ASHP Acoustic Mitigation

Project Brief

PDA Ltd were commissioned to provide acoustic design for the installation of new Air Source Heat Pumps (ASHPs) in the wider aim of supporting a net zero hotel. The project required compliance with Manchester City Council’s (MCC) planning guidance which stipulates that fixed plant should be 5dB below the existing typical background sound level at the surrounding residential receivers when assessed in accordance with BS4142 ‘Methods for rating and assessing industrial and commercial sound’.
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1. Initial Assessment and Planning

PDA Ltd conducted a Noise Impact Assessment (NIA) which established the typical background sound levels at the nearest noise-sensitive receivers and predicted the noise levels from the proposed ASHPs utilising manufacturers test data and 3-dimensional modelling software.
The initial findings were that acoustic screening would be required to mitigate the noise levels in addition to absorbative internal linings to reduce reflections back over the screening in opposing directions. However, the effectiveness of acoustic screening would be limited due to the elevated heights of the surrounding receivers which in turn increased the line of sight to the ASHPs.
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2. On-Site Investigation and Noise Verification

An on-site noise level investigation was carried out once the ASHPs were in-situ but not yet in operation and prior to the screening install. The ASHPs were measured under various operational test loads to obtain the noise emissions which could then be compared to the manufacturers data in the initial assessment. The results of the investigation found that the noise levels were within 1 – 2dB of the manufacturers data and therefore demonstrated a strong correlation.

Notably, during the investigation it was found that the ASHP noise emissions were dominated by the compressor located towards the bottom of the unit with much lesser contributions from the fan openings to the top. As a result the noise model was revised in order to more accurately represent the noise emissions from the compressor and fan openings of each ASHP and subsequently the effect of the proposed screening would have given that the dominant noise source (compressor) was located lower than initially assumed. This can be seen in the Figures below:

3. Bespoke Mitigation and Project Compliance

The updated modelling assessment demonstrated that whilst the compressor was screened more effectively than initially predicted, the upper levels of the surrounding receivers were still at risk of exceeding the MCC criteria during the night-time. The findings concluded that to avoid an over engineered and potentially costly full acoustic enclosure, the noise may be mitigated at source in the form of an insulated jacket for the compressor which was designed in collaboration with a third-party industrial noise, vibration and thermal management manufacturer. A bespoke insulated jacket for the compressor units was produced which was tested on-site by PDA and was shown to provide a significant reduction to the overall noise emissions from the ASHP units.

Our assessment and involvement in the design process enabled the noise emissions from the ASHP units to be effectively mitigated, allowing the development to achieve compliance with MCC requirements whilst delivering a pragmatic and cost effective outcome for the client. Additionally, the project outcome enables the net-zero operation of the hotel promoting a positive sustainable approach.