Pipe sizing: The CIPHE loading units method

Frequency of use

When designing or installing hot and cold water pipework in non-residential buildings, it is recommended that the loading unit method within the CIPHE Plumbing Engineering Services Design Guideis used. The guide can be viewed online by members of the CIPHE at www.ciphe.org.uk/professional-members/publications.

The key difference between the CIPHE method and the two sizing methods demonstrated in the last two issues ofP&H Engineeringis that the CIPHE method gives the designer the choice of different frequency-of-use factors: low, medium and high. An example of high frequency of use is a theatre at peak times; an example of medium use is a standard office building. This article illustrates how frequency measures affect overall pipe size.

Origin of UK loading units

During the 1960s, a past president of the Institute of Plumbing (IOP) called Harry Howick created a loading unit system method which was derived from an idea developed in the USA based on the Hunter fixture unit. Roy B. Hunter authored a book called Methods of Estimating Loads in Plumbing Systems, which explains the original theory. Hunter’s method of estimating the load in a plumbing system is based upon assigning a discharge value to each plumbing fixture outlet when compared with a base appliance of 1 LU. The CIPHE design guide includes a table showing the base data for simultaneous demand (Table 14, not shown), which highlights a wash basin with a frequency of 1,200 seconds as the base appliance, with all other outlets being given a greater value. The data in this table was used to create the single probability graphs in the guide for both copper (figure 1, overleaf) and plastic that enables the user to convert the total LU of the appliances into the simultaneous design flow. This new method from the IOP was included in the 1965 version of the BS CP 310 standard, which was subsequently published in the design guide.

Simultaneous demand

The simultaneous demand is the total number of appliances that are operating at any one time, which can be calculated using probability theory. The factors that have to be taken into account are: capacity of appliance in litres; draw-off flow rate in litres per second (l/s); draw-off period in seconds (time to fill appliance); and frequency of use. These factors differ for different buildings.

Gauging frequency of use

The frequency of use is the time difference between the use of the appliance. The CIPHE loading units method, which differs to BS EN 8558 and BS EN 805, is that you have the option to choose between low, medium and high use (figure 2). Low use is deemed to have a time difference of 1,200 seconds (20 minutes) between each use, compared to 10 minutes for medium and five minutes for high use. Low use occurs where there is a single person or small group; high use occurs at buildings such as theatres or buildings used for sports events.

Velocity and pressure loss

According to figure 4, when sizing the pipework, it is important to ensure that within any circulation areas, the water velocity does not exceed 1.5 m/s due to the risk of noise – however, according to the guide, erosion and corrosion can become a risk with velocities in excess of 2.5 m/s. In figure 3, taken from table 17 in the CIPHE design guide, it states the maximum pipework velocities for different locations. By contrast, pipework that is oversized, creating laminar flow regions, may create other water-related risks.

In addition to velocity, the available dynamic flow pressure at each outlet needs to be no less than 1 Bar dynamic or 5 Bar static, according to BS EN 806:3, depending upon the manufacturers’ recommendations. Therefore it is critical to firstly ascertain the available pressure at the inlet to the building to ensure that the selected pipe size minimises the pressure being lost through the connecting pipework.

Calculating pipe size

Using figure 2, (Table 15 from the CIPHE design guide), the designer needs to take care when selecting whether the outlet and building application is suitable for low, medium or high usage. Once the total number of loading units has been attributed to the supply pipe, the designer would then use figure 1 to plot the total number of loading units onto the graph to choose the suitable pipe size, based upon a suitable pipe velocity and maximum pressure loss per metre of pipe. As sizing hot and cold water pipework for any commercial building is a complex calculation, which takes into consideration many other factors, it is critical that a suitably qualified engineer, such as an Incorporated or Chartered Engineer, in this area of expertise is consulted.

Step 1

Draw a simple schematic

The simple example below has been created for illustration purposes only to demonstrate scenario one, high frequency-of-use theatre (figure 4). You could go through the same process to illustrate scenario two, medium frequency-of-use office building.

Step 2

Calculate the total number of loading units

Calculate the total number of loading units for the application and ascertain whether to select low, medium or high frequency of use. Label each outlet with the corresponding loading unit. For the purposes of the illustrative example, scenario one (theatre) uses a loading unit of 4 for each basin; if you were working with scenario two (office building), you would select a loading unit of 2.

Step 3

Select the required pipe size

Taking a look at figure 1, the total number of loading units of 100 (scenario 1) and 50 (scenario 2) have been plotted onto the graph. Figure 5 is a summary of the differences in the selected pipe size for both scenarios.

Where next for the industry?

The CIPHE is a member of the stage 2 LUNA steering group in collaboration with CIBSE researching a new method for pipe sizing hot and cold-water services. The report from stage 1 can be accessed on the CIPHE website at www.ciphe.org.uk.

Find out more

For further information on how to expand you career as a plumbing engineering designer, contact membership@ciphe.org.uk for advice on what qualifications and experience is expected to achieve Engineering Council registration for Engineering Technician (Eng Tech), Incorporated Engineer (IEng) and Chartered Engineer (CEng).

This article first appeared in the March/April 2019 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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