Do I need MVHR in a London extension?

Building Regulations Approved Document F requires whole-dwelling ventilation strategy. Continuous extract (PIV or dMEV) is the minimum for most retrofits. MVHR is required only where: new dwelling targeting <3 m³/h/m² airtightness; Passivhaus/EnerPHit certification; non-domestic rated NSO conversions. For a typical kitchen extension, kitchen mechanical extract + trickle vents satisfies regs — MVHR is a choice driven by indoor air quality and energy ambition, not a code mandate.

Will MVHR fix damp and mould?

Yes — MVHR continuously extracts moisture from wet rooms and delivers dry filtered air to bedrooms, eliminating the condensation conditions that drive mould. Effective in airtight homes; less effective in leaky envelopes (humidity drifts back in via gaps). PIV is the simpler damp-fix in older non-airtight houses. Always combine with damp source elimination — fixing leaks, repairing rendering, treating rising damp — ventilation is the second line, not the first.

Can I retrofit MVHR in a Victorian terrace?

Yes but complex. Routes typically through under-stairs cupboards, behind chimney boxing, in suspended ceiling zones, or via dropped ceilings in hallways. Period houses often have generous floor voids (250mm joist depth) that accommodate ducting if floorboards are being lifted anyway during a full renovation. Expect £8,500–£14,500 for whole-house Victorian retrofit. dMEV per-room is often a more pragmatic alternative.

Does PIV work in a London flat?

Yes — PIV is particularly suited to single-aspect London flats (lower-ground basement, ex-local authority, conversion flats) where bathroom and kitchen extract is inadequate and condensation drives mould. Loft access not required: hallway-mounted PIV units (Nuaire Drimaster Heat) install in any ceiling void. Cost £550–£950 in a flat. Verify with managing agent before installing — most leasehold consents are routine.

MVHR vs PIV London Renovation 2026 | Builderr

Q: Will MVHR fix damp and mould?

Yes — MVHR continuously extracts moisture from wet rooms and delivers dry filtered air to bedrooms, eliminating the condensation conditions that drive mould. Effective in airtight homes; less effective in leaky envelopes (humidity drifts back in via gaps). PIV is the simpler damp-fix in older non-airtight houses. Always combine with damp source elimination — fixing leaks, repairing rendering, treating rising damp — ventilation is the second line, not the first.

Q: Can I retrofit MVHR in a Victorian terrace?

Yes but complex. Routes typically through under-stairs cupboards, behind chimney boxing, in suspended ceiling zones, or via dropped ceilings in hallways. Period houses often have generous floor voids (250mm joist depth) that accommodate ducting if floorboards are being lifted anyway during a full renovation. Expect £8,500–£14,500 for whole-house Victorian retrofit. dMEV per-room is often a more pragmatic alternative.

Q: Does PIV work in a London flat?

Yes — PIV is particularly suited to single-aspect London flats (lower-ground basement, ex-local authority, conversion flats) where bathroom and kitchen extract is inadequate and condensation drives mould. Loft access not required: hallway-mounted PIV units (Nuaire Drimaster Heat) install in any ceiling void. Cost £550–£950 in a flat. Verify with managing agent before installing — most leasehold consents are routine.

MVHR — how it works and when it's right

Mechanical Ventilation with Heat Recovery extracts stale humid air from wet rooms (kitchens, bathrooms, utility) and supplies fresh filtered air to dry rooms (bedrooms, living rooms) via a central unit with a counter-flow heat exchanger. Heat exchanger transfers 88–92% of outgoing heat to incoming fresh air — so winter supply air is 18–20°C rather than 0–5°C, reducing space heating demand by 15–25% in airtight homes. Install cost £4,500–£12,000 depending on house size: typical 4-bed London house with 8–10 extract/supply points £6,800–£8,500. Components: central unit (Zehnder ComfoAir Q450, Vent-Axia Sentinel Kinetic Advance) in plant cupboard or loft; semi-rigid 75mm or 90mm radial ducting; manifold; valves; intake/exhaust through external wall or roof. Filters: G4 standard + F7 pollen, change every 6 months £35–£85/set. Right for: new extensions with exposed ceiling voids (ducting easy), deep retrofits achieving <3 m³/h/m² @50Pa airtightness, basement conversions (no operable windows), Passivhaus and EnerPHit projects. Wrong for: leaky old houses (heat recovery wasted if air leaks elsewhere), retrofits with no duct routes, budgets under £5,000.

PIV — the simple damp-fix alternative

Positive Input Ventilation pressurises the house slightly by pushing filtered outside air in through a single loft-mounted or hallway-mounted unit. Stale humid air is forced out through natural gaps in the building envelope (window frames, letter boxes, extract fans). Install cost £450–£1,200 including unit (Nuaire Drimaster Eco, Vent-Axia Pure Air Home) and basic electrical connection — typically a 1–2 day install. No ducting required — single ceiling diffuser feeds whole house. Right for: condensation and mould problems in Victorian/Edwardian terraces with poor airtightness, basement flats with damp, ex-local-authority flats with single-aspect bathrooms, listed buildings where ducting routes prohibited. Heat recovery: none — but typically only 200–400W input fan operating cost £35–£60/year so impact on energy bills is minor. Limitations: doesn't work in highly airtight homes (<5 m³/h/m² @50Pa) because there's nowhere for displaced air to exit; tenants must keep extract fans/trickle vents functional; over-pressurising risks driving warm humid air into cold cavity walls causing interstitial condensation. Always survey for these risks.

Retrofit feasibility — the duct problem

MVHR retrofit feasibility is dominated by the duct routing question. New build or extension: easy — design ductwork into floor voids and ceiling zones from day one. Loft conversion: routes from loft-mounted central unit down to ground floor wet rooms typically require dropping ducting through cupboards, behind boxed-in chimney breasts, or in suspended ceiling zones (60–80mm depth needed for semi-rigid 90mm duct). Full whole-house retrofit: requires ceiling zone of 100mm+ in dry rooms — usually means lowering ceilings, lifting first floor floorboards, or running through cupboards. Listed building or conservation area: external intake/exhaust grilles may require LBC/planning. Typical retrofit cost adder £2,500–£6,500 over and above standard new-build install due to construction complexity. Where MVHR retrofit is impractical: use dMEV (decentralised mechanical extract ventilation) — individual continuous-running extract fans in each wet room £180–£350 each; or PIV.

Cost comparison and lifecycle

MVHR new-build/extension £4,500–£8,500 install + £150/year filters and electricity = £8,000–£12,000 over 15 years. Heat recovery saves £180–£320/year on heating bill — net 15-year cost £5,300–£8,400. PIV £450–£1,200 install + £40/year electricity = £1,050–£1,800 over 15 years; no heat recovery savings. dMEV £550–£1,400 install per dwelling + £30/year = £950–£1,850 over 15 years; no heat recovery. MVHR pays back in airtight homes only — at 6 ACH/h leakage MVHR delivers nominal heat recovery, at <3 ACH/h heat recovery is significant. Heat recovery efficiency degrades 8–15% if filters not changed every 6 months. MVHR commissioning is mandatory under Approved Document F — installer measures flow rates at every grille and provides commissioning certificate.

MVHR vs PIV for London Renovation: Which Ventilation System?