MVHR Unit Sizing Calculator — Balanced Ventilation

Calculate the total continuous extract rate from all wet rooms per ADF 2021 Table 1.2 (kitchen 13 L/s, bathroom 8 L/s, WC 6 L/s, utility 8 L/s). Every wet room — including the kitchen — needs its own continuous MVHR extract terminal at the Table 1.2 high rate (ADF 2021 para 1.70). Calculate the whole-dwelling ventilation rate from ADF 2021 Table 1.3 (13 + 6 × number of bedrooms L/s, i.e. 1 bed 19, 2 bed 25, 3 bed 31, 4 bed 37, 5 bed 43; never less than 0.3 L/s per m² of floor area). The MVHR unit must handle whichever is greater, balanced between supply and extract. A boost allowance of roughly 50% is a common design rule of thumb, but ADF does not define a flat multiplier — size the boost against the unit manufacturer’s commissioning data so every wet room can reach its Table 1.2 high rate at once.

ADF 2021 Table 1.3 specifies the minimum whole-dwelling ventilation rate based on the number of bedrooms: 1 bed = 19 L/s, 2 bed = 25 L/s, 3 bed = 31 L/s, 4 bed = 37 L/s, 5 bed = 43 L/s (add 6 L/s for each further bedroom). If the floor area is large, use 0.3 L/s per m² if this gives a higher value. This total must be distributed to supply rooms.

Boost mode temporarily increases the MVHR flow rate (commonly by 50-100% as a design convention — this is not an ADF figure) when wet rooms are in use, for example during cooking or bathing. It can be triggered manually by a switch, automatically by humidity sensors, or by a CO2 sensor. In regulatory terms the boost requirement is that every wet room can reach its ADF 2021 Table 1.2 high rate (kitchen 13, bathroom/utility 8, WC 6 L/s) at the same time. The unit must be sized to deliver this without excessive noise or exceeding SFP limits — confirm against the manufacturer’s commissioning data.

Yes, MVHR systems should be balanced so that total supply airflow equals total extract airflow. This prevents the dwelling from being pressurised (which can push moisture into the building fabric) or depressurised (which increases infiltration). Commissioning should achieve balance within ±10% at each terminal.

Yes. A cooker hood is a separate, supplementary device and does NOT remove the kitchen’s MVHR extract requirement — under ADF 2021 para 1.70 the kitchen is a wet room and must keep its 13 L/s continuous MVHR extract terminal whether or not a hood is fitted. A ducted hood is an intermittent extract (ADF Table 1.1: 30 L/s if it ducts to outside, 60 L/s if recirculating or absent) and needs make-up air; it should not run at the same time as the kitchen MVHR valve, as that imbalances the system. A recirculating hood does not count as ventilation for Part F at all. Many designs simply use the MVHR (with boost) for kitchen extraction and fit a recirculating hood for grease/odour only.