Size a continuous mechanical supply-and-extract heat-recovery system (MVHR) to Approved Document F 2021, the whole-dwelling minimum ventilation rate against each wet room's Table 1.2 boost extract requirement.
Tender-stage only. Verify final design to project spec and Approved Document F, Volume 1: Dwellings.
For design guidance only. Always verify with a qualified engineer.
About this MVHR sizing calculator
This free MVHR sizing calculator sizes a continuous mechanical ventilation with heat recovery (MVHR) system for a dwelling to Approved Document F, Volume 1 (2021 edition). It is aimed at ventilation designers, M&E contractors and building-regs consultants establishing the whole-house design airflow for a balanced supply-and-extract unit. Enter the number of bedrooms and internal floor area, and how many kitchens, utility rooms, bathrooms and WCs the dwelling has, and it returns the whole-dwelling minimum supply rate, the total wet-room boost extract, which side governs and the design airflow to size the unit to. Everything runs in your browser. Nothing is uploaded.
How MVHR sizing is calculated
The whole-dwelling minimum supply is the greater of 0.3 l/s per m² of floor area (para 1.24a) and the Table 1.3 bedroom-count rate (19 l/s for 1 bed up to 43 l/s for 5, +6 l/s per bedroom beyond 5). The extract side sums each wet room’s Table 1.2 high-rate boost, 13 l/s kitchen, 8 l/s utility, 8 l/s bathroom, 6 l/s WC, times its room count. The design airflow is the greater of the two, since a balanced MVHR must satisfy both supply and extract (Appendix A; BS EN 13141-7), and the tool reports whether supply or extract governs.
Frequently asked questions
Does floor area or bedroom count drive the supply rate?
Whichever gives the larger figure. The standard requires both conditions to be met, so the calculator takes the greater of the 0.3 l/s/m² floor-area rate and the bedroom-count rate from Table 1.3. A large open-plan flat can be floor-area-driven, while a compact multi-bedroom house is usually bedroom-driven.
Why does the extract side sometimes govern?
Because the design airflow is the greater of the whole-dwelling supply minimum and the total wet-room boost extract. A dwelling with several wet rooms (a kitchen plus multiple bathrooms) can need more boost extract than its supply minimum, so extract governs and sizes the unit; the tool tells you which side is binding.
What is not covered here?
This is the airflow sizing only. You still need to confirm the unit against a Specific Fan Power calculation (Approved Document L, Table 6.9) and its heat-recovery efficiency to BS EN 13141-7, provide purge ventilation of at least 4 air changes per hour to habitable rooms, and note that MVHR dwellings should not also have background trickle vents. Verify the final design against Approved Document F.