Ventilation in London Home Renovations in 2025: Part F, MVHR, dMEV, and Air Quality

Part F ventilation requirements, whole-house MVHR systems, decentralised MEV, and PIV for London home renovations in 2025: PART F BUILDING REGULATIONS 2021 (ENGLAND) — VENTILATION REQUIREMENTS FOR DWELLINGS: Part F (Means of Ventilation) of the Building Regulations applies to ALL WORK on dwellings (not just new buildings) where the work affects the ventilation of the property; the most significant update in the PART F 2021 (Approved Document F, Volume 1: Dwellings — effective from 15 June 2022 for England) is: INCREASED MINIMUM VENTILATION RATES (particularly for whole-house ventilation systems); INCREASED TRICKLE VENT REQUIREMENTS (minimum EQUIVALENT AREA in replacement windows increased from 4,000mm² to 8,000mm² for ventilated rooms in dwellings undergoing renovation — IMPORTANT: this applies when the window is being replaced); NEW REQUIREMENT FOR VENTILATION PURGE: every habitable room must have the ability to be PURGE VENTILATED (either a window that opens to provide at least 1/20th of the floor area of the room, or a ventilation system capable of purge rates); FOUR VENTILATION STRATEGIES DEFINED IN PART F (SYSTEMS A-D): SYSTEM A (BACKGROUND AND INTERMITTENT EXTRACT — THE TRADITIONAL APPROACH): BACKGROUND VENTILATION: TRICKLE VENTILATORS in windows and doors (the small slots or grilles at the top of window frames that provide CONTINUOUS BACKGROUND AIRFLOW); INTERMITTENT EXTRACT: fans in kitchen and bathroom that are used when cooking or showering; this is the most common system in existing London dwellings; BACKGROUND VENTILATION RATES REQUIRED UNDER PART F 2021: for a REPLACEMENT WINDOW (HABITABLE ROOM): minimum 8,000mm² EQUIVALENT AREA trickle vent (up from 4,000mm² in the 2010 version of Part F); for NEW WINDOWS (IN A NEW EXTENSION): 8,000mm² per habitable room; KITCHEN: 8,000mm²; WET ROOMS (BATHROOM, WC, UTILITY): 4,000mm²; INTERMITTENT EXTRACT RATES REQUIRED: KITCHEN: 13 litres/second (l/s) continuous; or 30 l/s intermittent with boost; WET ROOM (BATHROOM WITH SHOWER OR BATH): 8 l/s continuous; or 15 l/s intermittent with boost; SYSTEM B (PSV — PASSIVE STACK VENTILATION): a PASSIVE SYSTEM that uses the TEMPERATURE DIFFERENCE and WIND EFFECT to draw air up from the rooms to outlets at the roof; does not require electricity; uncommon in new London renovation projects (most modern buildings rely on mechanical systems); SYSTEM C (CONTINUOUS MECHANICAL EXTRACT — WHOLE-HOUSE MEV): a CENTRAL OR DECENTRALISED MECHANICAL EXTRACT SYSTEM running continuously at LOW RATE (providing background extract); can boost to higher rate in kitchens and bathrooms when sensors detect humidity or CO2; WHOLE-HOUSE MVHR (MECHANICAL VENTILATION WITH HEAT RECOVERY — SYSTEM D): WHAT IS MVHR: a whole-house ventilation system that extracts STALE, HUMID AIR from wet rooms (kitchen, bathrooms, utility) and supplies FRESH, FILTERED AIR to living rooms and bedrooms; the extract and supply air streams pass through a HEAT EXCHANGER in the MVHR UNIT, transferring HEAT from the outgoing stale air to the incoming fresh air (typically 85-95% heat recovery efficiency); the result is FRESH AIR supplied at close to indoor temperature (no draughts) while maintaining indoor air quality; KEY BENEFITS OF MVHR IN A LONDON HOME RENOVATION: ENERGY EFFICIENCY: recovers 85-95% of the heat from extracted air (less heating energy needed to warm incoming fresh air); INDOOR AIR QUALITY: continuous filtered fresh air supply removes CO2, cooking odours, VOCs from building materials, and external pollutants; F7 HEPA-class filters can filter out particulate matter (PM2.5) from London traffic air pollution; HUMIDITY CONTROL: continuous extract from wet rooms controls humidity (reduces condensation and mould risk); ACOUSTIC BENEFIT: all ventilation through the MVHR ductwork (no open trickle vents in windows) — better acoustic performance in a London location near roads; MVHR SUITABILITY FOR LONDON RENOVATIONS: IDEAL FOR: a NEW BUILD EXTENSION that is being built to HIGH THERMAL PERFORMANCE (Part L compliant air permeability <5m³/h.m² — which is tight enough that background ventilation through trickle vents and draughts is inadequate); a LOFT CONVERSION where the whole house is being brought to improved thermal performance; LESS SUITABLE FOR: an existing Victorian terrace where the main house is NOT being fully upgraded (the background infiltration through gaps in the old fabric means the MVHR cannot control the overall air flows effectively — it becomes a kitchen and bathroom extract rather than a whole-house system); MVHR DUCTWORK IN LONDON RENOVATIONS: RIGID DUCT (SPIRALLY WOUND GALVANISED STEEL — THE PERFORMANCE CHOICE): minimum friction loss; the most efficient duct type; FLEXIBLE DUCT (SEMI-RIGID OR FLEXIBLE PVC SPIRAL DUCT — WIDELY USED BUT WITH A PERFORMANCE PENALTY): if flexible duct is specified, it must be SUPPORTED THROUGHOUT ITS LENGTH and must have NO KINKS OR BENDS (each unintended kink in a flexible duct reduces the system airflow significantly); IMPORTANT INSTALLATION NOTE: MVHR SYSTEMS FREQUENTLY UNDERPERFORM IN LONDON INSTALLATIONS BECAUSE THE FLEXIBLE DUCTWORK IS POORLY INSTALLED (kinked, unsupported, overly long runs) — the resulting system airflow is well below design; always commission a DUCTWORK AIR FLOW TEST after installation; MVHR UNIT TYPES AND BRANDS IN LONDON 2025: ZEHNDER COMFOAIR Q (THE PREMIUM CHOICE — VERY QUIET OPERATION, EFFICIENT FANS, INTEGRATED FILTERS): £1,500-£3,000 unit cost; PAUL NOVUS 300 OR 450: excellent performance; VENT-AXIA SENTINEL KINETIC PLUS; DRIMASTER PIV UNITS (not MVHR — see below); DECENTRALISED MEV (dMEV — DECENTRALISED MECHANICAL EXTRACT VENTILATION): WHAT IS dMEV: a WET ROOM FAN (IN KITCHEN, BATHROOM, OR WC) that runs CONTINUOUSLY at a LOW SPEED (typically 3-8 l/s continuous) and BOOSTS to a higher speed (15-30 l/s) when a HUMIDITY SENSOR detects elevated moisture or when a WALL SWITCH or PULL CORD is operated; the most widely used SYSTEM C COMPLIANT product in UK homes (every kitchen and bathroom has its own fan rather than one central unit); HUMIDITY-CONTROLLED dMEV FANS (THE STANDARD FOR COMPLIANCE WITH PART F SYSTEM A IN LONDON RENOVATION): VENT-AXIA SILENT FAN WITH HUMIDITY SENSOR (LO-CARBON SENTINEL KINETIC): the market-leading humidity-controlled bathroom fan in UK renovation; MANROSE MF100T OR MF100H: timer or humidity sensor; ENVIROVENT SILENT: very low noise (useful in London bedrooms with adjacent en suites); THE KEY REQUIREMENT: the fan must RUN CONTINUOUSLY when the dwelling is occupied (the Part F 2021 System A requires continuous background ventilation — an intermittent fan that only operates when the room is occupied or when the switch is on does NOT meet the continuous ventilation requirement without a background trickle vent complement); PIV (POSITIVE INPUT VENTILATION): WHAT IS PIV: a unit installed in the LOFT or in the HALLWAY that draws air from the loft space (through the building fabric) or from outside and gently PUSHES IT INTO THE DWELLING at LOW RATE; the positive pressure in the dwelling drives stale, humid air out through gaps and trickle vents; DRIMASTER ECO (THE DOMINANT PRODUCT): installed in the loft; draws air through a filter; WIDELY USED AS A RETROFIT SOLUTION FOR CONDENSATION AND MOULD IN LONDON VICTORIAN TERRACES; PIV is NOT a ventilation strategy recognised under Part F System D (MVHR) but may be used as a supplementary system to improve condensation control; the limitation of PIV in London is that PUSHING AIR FROM THE LOFT IN WINTER can introduce cold air into the hallway unless the unit has a FROST PROTECTION HEATER.

Kitchen extraction, trickle vent specification, indoor air quality, and realistic ventilation costs for London home renovations in 2025: KITCHEN EXTRACTION IN LONDON HOME RENOVATIONS: KITCHEN COOKER HOOD OR CEILING EXTRACTOR IS THE PRIMARY SOURCE OF MOISTURE AND COOKING ODOUR EXTRACT IN THE KITCHEN: TYPES OF KITCHEN EXTRACTION: DUCTED EXTRACTION (STRONGLY PREFERRED): the extracted air is carried through DUCTWORK to the OUTSIDE of the building; this removes moisture, grease, and odours from the kitchen permanently; EXTRACTION RATE: for a LONDON KITCHEN RENOVATION with a gas hob (even though gas hobs are increasingly unusual in new London kitchens — see FUTURE OF GAS IN LONDON HOMES below) or electric hob, the extraction rate should be: MINIMUM 30 L/S (PART F INTERMITTENT EXTRACT RATE FOR A KITCHEN) for a simple extract fan; RECOMMENDED FOR A LONDON KITCHEN RENOVATION WITH COOKING: 50-100 L/S (EFFECTIVE EXTRACTION CAPACITY) for a cooker hood over a standard 600mm hob; more powerful extraction (150-300 l/s) for large hobs, induction hobs with high peak cooking loads, or open-plan kitchens where cooking odour management is important; RECIRCULATING EXTRACTION (NOT SUITABLE AS THE SOLE MEANS OF EXTRACT): RECIRCULATING HOODS use a CARBON FILTER to remove grease and some odours and return the air to the kitchen; they do NOT extract moisture; a recirculating hood does NOT satisfy the Part F ventilation requirement for a kitchen (because it does not remove moisture); may be used SUPPLEMENTARY to a ducted system or background trickle ventilation; USED ONLY WHERE DUCTING TO OUTSIDE IS GENUINELY IMPOSSIBLE (e.g. an island hob in a basement extension where routing the duct to outside is structurally impossible); DUCT ROUTING CONSIDERATIONS FOR LONDON KITCHEN EXTRACTIONS: the duct must be as SHORT AS POSSIBLE and with as FEW BENDS AS POSSIBLE (each bend reduces extraction efficiency); a maximum of approximately 3m of RIGID SMOOTH BORE DUCT and 1-2 gentle 90° bends for a standard wall-terminated extract; DUCT DIAMETER: typically 125mm or 150mm for a standard kitchen hood; 150mm or 200mm for higher-power extract; EXTRACT TERMINATION: the duct must terminate through the EXTERNAL WALL or through the ROOF (with appropriate weatherproofing — a WALL GRILLE with a DAMPER FLAP to prevent back-draught); in London rear extension kitchens, the duct often exits through the REAR WALL (the shortest route) or through the FLAT ROOF (an upward route through the flat roof structure); OPEN-PLAN KITCHEN IN LONDON EXTENSIONS (THE DOMINANT LAYOUT IN 2025): the OPEN-PLAN KITCHEN/DINING/LIVING ROOM in the rear ground floor of a London Victorian terrace is the most common renovation outcome in 2025; for OPEN-PLAN SPACES, kitchen extraction is particularly important because cooking odours spread through the full open-plan area; the cooker hood should be positioned DIRECTLY OVER THE HOB (not displaced to the side — a 600mm hob should have a 600mm or 900mm canopy positioned directly above) and should have SUFFICIENT EXTRACTION POWER (100 l/s minimum for an open-plan kitchen); FUTURE OF GAS COOKING IN LONDON: the UK government has been moving towards ENDING THE INSTALLATION OF GAS HOBS IN NEW HOMES (through updated Part L and building regulations — check the current status as policy has been evolving); many London renovations in 2025 are specifying INDUCTION HOBS (electric) in new kitchens; induction has different extraction requirements (no combustion products, but still significant steam, grease, and odour from cooking); TRICKLE VENT REQUIREMENTS IN LONDON HOME RENOVATIONS 2025: WHAT ARE TRICKLE VENTS: trickle ventilators are small SLOTS OR GRILLES fitted in the frame of windows or external doors that provide CONTINUOUS BACKGROUND VENTILATION (the minimum airflow needed to remove moisture and maintain CO2 at acceptable levels) without needing any mechanical fan; PART F 2021 TRICKLE VENT REQUIREMENTS (ENGLAND): when replacing a window in a habitable room in an existing dwelling: minimum EQUIVALENT AREA of 8,000mm² per replacement window (up from 4,000mm² in the 2010 edition); for new windows in a new extension or loft conversion: 8,000mm² per habitable room window; for bathrooms/WC/kitchen: 4,000mm²; EQUIVALENT AREA vs GEOMETRIC AREA: EQUIVALENT AREA is the measured airflow performance of the vent (in mm²) rather than the physical size of the slot — it is measured by the manufacturer; a vent with a 8,000mm² equivalent area may have a much larger physical slot (the flow path through the vent is not frictionless); WHAT HAPPENS IF A REPLACEMENT WINDOW DOES NOT INCLUDE TRICKLE VENTS: where the EXISTING WINDOW HAD NO TRICKLE VENT, and the replacement window is installed in a BUILDING THAT HAS OTHER ADEQUATE MEANS OF VENTILATION (e.g. a wet-room extract fan that runs continuously), a trickle vent may not be required — BUILDING CONTROL SHOULD BE CONSULTED before omitting trickle vents from replacement windows; PLANNING NOTE ON TRICKLE VENTS: trickle vents fitted into TIMBER SASH WINDOWS in London conservation areas should be CONCEALED WITHIN THE MEETING RAIL (the horizontal rail where the two sashes meet) or fitted as INTEGRAL SLOT VENTS in the frame members — not as prominent applied-on grille vents that alter the character of the window; INDOOR AIR QUALITY IN LONDON HOMES — THE SPECIFIC CHALLENGE: LONDON'S OUTDOOR AIR POLLUTION: London regularly exceeds WHO guidelines for PM2.5 (particulate matter from diesel vehicles and other sources) and NO2; bringing UNFILTERED OUTDOOR AIR INTO A LONDON HOME via trickle vents and basic extract fans may introduce polluted outdoor air into the house; MVHR WITH F7 HEPA-CLASS FILTERS filters out PM2.5 from outdoor air before it enters the living space — this is a significant benefit for London homes close to busy roads (zones with high traffic volumes, bus routes, and HGV movements); AIR QUALITY SENSORS IN LONDON HOMES: INDOOR AIR QUALITY MONITORS (AIRTHINGS WAVE PLUS, NETATMO HEALTHY HOME COACH, FOOBOT): measure CO2, VOC (volatile organic compounds from off-gassing building materials, paints, and furniture), humidity, PM2.5; CO2 as an indicator of ventilation effectiveness: CO2 below 800 ppm = well-ventilated; 800-1,200 ppm = acceptable; above 1,200 ppm = poor ventilation (typical of an unventilated London bedroom with windows closed in winter); RADON IN LONDON: RADON (a radioactive gas emitted from certain geological formations) is not a significant risk in most of London (the geology is generally low-radon); some areas of London — particularly OUTER SOUTH-EAST LONDON and PARTS OF BEXLEY, BROMLEY, and CROYDON — may have elevated radon potential; CHECK THE UK HEALTH SECURITY AGENCY RADON MAP (ukhsa.gov.uk) for the specific property postcode if in an outer London area; ventilation costs: BATHROOM HUMIDITY-CONTROLLED dMEV FAN (SUPPLY AND INSTALL — VENT-AXIA SILENT OR EQUIVALENT): approximately £150-£400; KITCHEN COOKER HOOD (SUPPLY AND INSTALL — EXCLUDING DUCT ROUTE): approximately £200-£1,500+ (wide range depending on product specification); DUCT ROUTE FOR KITCHEN EXTRACTOR (THROUGH WALL — LABOUR AND MATERIALS — STANDARD 125mm-150mm RIGID DUCT ROUTE): approximately £200-£600; MVHR UNIT (ZEHNDER COMFOAIR Q OR EQUIVALENT — SUPPLY ONLY): approximately £1,500-£3,000; MVHR FULL INSTALLATION (UNIT, DUCTWORK, COMMISSIONING — 3-BED TERRACE): approximately £5,000-£12,000; POSITIVE INPUT VENTILATION UNIT (DRIMASTER ECO — SUPPLY AND INSTALL): approximately £300-£700.