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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.
Total internal floor area of the dwelling
Each bedroom gets a supply air valve
Full bathrooms: 8 l/s extract each
Separate shower rooms: 8 l/s each
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 m² 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.
Frequently Asked Questions
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.