Retrofitting MVHR: Can You Install Heat Recovery Ventilation in an Older Property?

Modern new-builds are designed around mechanical ventilation from the outset. Ductwork routes are planned into the building fabric, plant rooms accommodate air-handling units, and the airtightness of the envelope is tested to confirm the system will perform as designed.

Pre-1960s UK housing was built on a fundamentally different premise: natural infiltration through gaps, chimney stacks, suspended timber floors, and single-glazed windows provided incidental ventilation. Retrofitting MVHR into this fabric requires solving four specific engineering challenges:

• Routing supply and extract ductwork to every habitable room and wet room without unacceptable visual impact or structural compromise
• Finding a location for the air-handling unit that is accessible for maintenance and connected to an external discharge point
• Assessing airtightness to determine whether the envelope is tight enough for MVHR to be effective
• Managing condensation within insulated duct runs passing through cold roof voids or external walls

Victorian Terraced Houses

The Victorian terrace's greatest retrofit asset is its chimney stack. Where an original chimney breast has been removed, the flue void often provides a workable vertical duct route connecting multiple floors. Party walls, however, cannot legally be penetrated without formal agreement under the Party Wall Act, which limits cross-building routing options.

Ceiling heights in Victorian properties—often 2.8 m to 3.2 m—do provide more void space above boxed-out ductwork than in 1970s stock. Many Flexivent engineers achieve minimal visual impact by routing flat-oval ductwork within purpose-built boxings that double as architectural features or by integrating runs within refurbished cupboards.

1930s Semi-Detached Homes

The 1930s semi presents a different set of opportunities. The characteristic shallow roof pitch and large loft space make this property type one of the more retrofit-friendly older build forms. The air-handling unit can typically be located in the loft, with relatively short supply duct runs to first-floor bedrooms and longer but manageable drops to ground-floor living spaces.

The cavity-wall construction of most 1930s properties—unlike the solid brick of Victorian stock—simplifies the creation of external termination points for supply and exhaust. Correctly designed terminals must be positioned to avoid short-circuiting (supply air re-entering the exhaust terminal), a fundamental design requirement that is often overlooked in lower-cost installations.

MVHR performance is directly coupled to envelope airtightness. In a leaky building, the ducted airflow rates become a small proportion of total air change, and the heat recovery benefit is diluted by uncontrolled cold infiltration. CIBSE guidance suggests that MVHR is most cost-effective in properties with an air permeability below 5 m³/(h.m²) at 50 Pa—a standard readily achievable in modern construction but requiring deliberate intervention in older stock.

Based on our project experience, MVHR retrofit works best in older properties when:

• The property is undergoing a substantial refurbishment, making duct installation less disruptive and more cost-effective
• The client has specific health requirements—clinical allergy diagnoses, respiratory conditions—where the filtration benefit of MVHR is clinically significant
• The property has already been improved to a reasonable airtightness standard through insulation and window replacement programmes
• There is an accessible loft or basement space for the air-handling unit, with a clear path to an external termination point

Flexivent engineers do not recommend MVHR as a default for every older property. Where a full retrofit is impractical or the cost-benefit case is marginal, we frequently specify:

• Positive Input Ventilation (PIV) — highly effective at eliminating condensation and mould in older properties, with a single loft installation point and no distributed ductwork requirement
• Centralised Mechanical Extract Ventilation (cMEV) — paired with trickle ventilators in habitable rooms, providing continuous wet-room extraction without whole-house ductwork
• Hybrid approaches — PIV combined with high-performance extract fans in kitchens and bathrooms, commissioned as an integrated system to achieve Part F compliance

Our pre-design survey for older properties covers: measured survey of ceiling voids and roof space; airtightness assessment; thermal imaging to identify cold bridges near proposed duct routes; occupant interview to establish moisture load and health priorities; and a preliminary system schematic with two or three design options presented at different investment levels.

This is not a sales visit. It is an engineering consultation, and the output is a written technical recommendation you can act on confidently.

Contact Flexivent to arrange your retrofit survey today. We serve clients across the UK and specialise in finding the most effective ventilation solution for the specific constraints of your property.