What is MVHR and how does it work?

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.

What are the Part F requirements for MVHR (System 4)?

Approved Document F System 4 requires a minimum whole-dwelling ventilation rate of 0.3 l/s per m² of internal floor area, or the total of all extract rates from wet rooms, whichever is greater. Supply air must be delivered to every habitable room, and extract air removed from every wet room. The MVHR unit must have a specific fan power (SFP) of no more than 1.0 W/(l/s) and a heat recovery efficiency of at least 73%.

Where should an MVHR unit be installed?

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.

What duct sizes are used for MVHR?

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.

How much can MVHR save on heating bills?

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.

Heat Recovery Ventilation Calculator — MVHR Sizing

Size an MVHR (Mechanical Ventilation with Heat Recovery) unit for a UK dwelling. Calculates flow rates, duct sizes, and supply/extract points per Approved Document F System 4.

Dwelling Type

Pre-fills typical values or select Custom

Internal Floor Area

Total internal floor area of the dwelling

Number of Bedrooms

Each bedroom gets a supply air valve

Number of Bathrooms

Full bathrooms: 8 l/s extract each

Number of Shower Rooms

Separate shower rooms: 8 l/s each

Number of Separate WCs

Separate WCs: 6 l/s each

How We Calculate This

This calculator sizes an MVHR system for a UK dwelling using the methodology in Approved Document F (2021 edition) for System 4 — continuous supply and extract with heat recovery.

Design flow rate

The design flow rate is the higher of two calculations:

Whole-dwelling rate: 0.3 l/s per m\u00B2 of internal floor area
Total extract rate: Sum of minimum extract rates from all wet rooms (kitchen 13 l/s, bathroom 8 l/s, WC 6 l/s, utility 8 l/s)

Supply and extract points

Supply air is delivered to each habitable room (bedrooms and living room). Extract air is removed from each wet room (kitchen, bathrooms, WCs, utility). The total supply must equal the total extract to maintain pressure balance.

Energy savings

Annual energy savings are estimated using UK average heating degree days (2200), the specified heat recovery efficiency, and the design flow rate. Savings are converted to GBP using current gas prices.