Approved Document F: Ventilation Requirements for Dwellings

Summary

Ventilation is the second most misunderstood building regulation after insulation, and the 2021 revision of Approved Document F significantly increased the requirements — particularly for airtight new-build dwellings where inadequate ventilation causes condensation, mould growth, and poor indoor air quality.

The fundamental principle of Part F is a two-tier system: background ventilation (continuous low-level air exchange through trickle vents and purpose-provided openings) plus rapid ventilation (openable windows or extract fans for wet rooms and cooking areas). For new buildings, the whole-dwelling ventilation rate must be calculated and documented. For existing dwellings, when ventilation is modified or replaced, the replacement must meet current standards.

For tradespeople, Part F is most relevant when: fitting or replacing kitchen extractor fans or cooker hoods, installing bathroom extract fans, fitting replacement windows (trickle vent requirements), and installing whole-house MVHR systems. Understanding the minimum extract rates and background ventilator requirements prevents costly remedial work.

Key Facts

• ADF 2021 (Volume 1: Dwellings) — In force from 15 June 2022. Supersedes ADF 2010. Significant updates: higher whole-dwelling ventilation rates, specific new-build requirements, new commissioning and testing requirements
• Four ventilation strategies — System 1 (intermittent extract), System 2 (passive stack), System 3 (MEV — continuous extract), System 4 (MVHR — balanced continuous)
• Minimum whole-dwelling ventilation:
  • 1–2 bedroom dwelling: 19 l/s continuous
  • 3 bedroom: 25 l/s continuous
  • 4 bedroom: 31 l/s continuous
  • (Or 0.3 l/s per m² gross floor area, whichever is greater)
• Extract fan minimum rates (intermittent — System 1):
  • Kitchen (cooker hood): 30 l/s adjacent to hob, or 60 l/s elsewhere in kitchen
  • Utility room: 30 l/s
  • Bathroom: 15 l/s
  • WC/cloakroom: 6 l/s
• Overrun (run-on) timer — Extract fans must have a run-on timer of minimum 15 minutes to extract residual moisture after use
• Background ventilators (trickle vents) — When replacing windows, trickle vents must be fitted if the existing windows had them. Minimum equivalent area (EA) for habitable rooms: 8000 mm² (5000 mm² in wet rooms). Check: does the room have a background ventilator?
• Commissioning — MVHR systems must be commissioned and a commissioning certificate issued. MEV systems must be set to design airflow rates. Commissioning is a Building Regulations requirement for new build
• Part F and airtightness — In new builds where airtightness is ≤5 m³/(h·m²) at 50 Pa, a continuous whole-dwelling ventilation strategy (System 3 or 4) is strongly recommended. Intermittent fans alone are insufficient in airtight buildings

Quick Reference Table

Detailed Guidance

System 1 — Intermittent Extract Fans

System 1 is the most common domestic ventilation strategy. It relies on:

• Extract fans in wet rooms (kitchen, bathroom, WC)
• Background ventilators (trickle vents in windows, or purpose-provided wall ventilators) in habitable rooms
• Opening windows for rapid/purge ventilation

Fan specification: For a bathroom, the minimum 15 l/s means the fan's certified flow rate (from the manufacturer's data sheet, measured at a typical installed resistance) must be ≥15 l/s. Do not rely on the nominal rating alone — measure or verify against resistance in the installed duct run.

Run-on timer: All intermittent fans in bathrooms and WCs must have a minimum 15-minute run-on timer (overrun). Humidity-sensing fans that continue to run until humidity drops below a setpoint are an alternative. Pure on/off fans with no run-on timer do not comply with ADF 2021.

Kitchen extract: A cooker hood above the hob must extract at least 30 l/s. A wall or ceiling fan located elsewhere in the kitchen (not adjacent to the cooking source) must extract at least 60 l/s. This is why a 100mm (4 inch) fan in a kitchen is marginal — a 150mm (6 inch) fan delivering 60 l/s is preferred.

Duct length and resistance: Centrifugal fans in extract fans work against the duct resistance. A long duct run (over 3m) with bends will significantly reduce flow rate. For each 90° bend, add an equivalent length of 1.5m to the duct. For metal duct (150mm diameter) with a 5m run and two bends (8m equivalent), the actual flow rate may be 25% lower than rated. Use larger-diameter duct or a more powerful fan to compensate.

Recirculation vs ducted cooker hoods: A recirculation cooker hood does NOT provide Part F ventilation — it filters and recirculates kitchen air without removing moisture. Ducted extraction is required for Part F compliance. For retrofit where ducting is impractical, a System 1 compliant alternative must be provided.

System 4 — MVHR (Mechanical Ventilation with Heat Recovery)

MVHR is increasingly common in new-build housing (especially detached and semi-detached housing where duct runs are shorter) and in Passivhaus or EnerPHit retrofit projects.

How it works: A central unit with two fans draws stale air from wet rooms (kitchen, bathroom, WC) and supplies fresh air to habitable rooms (bedrooms, living rooms). A heat exchanger in the unit transfers 75–93% of the heat from extract air to supply air — avoiding the heat loss penalty of conventional extract ventilation.

Design requirements:

• MVHR must be designed by a competent person using BRE Digest 498 or equivalent software
• Supply air must be filtered (minimum F7 filter class for urban areas)
• The system must be balanced: supply airflow rate = extract airflow rate ±10%
• Commissioning: certified to design flow rates using calibrated airflow measurement equipment

Duct design:

• Rigid circular ductwork is more efficient than semi-rigid flexible duct — use rigid for long runs, flexible for final connections only (maximum 1m)
• Minimum 100mm diameter for individual room branches; 150–200mm for main trunk duct
• All ductwork joints must be sealed to prevent air leakage reducing efficiency and contaminating supply air
• External air intake and exhaust must be separated by at least 1m and positioned to avoid short-circuiting (intake to windward side; exhaust to leeward, or use separated duct terminals)

Commissioning certificate: ADF 2021 requires a commissioning certificate for MVHR systems, which must be provided to the building control inspector on completion.

Trickle Vents (Background Ventilators)

Background ventilators allow a low level of continuous air exchange even when windows are closed. The most common form is the trickle vent in a window frame — a small controllable slot ventilator, typically 4000–8000 mm² equivalent area (EA), in the window head or frame.

When do trickle vents apply?

• New build: Always required. Habitable rooms: minimum 8000 mm² EA. Wet rooms: 5000 mm² EA (or the extract fan acts as the background ventilator via a passive opening)
• Replacement windows: If the windows being replaced had trickle vents, the replacement windows must have trickle vents. Equivalent area must be equal to or greater than what was removed
• Existing windows (no replacement): If windows are not being replaced, trickle vents in existing windows need not be upgraded (though it is good practice)

Part F and window replacement: Window replacements are notifiable under Part L (energy efficiency) when the work involves more than one window, and Part F trickle vent compliance must also be demonstrated. A window installer who removes trickle vents on replacement windows without providing equivalent background ventilation is creating a Part F defect.

Part F and Existing Dwellings (Renovation)

In existing dwellings, Part F applies to replacement ventilation systems (new extract fans replacing old ones) and to new extensions that create new habitable or wet rooms. The key rule: any new or replacement ventilation element must meet ADF 2021 standards.

For renovations and extensions:

• New bathroom → Part F extract fan required (minimum 15 l/s, run-on timer)
• New kitchen extension → Part F extract fan required (30 l/s adjacent to hob, or 60 l/s elsewhere)
• Extension creating additional floor area → the whole-dwelling ventilation rate recalculation may be required
• Re-roofing or major fabric work → triggers a check on current ventilation provision, with a requirement to improve if the work disturbs the existing ventilation

Note: Part F does not typically require wholesale ventilation upgrades in a like-for-like renovation. The trigger is disturbance or change — replacing like with like (e.g., new extract fan of equivalent or better performance) is compliant.

Frequently Asked Questions

Does a new bathroom in an extension need an extract fan if there's an openable window?

Yes — under ADF 2021, a mechanical extract fan is required in all bathrooms in new build, and in new rooms created in extensions. An openable window provides rapid ventilation but does not substitute for the mechanical extract fan in a bathroom or WC. The fan must achieve 15 l/s minimum and must have a run-on timer.

I've installed a new cooker hood that only recirculates. Is this compliant?

No. ADF 2021 specifically states that recirculation systems do not provide the extract ventilation required by Part F. Recirculation hoods filter cooking fumes and odour but do not remove moisture or provide air exchange. If ducting is genuinely not possible, you must provide a separate extract fan in the kitchen (minimum 60 l/s if not adjacent to the hob) to meet Part F requirements.

What's the difference between Part F and Part L on window replacement?

Part L1B deals with the thermal performance of the replacement window (U-value ≤1.4 W/m²K for the whole window). Part F deals with trickle vents (background ventilators). Both apply to replacement windows. The FENSA or CERTASS scheme self-certification covers both — the installer must confirm compliance with both parts. Removing trickle vents on replacement windows without equivalent provision is a Part F defect.

My MVHR unit needs servicing. What's involved?

MVHR servicing typically every 12 months:

1. Clean or replace supply air filters (G4/F7 filter pads)
2. Clean heat exchanger core (remove and brush or wash according to manufacturer's guidance)
3. Clean extract grilles and supply diffusers
4. Check and clean condensate drain from heat exchanger
5. Check and clean external air intake and exhaust terminals
6. Verify airflow rates using an airflow measurement hood (commissioned units should be re-verified) Record the service in a commissioning/service log.

Regulations & Standards

• Approved Document F (2021 edition) — Ventilation: Volume 1 Dwellings (in force June 2022)

• BS EN 13141 — Performance testing of ventilation components/products for residential buildings

• BRE Digest 498 — Selecting ventilation systems for housing

• BS 5250:2021 — Code of practice for control of condensation in buildings (linked to Part F intent)

• CIBSE TM61 — Ventilation commissioning (referenced in ADF 2021 commissioning requirements)

• GOV.UK — Approved Document F (2021) — Full text of the 2021 revision

• NHBC Technical Guidance — Ventilation — Practical guidance for housebuilders on ADF 2021 compliance

• BRE Group — Ventilation in Housing — BRE Digest 498 and related research

• bathroom fan wiring — Extract fan wiring and timer connections

• kitchen appliance circuits — Kitchen extract fan circuits and FCU connections

• airtightness — Airtightness and its interaction with Part F

• part l energy — Part L energy efficiency requirements for windows and replacement work