Acoustic Design: Enclosure for Air Source Heat Pump / Condenser Units

Acoustic Design:  Enclosure for Air Source Heat Pump / Condenser Units

1. Project Brief

PDA Ltd was commissioned by Watkin Jones Construction to undertake the acoustic design of a four storey academic building in Stratford, London. The proposed facility consisted of an academic café, reception, library and ancillary areas. a 151 seat lecture theatre and 23 classrooms and academic rooms,

The development also had an internal courtyard containing 4 condenser units that were designed to provide the heating and cooling for the building.  The noise break-out from these units was assessed by PDA Ltd and was noted as needing significant attenuation requiring an insertion loss in the region of  15-20dB(A) to meet project requirements.  However, due to long lead times arising from the Covid19 pandemic the available attenuator manufacturers were not able to guarantee manufacture of attenuating enclosures in time to meet project handover.

2. Condenser Enclosure Design

In light of the said lead time limitations PDA Ltd were asked to provide an acoustic enclosure design to achieve acoustic and airflow requirements.  The required turnaround for the design was less than a week such that fabricators could produce the enclosure in time for project handover within a total 6 week period. 

In due course PDA Ltd met the timescale and designed the enclosure and acoustic attenuators and assessed and approved the proposed enclosure cladding to provide the required acoustic performance based on their own noise assessment criteria.  In addition PDA Director Dr Max de Salis used his Building Services background to provide design and calculations of static pressure losses for the entire airflow system to ensure that this was within the limits of the static pressure available from the units.  

Figure 1 below shows the installation in progress and Figures 2 and 3 show the final installation: 

Figure 1 Enclosure Build in Progress

Figure 2 Enclosure in Courtyard

Figure 3 Enclosure in Courtyard from Above

The design (in combination with Glidefield), installation (Configured Platforms) and attenuator fabrication to PDA Ltd design (Glidefield) was all provided within a six week turnaround to meet project handover and preventing the contractor from incurring significant penalties. 

3. Condenser Commissioning

The noise measurements with all 4 condenser units at full duty were measured at 56 dB(A) Leq free field at the nearest window (500mm from the attenuator outlet as shown in Figures 1-3) and 53 dB(A) Leq in the centre of the courtyard.  

This comfortably met the 60 dB(A) Leq upper limit at the nearest window of the mechanically ventilated apartment above as shown in Figures 1 and 2.

The predicted noise level without the enclosure was 73-76 dB(A) Leq at the nearest window.

As such the attenuation achieved was estimated at between 17 and 20 dB(A) whilst still achieving adequate airflow to the units.

To design, fabricate and install such an enclosure to achieve noise level reduction for static pressure sensitive units such as condensers especially in such a short window, is a huge achievement and PDA Ltd, Configured Platforms and Glidefield should be incredibly proud of achieving this outcome. In addition a large cost saving was achieved compared to available turn key and bespoke enclosure solutions.