Heat Recovery Ventilation Calculator — MVHR Sizing

MVHR (Mechanical Ventilation with Heat Recovery) is a whole-house ventilation system that continuously supplies fresh air to habitable rooms (bedrooms, living room) and extracts stale air from wet rooms (kitchen, bathrooms, WCs). A heat exchanger in the central unit recovers 85-95% of the heat from the outgoing air and transfers it to the incoming fresh air. This provides ventilation without the heat loss of opening windows.

Approved Document F (2021, Vol 1) System 4 requires a minimum whole-dwelling ventilation rate of 0.3 l/s per m² of internal floor area, or the total of the wet-room extract high rates (Table 1.2: kitchen 13, bathroom/utility 8, WC 6 l/s), whichever is greater. Supply air must be delivered to every habitable room (distributed by room volume) and extract air removed from every wet room. Part F itself sets no specific fan power or heat-recovery efficiency figure — those come from Part L: under the Domestic Building Services Compliance Guide the maximum specific fan power (SFP) for a continuous balanced MVHR is 1.5 W/(l/s), and the minimum heat-recovery efficiency is about 73% (Part L / Ecodesign ErP Regulation 1253/2014).

The MVHR unit should ideally be installed centrally within the dwelling to minimise duct run lengths — a loft, utility room, or central cupboard is typical. It must be accessible for filter changes (every 3-6 months) and servicing. The unit needs two external connections: one for fresh air intake and one for exhaust air discharge, ideally on opposite sides of the building to avoid short-circuiting.

Main trunk ducts are typically 125-200mm diameter depending on the total flow rate. Branch ducts to individual rooms are usually 75mm or 100mm diameter semi-rigid duct. Radial (octopus) distribution systems use 75mm ducts from a central plenum, while trunk-and-branch systems use larger main ducts with 100mm branches. Duct velocity should be kept below 3-4 m/s for quiet operation.

A typical 3-bedroom UK house with MVHR can save approximately 1,500-2,500 kWh per year compared to opening windows for ventilation, equating to roughly £100-£180 per year on gas heating at current prices. The actual saving depends on the house size, air tightness, local climate, and MVHR efficiency. MVHR is most cost-effective in well-insulated, airtight new builds where air tightness is below 3 m³/h.m² at 50 Pa.