External terminations for heat recovery ventilation
Two holes in the envelope, a separation requirement and an elevation an officer is judging. Heat recovery terminations are a consent question long before they are a services one.
Every mechanical ventilation with heat recovery installation needs two holes in the building envelope: a fresh air intake and an exhaust discharge. On a new house that is a coordination job. On a conservation area terrace or a listed building it is a consent question, and MVHR planning terminations are refused often enough to be worth designing properly at planning stage rather than at Stage 4.
The grilles are small. The trouble they cause is out of proportion to their size, because they appear on an elevation drawing that an officer is assessing for its effect on the street.
Why terminations become a planning issue
On a new building the terminations form part of the external appearance shown on the approved elevations. Change their position or type afterwards and you are amending an approved drawing, which means a non-material amendment application at best.
On an existing dwelling the position is less tidy. Class G of Part 1 of the General Permitted Development Order 2015 permits the installation, alteration or replacement of a chimney, flue or soil and vent pipe on a dwellinghouse, within limits, and with tighter restrictions on Article 2(3) land such as conservation areas. Whether a ventilation terminal for a heat recovery unit counts as a flue for those purposes is not always obvious, and the answer can depend on the specifics.
Where it matters, the only formal way to settle it is a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990. On a straightforward rear elevation nobody will ever ask. On a front elevation in a conservation area, someone will.
For a listed building, listed building consent is required for works affecting its character as a building of special interest, and cutting through historic fabric to install a terminal is squarely within that.
Grille positions and elevational effect
Officers react to three things: how many, how big, and where.
Number is the one designers control most easily. A single MVHR unit serving a flat needs two terminals. Add a cooker hood discharging separately, a bathroom fan, a heat pump condensate route and a boiler flue and you are at five or six openings on a small elevation.
The fixes are straightforward once you look for them. Route the cooker extract through the MVHR system where the manufacturer allows recirculating hoods. Combine terminations in a single architectural element, such as a recessed reveal or a shared louvre panel. Or use a roof terminal for the exhaust so that only the intake appears on the wall.
Position matters more than size. A grille centred between two windows on a principal elevation reads as a mistake. The same grille tucked under a cill, behind a downpipe line, or aligned with a horizontal joint in the cladding disappears. Draw them on the elevation at the right scale and you will see the answer.
Colour is the third variable and the least expensive. Specify the grille in a colour matched to the wall or to the window frames, give the reference on the drawing, and the material condition becomes easy to discharge.
Conservation areas and listed buildings
In a conservation area the test is whether the works preserve or enhance the character and appearance of the area. Vents on a rear elevation almost never engage it. Vents on the front of a terrace with an intact frontage almost always do.
Approaches that succeed on historic buildings:
- Take the terminations through an existing opening: a redundant flue, an old air brick position, a former ventilation grille.
- Discharge at roof level through a slate vent or an existing chimney pot, where the run length and pressure drop allow.
- Use a cast iron or painted metal grille of a traditional pattern rather than a moulded plastic cowl.
- Group terminations on a secondary elevation or within a rear extension, accepting a longer duct run.
What rarely gets consent is a white plastic hooded cowl on a principal elevation. It is worth saying to a client early, because the ductwork strategy has to change, not the grille specification.
External wall insulation raises a related set of issues on the same buildings, and our note on external wall insulation in a conservation area covers the overlap.
Separation between supply and extract
The technical constraint that shapes the elevation is short-circuiting. Put the intake too close to the exhaust and the system draws its own stale air back in, which destroys the efficiency the installation was bought for.
Approved Document F and the unit manufacturer both give minimum separations, and they are not identical. Take the greater of the two, and check the manufacturer's position on the arrangement you are proposing, because vertical separation, horizontal separation and separation around a corner do not behave the same way.
Intake location has its own rules. Keep it away from combustion flue terminals, bin stores, car parking, service yards, loading bays and, on an urban site, the road frontage where it will draw in traffic pollution. On a deep plan the intake sometimes has to go on the opposite elevation from the unit, which changes the riser strategy.
| Factor | Effect on elevation design |
|---|---|
| Minimum intake to exhaust separation | Fixes the spacing between the two grilles |
| Intake away from pollution sources | May force the intake onto a different elevation |
| Duct run length and bends | Longer runs need larger ducts and bigger grilles |
| Weather louvre and insect mesh | Reduce free area, so the grille grows |
| Condensate and staining | Affects position relative to the material below |
The free area point catches people. A louvre with a mesh may deliver well under half the free area of the nominal opening, so the grille on the drawing is larger than the duct diameter suggests. Size it from the free area, not the duct.
Noise breakout at the terminal
Two noise paths. The unit itself, transmitted into the dwelling and through the structure. And regenerated noise at the terminal, radiating outwards to neighbours and to the building's own windows.
Neither is usually decisive on a house. On a block of flats with a communal system, or where a terminal sits close to a neighbour's window, the environmental health officer may ask for an assessment against the usual industrial and commercial sound methodology, and the terminal position becomes a noise decision as well as a visual one.
Attenuators take space. If one is likely to be needed, allow for it in the ceiling void or the riser at planning stage. Adding it later means either losing head height or losing the attenuator, and the second option is what actually happens.
Coordinating services with the facade drawing
The practical sequence that avoids rework:
- Fix the ventilation strategy before the elevations are drawn, including whether the cooker extract is separate.
- Produce a grille schedule with type, size, free area, colour reference and position, and put it on the planning drawings.
- Show every termination on every elevation at the right scale, including the ones on the rear.
- Check duct routes against the insulation line, cavity barriers and fire stopping before the layouts are fixed.
- Confirm separations against both Approved Document F and the manufacturer's data.
Point two is the one that pays for itself. An energy statement that describes a heat recovery strategy while the elevations show no terminations is an inconsistency a case officer will notice, and it invites a condition you then have to discharge.
Where the scheme involves commercial cooking or process extraction as well, a ventilation and extraction statement will be needed alongside, and the two documents should agree.
Detailing that survives a condition
Expect a condition requiring details or samples of external materials, and expect it to be read as covering vents and grilles.
Discharge it in one submission by providing an annotated elevation showing each termination, a product data sheet for the grille, the colour reference, and a detail section showing how the grille meets the wall build-up, including the reveal depth and the weathering.
Two details that get queried. Staining below a terminal on a light-coloured render, which a drip or a projecting cill solves. And the appearance of a terminal within a reveal, where the duct is visible from an oblique angle unless a blanking plate is specified.
The decision to go all-electric changes the termination count on the elevation, since a heat pump replaces a boiler flue with a different set of external items, as our note on the all-electric new homes decision explains.
Enter your postcode on PlanScope to see which supporting documents your scheme is likely to need.
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 MVHR vents need planning permission?
On a new building they form part of the approved elevations, so changing them later means amending an approved drawing. On an existing dwelling, Class G of Part 1 of the GPDO 2015 permits chimneys, flues and soil and vent pipes within limits, and whether a ventilation terminal qualifies is not always clear. A Lawful Development Certificate settles it.
How far apart must intake and exhaust be?
Approved Document F and the unit manufacturer both give minimum separations, and they are not always the same. Take the greater figure and check the manufacturer's position on your specific arrangement, because vertical, horizontal and around-a-corner separations do not behave alike. Too close and the system draws its own exhaust back in.
What works on a listed building?
Reusing an existing opening such as a redundant flue or old air brick, discharging at roof level through a slate vent or existing pot, and using a painted metal grille of traditional pattern rather than a moulded plastic cowl. Listed building consent is required where the works affect the building's special interest.
Why is the grille bigger than the duct?
Because a weather louvre with an insect mesh delivers considerably less free area than the nominal opening, sometimes well under half. Size the grille from the free area required rather than from the duct diameter, and note the free area on the grille schedule so the elevation drawings are right first time.
Will a materials condition cover vents and grilles?
Usually yes. Discharge it in a single submission with an annotated elevation showing each termination, a product data sheet, the colour reference, and a detail section showing how the grille meets the wall build-up including reveal depth and weathering. Add a drip or projecting cill where staining below the terminal is likely.
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.