In the UK Air Source Heat Pumps (ASHPs) and Air Conditioning Units are becoming more commonplace. This is especially true as the country moves towards more energy-efficient ways of heating and cooling homes and the green revolution. However, the noise from these units is often overlooked.

Do I need planning permission to install an AC unit or Air Source Heat Pump?

With the implementation of these small plant units comes the inevitable increase in noise levels due to the operation of these plant units. Because of this, it is sometimes the case that in order to install an Air Conditioning Unit or an Air Source Heat Pump planning permission from your local planning authority would be required.

During this process, the local planning authority may request a noise impact assessment in order to analyse the potential for adverse noise impact on nearby noise-sensitive receptors such as residential dwellings or other sensitive buildings such as schools.

How do I assess the noise from Air Conditioning Units?

In the UK there is a standardised method for assessing noise from mechanical plant such as AC or ASHP units, this is outlined in British Standard BS412. A BS4142 assessment is a comparative assessment and is undertaken by comparing the background sound level of a given area to the expected noise emissions of the proposed AC or ASHP unit [1]. The magnitude by which the noise from the unit exceeds the background sound level defines the potential for noise Impact.

In order to assess the noise impact from the unit and undertake a BS4142 assessment the first step is to do a noise survey at the location of the proposed units and the closest Noise Sensitive Receptor. During the survey, the LA90 acoustic parameter should be measured this parameter is defined in BS4142 as the underlying background sound level of a given area.

Once the background sound level has been established the next step is to calculate the noise level from the AC or ASHP unit at the closest residential window. Typically these types of assessments are undertaken before the unit is installed and therefore in order to do this manufacturer's noise data can be used.  Nearly all manufacturers will supply the noise levels of the unit in the technical specification sheet which can generally be found on their websites. Using the sound power levels (LwA) stated by the manufacturer a distance correction can be applied to the sound power level to predict the specific sound pressure levels at the closest Noise Sensitive Receptor using the following formula:

Where:

LpA = A-Weighted Sound Pressure Levels at NSR

LwA = Manufactures sound power level

r = Distance between the location of the unit and residential NSR

Once the specific sound pressure level of the unit has been predicted at the closest NSR BS4142 stipulates that rating penalties should be applied to account for any likely acoustic characteristics associated with the noise source. Acoustic characteristics can include tonality, intermittency or impulsivity.

The rating noise level associated with the AC unit or ASHP is then compared to the underlying background sound level LA90 established during the noise survey of the site. If the rating noise level from the unit falls below the background sound level this is an indication of ‘Low Impact depending on Context’ when assessed in accordance with BS4142 and generally is the outcome any local authority would be looking for. When compared to the National Planning Policy Framework [2] and the Noise Policy Statement for England this would equate to ‘No Observed Effect Level’ [3] and again would be a strong indication to local planners the installation of the unit would not have an adverse effect on nearby sensitive receptors.  If the rating noise level from the unit falls above the background sound level this is an indication of ‘Adverse Impact’ or ‘Significant Adverse Impact’ depending on the level of exceedance above the background sound level.

How do I reduce noise from an Air Conditioning unit or Air Source Heat Pump?

There are a few simple ways of reducing noise from small pieces of mechanical plant such as Air Conditioning units or Air Source Heat Pumps.

The first of these is to move the unit further away from the closest noise-sensitive receptor. Typically, AC Units and ASHPs would be considered as point sources and therefore for every doubling of distance between the source and received a drop of 6 dB would be achieved. For example, if the AC unit was positioned at 3m from the closest residential window, moving it a further 3m away, totalling 6m from the closest window, the noise level at the window would drop by 6 dB.

The second approach to reducing noise from AC and ASHP units is to provide screening in the form of noise barriers such as fencing etc. To devise how much a noise barrier can attenuate the noise from the unit a detailed noise barrier calculation should be undertaken. However, a general rule of thumb is if the unit is shielded from the direct line of site you can expect a 10 dB reduction in noise levels and if the unit is only partially shielded then you can expect a 5 dB reduction.

The final method for reducing noise from these types of mechanical plant is to position the AC unit or Air source heat pump within an acoustic enclosure. These enclosures can come in many forms and provide varying levels of sound reduction therefore it is advised that you speak to a specialist company when considering this option as positioning units within an enclosure can affect their performance due to static pressure issues and reduce airflow.

For a noise impact assessment of your air conditioning unit or air source heat pump contact a consultant today.

[1] British Standard BS4142:2014+A1:2019

[2] National Planning Policy Framework – https://www.gov.uk/government/publications/national-planning-policy-framework--2

[3] Noise Policy Statement for England - https://www.gov.uk/government/publications/noise-policy-statement-for-england