MVHR Unit Sizing Calculator — Balanced Ventilation
Size MVHR units by calculating total extract rates from wet rooms and matching supply to habitable rooms per Approved Document F (2021) Table 1.3.
8 L/s extract each
6 L/s extract each
13 L/s extract (if no separate hood)
How We Calculate This
This calculator sizes MVHR units (System 4 per Approved Document F) by balancing the extract demand from wet rooms against the supply to habitable rooms.
Sizing methodology
- Sum continuous extract rates from all wet rooms (ADF 2021 Table 1.2)
- Calculate whole-dwelling rate (ADF 2021 Table 1.3)
- Unit capacity = max(total extract, whole dwelling rate)
- Boost mode = trickle rate × boost factor (typically 1.5×)
- Supply air distributed equally to bedrooms and living rooms
ADF 2021 Table 1.3 — whole-dwelling rates
- 1 bedroom: 19 L/s
- 2 bedrooms: 25 L/s
- 3 bedrooms: 31 L/s
- 4 bedrooms: 37 L/s
- 5 bedrooms: 43 L/s (add 6 L/s per further bedroom)
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.