Heat recovery ventilation in a planning submission

Mechanical ventilation with heat recovery is a Building Regulations matter, but it shows up on a planning drawing in three places: the external terminals on the elevations, the storey height needed to run ducts, and the acoustic case where windows have to stay shut. Councils do not assess ventilation performance. They do assess a facade covered in grilles, a building 300 mm taller per floor than the context, and a noise argument that depends on a system nobody has drawn.

Why ventilation appears in a planning document at all

Approved Document F to the Building Regulations sets out four ventilation systems for dwellings, and the continuous mechanical supply and extract system with heat recovery is the fourth. Selecting it is a compliance decision made by the designer, not a planning one.

The planning consequences are indirect and they are real.

Where a site is noisy, the acoustic case often rests on windows being closed. That immediately requires a whole-dwelling ventilation system that works without opening windows, and the officer's environmental health colleague will ask what it is and where its terminals are. Without that answer, a Noise Impact Assessment recommending closed windows is incomplete.

Where the scheme is tall or in a sensitive setting, the ductwork zone adds to storey height, which adds to building height, which is assessed. And the terminals themselves are external alterations on a facade that the design has been assessed against.

So the ventilation strategy belongs in the design documents as a short, drawn, stated approach, rather than as a line in the energy statement.

External terminals and their effect on elevations

Each dwelling with a heat recovery unit needs an air intake and an exhaust discharge to outside. In a house that means two terminals. In a block of flats it means two per dwelling, and that is where elevations get into trouble.

Approved Document F requires the intake to be positioned away from sources of pollution and requires separation between intake and exhaust so that discharged air is not drawn back in. Those functional requirements set minimum positions before any design intent is applied.

Four approaches are used, and each has a cost.

Facade-mounted grilles, which are the simplest and least costly option and produce the grid of louvres that planning officers object to. Terminals concealed within a recessed reveal or behind a perforated panel integrated into the facade design, which works well and needs coordination early. Terminals taken to roof level through a riser, which cleans up the elevation entirely and costs riser space and duct length. Or terminals grouped into a designed feature, such as a continuous slot at slab level, which reads as architecture rather than plant.

Pick the approach before the elevations are drawn. Retrofitting concealment to an approved elevation after a services engineer has located the terminals is one of the more common causes of a non-material amendment application.

Where a site has a flue or a plant discharge nearby, intake positioning interacts with the air quality case as well, and the tensions that creates are set out in our note on combined heat and power and the air quality objection.

Acoustic performance of the ventilation route

A ventilation duct is a hole through the acoustic envelope, and the acoustic case has to account for it.

Three paths matter. Sound entering from outside through the intake duct, which is the one the noise assessment addresses. Sound from the unit itself into the dwelling, which Approved Document F constrains through design noise levels, set lower for bedrooms and living rooms than for wet rooms. And cross-talk between rooms and between dwellings through shared ductwork, which is the path most often forgotten and the one that generates complaints after occupation.

The fixes are duct attenuators, generous duct lengths with bends rather than straight runs, resilient mounting of the unit, and separate ductwork per dwelling rather than shared risers. All of them take space, which brings us back to the storey height.

State in the acoustic report that ventilation is provided without opening windows, name the system type, and give the terminal positions. An assessment that assumes closed windows without identifying the ventilation route is the assessment an objector's consultant will target first.

Duct routes, ceiling voids and riser space

This is the coordination that decides whether the approved building can be built.

A heat recovery system needs a plant location for the unit with access on at least one side and a condensate drain, a supply and extract duct network reaching every habitable room and wet room, and a route to the two external terminals. In a flat that typically means a ceiling void through the circulation core of the dwelling and a cupboard for the unit.

The void depth needed depends on duct sizing and the number of crossings, and the honest range for a flat is a few hundred millimetres rather than the 150 mm an architect hopes for. Multiply that by the number of storeys and the building height changes measurably.

Three decisions reduce the impact. Plan the duct route along a single spine rather than radiating from a central unit, so crossings are minimised. Use semi-rigid ducting in a screed or a service zone where the layout suits it. And locate the unit close to the external wall so the two largest ducts are short.

Do this before the planning submission, not after. A scheme consented at a stated height and then found to need another 200 mm per floor has either to lose a storey or go back for an amendment.

Interaction with overheating and energy targets

Heat recovery ventilation is not cooling. It moves air at close to internal temperature, so it does very little for overheating, and treating it as mitigation in an overheating assessment is a mistake reviewers catch.

Approved Document O deals with overheating separately, through limiting solar gains and providing a means of removing excess heat. Its simplified method sets maximum glazing areas and minimum free openable areas, which means window openable area becomes an elevation matter too. The dynamic modelling route uses the relevant CIBSE methodology and allows more design freedom at the cost of the modelling work. Either way, an Overheating Risk Assessment should state which route has been taken.

There is a genuine tension on a noisy site. Part O compliance through openable windows conflicts with an acoustic case built on closed windows, and the resolution is usually mechanical cooling or a ventilation strategy designed for both, with the trade-off explained. Pretending the conflict does not exist is what produces a condition requiring a revised strategy.

On the energy side, heat recovery reduces ventilation heat loss and therefore helps the regulated emissions figure in an Energy Statement. It also adds fan energy, which is regulated too, so the net benefit depends on the specific fan power of the unit selected. Quote the specific fan power you have assumed, because a modelled benefit based on an optimistic figure will not be delivered by a value-engineered unit. The same discipline about matching assumptions to specification applies to larger buildings, as our note on energy statements for warehouse and logistics schemes sets out.

Maintenance access and filter changes

Systems fail through neglect rather than design, and the neglect is designed in.

A unit in a loft hatch above a stair, or at the back of a boxed ceiling void, will not have its filters changed. Filters need replacing at intervals measured in months, and the access position determines whether that happens.

Specify the unit in a cupboard at standing height with a clear working space in front, a condensate route that does not require a pump, and filter access without tools. Where a managing agent will maintain the system across a block, locate the units so one visit can reach several, and provide a labelled schedule.

None of this is a planning matter in itself. It becomes one when a scheme's energy and acoustic performance were justified on a system that stops working, and a council with a post-occupancy monitoring policy will see that in the data.

What to show on the drawings

For a planning submission, five items cover it without over-documenting.

Terminal positions marked on every elevation, with the treatment shown and a note of the finish. A typical section through a dwelling showing the ceiling void or service zone depth and the storey height it produces. Plant locations on the floor plans with the cupboard dimensioned. A one-page ventilation strategy note naming the Approved Document F system type, the terminal arrangement and the acoustic treatment. And, where the acoustic case relies on closed windows, a cross-reference between the noise report and that note.

Keep the note short and specific. An officer is not assessing the system, they are checking that it exists, that it has been located, and that the noise and overheating arguments hang together with it.

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 ventilation, acoustic and overheating assessments each need a suitably qualified author.

Enter your postcode on PlanScope to see which energy and amenity reports your scheme is likely to need.