Attempt to improve the functioning of 7 air-supply ductworks connected to 257 high-pressure induction units – first real diagnosis compaign on a 23 years-old building

Purpose of the work 
This article presents the results of an HVAC diagnosis carried out in a large office building consisting of 257 high-pressure induction units (Spilotair®) connected to seven air-supply ductworks. This system is used for heating, cooling and ventilating purposes. This installation is located in France near the Bordeaux-Mérignac Airport and has never functioned properly for 23 years. As the name suggests, a high-pressure induction unit must be supplied with high pressure air (more than 250 Pa) - preheated or pre-cooled by a balanced Air Handling Unit. The entire ductwork is therefore subject to highpressures, which makes it all the more sensitive to possible air leakage from the ducts. 

Method of approach 
This paper presents the first results of an HVAC diagnosis started in October 2022 and allows to identify all the dysfunctions of the installation. The method and results are based on an experimental approach with on-site measurements (pressure and airflow rate in the ductwork, airtightness testing of the ductwork), pressure drops calculation and on-site observations (search for leaks in ductwork) in order to identify the causes of the HVAC malfunction. 

Content of the contribution 
The design heating and cooling loads of the HVAC system are in line with current needs of the building. However, the diagnosis showed that the major problem is due to both disproportionate pressure drops and leakage resulting in poor performance of the supply and extract ducts. This analysis helped to explain both the poor performance of the HVAC system observed by the building managers and local thermal discomfort reported by occupants. 

Results and assessment of their significance 
The diagnosis showed that the major problem is due to the poor performance of the 7 supply ductworks, and is not related to an undersizing of the 7 AHU, with a direct impact on the proper functioning of all high-pressure induction units. Reconditioning measures will be proposed by SNIA which has become the main contractor for the project, including the almost complete reconditioning of the ducts. 

Conclusions 
This presentation aims to show: 

1. Why the diagnosis of a malfunctioning HVAC system would require a full audit including the airtightness of the whole ductwork and not just a replacement of the fan motor as is often proposed.
2. Why the commissioning test after works can be useful to check all the installation -surprisingly, in this case, there has never been a global check of the installation.
3. When the ductwork designers respect some basic rules (especially low pressure drops and a very good airtightness) as recommended by the SPILOTAIR® manufacturer, the theory (target airflow ≈ measured airflow) works very well!

For further information please contact Fabrice Richieri at: fabrice.richieri@aviation-civile.gouv.fr