Heat pump arrays for flats: noise and roof design

An array of air source heat pumps serving a block of flats will almost always need planning permission, because the volume and siting limits in the permitted development classes were written for one unit on one house. Communal heat pump planning therefore turns on two questions a council asks early: what the array sounds like at the nearest window, and what it does to the roof when seen from the street and from the flats opposite.

Both are design problems with acoustic consequences, which is why they have to be settled with the mechanical engineer at concept stage rather than at technical design.

Where the plant can physically go

Three locations, each with a characteristic failure.

Roof mounted is the default on an infill block, and the constraint is competition for roof area. Photovoltaics, a green roof for biodiversity net gain, lift overrun, stair access, smoke ventilation and a maintenance zone all want the same surface. Heat pumps need clear airflow on the intake and discharge faces and separation between units so discharge from one is not drawn into the next, which uses more area than the unit footprint suggests. The tensions between competing roof uses are set out in our note on biosolar roofs combining panels and green roof.

At ground level in a compound, the acoustics are easier because there is more scope for a screen with mass, and the problems are security, leaf litter in the coils, and the ground floor flats whose windows are now next to the plant.

Distributed, with one small unit per flat on a balcony or bracket, removes the communal management burden and creates dozens of individual noise sources close to windows, plus a facade covered in condenser units. It also makes the scheme harder to assess, because the cumulative case now involves many sources at many heights.

Cumulative noise from multiple units

Doubling the number of identical units adds three decibels. Ten units add ten decibels over one. That arithmetic is the whole reason an array behaves differently from a single domestic installation.

Assessment for planning purposes normally follows BS 4142:2014+A1:2019, which compares the rating level of the specific sound with the background sound level measured at the assessment position. The standard treats a rating level around five decibels above background as an indication of an adverse effect and around ten decibels above as indicating a significant effect, with the context taken into account.

The rating level is not the measured level. Penalties are added for acoustic features, and heat pumps attract them: tonality from the compressor, intermittency as units cycle, and the impulsive character of a defrost cycle. A fan rated at a given sound power in a catalogue can end up several decibels worse as a rating level once those corrections are applied.

Night time is where schemes fail. Background levels in a residential street fall substantially after midnight, while heating and hot water demand does not stop, so the margin over background is worst when people are asleep with a window open. Design to the night time case and the daytime case looks after itself.

Two design responses reduce the cumulative level rather than mask it. Sizing the array so that units modulate at part load instead of cycling on and off, and sequencing the control strategy so the minimum number of units runs at night.

Screening, louvres and their acoustic effect

A screen works by mass and by geometry, and most proposed screens fail on one or the other.

For mass, a surface density of around 10 kilogrammes per square metre is the conventional minimum for a barrier to do acoustic work. Timber slatted screens with gaps between the boards look appropriate on a roofscape and do very little acoustically, because sound passes through the gaps. Where a slatted appearance is wanted, the usual answer is a solid acoustic panel behind the slats.

For geometry, a barrier only helps if it interrupts the line of sight between the source and the receiver. A screen at the parapet, shielding a unit mounted on a plinth above it, achieves nothing for a receptor in the upper floor of the building opposite. Insertion losses of five to ten decibels are achievable for a well placed solid barrier close to the source. Claims above that need modelling to support them.

Acoustic louvres are a different product from architectural louvres, heavier and deeper, and they impose a pressure drop. That pressure drop reduces airflow across the coil, which reduces capacity and efficiency, and the derating has to be taken from the manufacturer's data rather than assumed. A plant enclosure designed for appearance and then given acoustic performance requirements retrospectively is the most common cause of an undersized system.

Maintenance access and safety

Every unit needs access for filter and coil cleaning, refrigerant checks and eventual replacement, and a roof array means people working at height on a regular cycle.

Design in permanent access: a guarded walkway route, edge protection rather than reliance on fall arrest equipment, a hatch or stair large enough for a replacement unit, and a crane or hoist position identified on the plan. The Construction (Design and Management) Regulations 2015 require the designer to consider maintenance, and a roof plant layout that can only be serviced by a rope access team is a failure of that duty as well as a long term cost.

Noise during maintenance matters too. Cleaning and commissioning generate different sound from normal operation, and a management plan that restricts servicing to daytime hours is worth offering before a condition imposes something less convenient.

Visual effect on the roofscape

Planning officers read roof plant as part of the building's height and mass, and the drawing that settles it is a section with the plant enclosure on it, taken through the tallest element.

What tends to be accepted: plant set back from the parapet so it is not visible in the near view from the pavement opposite, enclosed in a single coherent element rather than as scattered boxes, with the enclosure treated as part of the architecture in the same material family as the upper storey. What tends to be resisted: an enclosure added above an agreed parapet line that raises the apparent height, a galvanised finish on a brick building, and separate screens around separate clusters.

Verified views from the agreed assessment points should include the plant. Omitting it from the visualisations and showing it on the sections is a discrepancy officers pick up, and on a scheme in a sensitive setting it can cost a round of consultation.

Resilience and the management arrangement

A communal system needs an owner. Councils increasingly ask how a block's heating will be maintained over time, and the answer belongs in the energy documents rather than being left to the management company's own arrangements.

Set out the redundancy strategy, because a single failed unit in an array serving 30 flats is a different event from a failed boiler in one flat. Explain the heat metering and billing arrangement, the maintenance contract assumption, and who holds responsibility for replacement at end of life. Where the scheme is all electric, the decision and its consequences are covered in our piece on the all electric new homes gas connection decision.

Where the heat pumps also provide cooling, the overheating position and the plant noise position interact, because cooling demand peaks in the evening of a hot day with windows open. That combination belongs in the Overheating Risk Assessment as well as the acoustic report.

Evidence the environmental health officer expects

A submission that gets through consultation without a further information request generally contains:

1. A noise assessment to BS 4142, with measured background levels including night time, at assessment positions agreed with the officer where possible.
2. Manufacturer sound power data for the proposed units, with the operating mode stated.
3. The cumulative calculation showing how the array total was derived, including any acoustic feature corrections applied.
4. A plant layout and section showing screen heights, mass and the line of sight to the nearest receptors.
5. The anti vibration mounting specification and a note on structural transmission paths to dwellings below.
6. A commissioning and verification commitment, with post installation measurement against the assessed rating level.

The verification commitment is the one applicants leave out and officers ask for, and offering it upfront usually converts a holding objection into a condition. The energy case behind the choice belongs in the Energy Statement.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and a BS 4142 assessment should be prepared by a suitably qualified acoustician. This is general guidance and not legal advice, so check the position with your local planning authority or a qualified professional before relying on it.