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CIBSE BSG Event – Overheating: Approved Document O

03 May 23
5 minutes
Building Simulation Group

The CIBSE Building Simulation Group hosted a hybrid event aimed at modellers tackling Part O assessments using the Dynamic Thermal Modelling approach.

The event was led by three experts: Ben Abel of Hilson Moran, Jack Harvie-Clark of Apex Acoustics, and Susie Diamond of Inkling, who shared their thoughts and recommendations on what they believe are the most challenging and confusing aspects of Part O assessments. They emphasised the need for consistent and appropriate approaches to these assessments, and highlighted some of the most confusing and tricky aspects of the analysis.

For more details, see the slides and to watch the whole event and very interesting Q&A head over to YouTube

One of the key issues discussed was the confusing terminology surrounding window geometry. The term "free area" is often used in Part O when what is really meant is "equivalent area." However, free area also has another definition, which is the total size of the opening. This can lead to confusion and errors when modelling windows for Part O assessments.

To address this issue, the experts recommended using "free area" and a calculated discharge coefficient when specifying natural ventilation openings and communicating with acousticians. They also reminded modellers to factor in the Part O max 650mm reach from the inside wall limit when assessing window openings.

Modelling opening windows is a complex task, as different terminology is used to describe flow performance through openings. The effective area is the product of the free area and the discharge coefficient (Cd), while the equivalent area is the area of a circular hole in a flat plate that achieves an equivalent flow performance as the opening being assessed. The flow performance of a partially open window will always have more friction, which means that the calculated discharge coefficient for any window opening is always less than 0.62.

Another important issue discussed was the impact of site night time noise limits on bedrooms. If these limits are exceeded, it is essential to understand from an acoustician where and by how much and to model accordingly. There is a hierarchy of possible solutions for night time noise depending on the severity of exceedance and the site location. Ideally, these solutions should be sized on night time bedroom demand rather than daytime for the whole unit.

To ensure consistency with acoustic calculations, the experts recommend entering the window as fully openable and reducing the discharge coefficient to the value calculated for the window. This method should create the same effective area within the models, which is what they base their air flow calculations on.

Finally, the experts advised modellers to use the Future Homes Hub (FHH) free spreadsheet developed for the simplified method to factor in the Part O 650mm reach limit. This spreadsheet asks for all the window dimensions and the "Distance [a] from inside wall to window frame (mm)," and then calculates the maximum opening angle within the 650mm reach limit and the consequent free area and discharge coefficients using the same algorithm as is embedded in the discharge coefficient calculator spreadsheet.

In conclusion, Part O assessments using the Dynamic Thermal Modelling approach can be challenging due to the confusing terminology and complex calculations involved. However, by following the experts' recommendations and using appropriate tools and spreadsheets, modellers can ensure that their assessments are accurate, consistent, and appropriate.

If you are interested to know more, read this interesting article written by one of speakers, Susie Diamond of INKLING.

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