial) where flushing occurs via a direct flow from the pipework without storage. These devices require a very high flow rate (up to 1.0 l/s per single fixture) over a very short time, making the system much more demanding on the network.

Using the correct curve helps avoid incorrect calculations that would lead to insufficient pipe diameters or sudden pressure drops during use of the facilities.

From design flow rate to peak management with i-TES

Correctly calculating the design flow rate according to UNI 9182 is the first step in understanding the building’s demand peaks. As we saw for EN 806, a high peak flow rate does not necessarily have to translate into an oversized heat generator.

i-TES thermal batteries fit perfectly into this design workflow:

Response to peaks: While UNI 9182 defines how much water must flow at the critical moment, the thermal battery provides the energy needed to heat it, drawing on the stored latent heat.
System efficiency: They allow the Heat Pump to be sized to the average load, letting the latent storage handle the typical demand “bursts” of systems calculated with high LUs.

This step is also essential for the correct sizing of a thermal storage system in buildings with high concentrated demand.

Conclusion

UNI 9182 is a precision tool which, thanks to the level of detail on loading units and the distinction between flushing systems, enables realistic modelling of draw-offs. Integrating these calculations with an intelligent storage strategy means designing systems that are not only compliant with standards, but also optimized for energy savings and space.