Heat pumps in flats: acoustics and plant space at scale
At scale the heat pump question is where the units go and how loud the array is at night. How the three system architectures differ on planning grounds.
On a block of flats the heat pump question is not about efficiency. It is about where 60 outdoor units go, what they sound like together at two in the morning, and who maintains them in year seven. Councils refuse or defer schemes on those points, and an energy strategy that answers them on the drawings moves much faster than one that treats plant as a services matter to be resolved later.
Three system architectures are in common use, and each fails in a different way if the planning application ignores it.
Individual, communal and central systems
Individual air source units serve one flat each, sited on a balcony, a terrace or a facade bracket. They are simple commercially, because each resident holds their own supply contract, and they are the hardest to accommodate physically.
Ambient loop systems run an uninsulated or lightly insulated water loop through the building, with a small water source heat pump inside each flat and heat rejection or collection at roof level or in boreholes. Outdoor units disappear from the elevations. Internal space is consumed instead, because each flat needs a plant cupboard and a cylinder.
Central plant with a heat network puts the generating equipment on a roof deck or in a plantroom, with heat interface units in each flat. Fewest outdoor units, most riser space, and a regulated billing relationship. Where a scheme sits inside a designated zone, connection may be required rather than optional, which is the subject of heat network zoning and the connection requirement.
| Approach | Outdoor units | Internal space per flat | Management burden |
|---|---|---|---|
| Individual air source | One per flat | Cylinder and small cupboard | Resident holds it |
| Ambient loop | Roof plant only | Cylinder plus plant cupboard | Management company |
| Central plant and network | Roof plant only | Heat interface unit and cylinder | Regulated heat supplier |
Where the units physically go
An outdoor monobloc needs clear air on the intake and discharge faces, condensate drainage that will not ice a walkway, and enough separation from the next unit that it is not ingesting its neighbour's exhaust. Manufacturers state clearances, and designers routinely shave them to fit a balcony.
Putting a unit on a balcony has three consequences worth drawing before the application: the usable amenity area reduces, sometimes below the local standard, the balcony role as solar shading in the overheating strategy may change, and the unit sits in the acoustic near field of the flat above and beside it.
Screening is where good intentions cause harm. A solid enclosure that restricts airflow lowers efficiency and pushes the unit to run at higher fan speed, and a hard reflective screen can raise levels at a receptor above. If you screen, make it acoustically absorptive on the inside and size the free area to the manufacturer's requirement.
Cumulative noise from an array
A single unit at 45 dB(A) at 1 metre is unremarkable. Thirty of them on one elevation are a different proposition, because sound powers add logarithmically and the receptors are the scheme's own residents rather than a neighbouring site.
Points that decide the assessment:
- Assess the whole array at the worst affected window, not one unit at a representative position.
- Use night-time operation as the design case. Heat pumps run when it is cold, which is when background is lowest and bedroom windows matter.
- Include defrost cycles, which are louder than steady running and occur exactly in the conditions that concern the officer.
- Account for reflection from facades and balcony soffits, which a free field calculation will miss.
- Set the criterion with the environmental health officer early. Councils commonly apply the BS 4142 methodology to fixed plant, and a Noise Impact Assessment on that basis is what they expect to receive.
Permitted development is a side issue at this scale, since plant forms part of the application. It still matters for retrofit, where Class G of Part 14 of Schedule 2 to the GPDO 2015 permits air source heat pumps on a dwellinghouse or block of flats subject to conditions including a noise test under the microgeneration planning standards and limits on size and siting. Those conditions have been amended more than once, so read the current text of the Order rather than an older summary.
Maintenance access and who carries it
An individual unit on a private balcony is the resident's to maintain, and in practice many will not. A unit on a roof deck is the management company's, and needs a safe access route, a lifting strategy for replacement, and a service charge line.
Write the maintenance strategy into the application. It should say where the access is, what the fall protection arrangement is, how a failed unit is craned or carried out, and who holds the refrigerant handling duties. Officers ask because they have seen schemes where the only route to the plant deck is through a private flat.
An illustrative example
Illustrative only: a 72 flat block on a corner site with a shallow plan. The first submission put individual monobloc units on every balcony. Environmental health objected on cumulative night-time levels at the inner corner, where units faced each other across a 9 metre reveal, and the amenity officer objected to the loss of balcony space.
The revised scheme moved to an ambient loop with roof mounted dry coolers behind an absorptive acoustic screen, added a 900mm deep plant cupboard to each flat plan, and lost two units from the top floor to create the plant deck. Predicted levels at the worst window fell well inside the agreed criterion, balconies stayed usable, and the scheme was approved.
The cost was two flats. Discovering the problem after approval would have cost the programme.
Billing, metering and the management structure
Communal and networked systems bring regulation with them. The Heat Network (Metering and Billing) Regulations 2014 impose notification, metering and billing duties on heat suppliers, and the Energy Act 2023 provides for heat networks to be regulated with Ofgem as the regulator, with consumer protection rules following. If your scheme creates a heat supplier, say who it will be and how residents are protected.
Individual systems avoid that, at the cost of losing central control over performance. There is no arrangement that is free of consequences, and planning officers respond well to an application that says so plainly rather than presenting one option as obviously correct.
What to submit
The plant case should be legible from four documents: elevations and plans showing every unit and its clearances, an acoustic assessment covering the cumulative array at night including defrost, a maintenance and access strategy, and an Energy Statement that reconciles the chosen system with the regulated energy calculation. The SAP outputs that statement has to show are set out in SAP outputs an energy statement has to show.
Keep the overheating work consistent with it, because a balcony that was shading in one report and a plant platform in another will be noticed, as explained in TM59 against Approved Document O.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Do heat pumps on a block of flats need planning permission?
On a new build they form part of the application, so the question is how they are assessed rather than whether permission is needed. For retrofit, Class G of Part 14 of Schedule 2 to the GPDO 2015 can permit air source units on a dwellinghouse or block of flats subject to conditions on noise, size and siting. Those conditions have changed, so check the current text.
How is cumulative noise from an array assessed?
At the worst affected window with all units running, rather than one unit at a nominal position. Sound powers add logarithmically, so thirty units are far from thirty times one unit but well above it. Assess night-time operation, include defrost cycles and account for reflections from facades and balcony soffits.
Does acoustic screening solve the problem?
Only if it is designed. A solid enclosure that restricts airflow reduces efficiency and drives the fan harder, and a hard reflective screen can increase levels at a window above. Use absorptive linings, size the free area to the manufacturer's clearance requirement, and model the screened condition rather than assuming a fixed reduction.
How much internal space does an ambient loop cost per flat?
Each flat needs a plant cupboard for the water source unit and space for a hot water cylinder, which has to be found within the floor area rather than added to it. Draw it on the unit plans at the design stage, because losing it later means either smaller rooms or a challenge against the space standard.
Who regulates billing on a communal heat system?
The Heat Network (Metering and Billing) Regulations 2014 impose notification, metering and billing duties on heat suppliers, and the Energy Act 2023 provides for heat network regulation with Ofgem in that role. If the scheme creates a heat supplier, identify who that will be and how residents are protected in the application documents.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.