How do I size an MVHR unit for a UK house?

Calculate the total extract rate from all wet rooms per ADF Table 5.1b (kitchen 13 L/s, bathroom 8 L/s, WC 6 L/s). Calculate the whole dwelling ventilation rate from Table 5.1a ((bedrooms + 1) × 7 L/s, or 0.3 L/s per m²). The MVHR unit must handle whichever is greater, balanced between supply and extract. Add 50% for boost mode.

What is the whole dwelling ventilation rate?

ADF Table 5.1a specifies the minimum ventilation rate for the entire dwelling based on the number of bedrooms: 1 bed = 14 L/s, 2 bed = 21 L/s, 3 bed = 28 L/s, 4 bed = 35 L/s, 5 bed = 42 L/s. 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.

What is boost mode on an MVHR?

Boost mode temporarily increases the MVHR flow rate (typically by 50-100%) 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. The unit must be sized to deliver the boost rate without excessive noise or exceeding SFP limits.

Should MVHR supply and extract be balanced?

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.

Can I use MVHR with a separate kitchen cooker hood?

Yes, but the kitchen MVHR extract terminal and the cooker hood should not both operate simultaneously as this creates imbalanced airflow. Options include: using the MVHR boost for kitchen extraction (no separate hood), or installing a recirculating hood that does not affect the MVHR balance. Ducted cooker hoods require make-up air.

TradeCalculator

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 Table 5.1a.

Dwelling Type

Bedrooms

Living Rooms

Bathrooms

8 L/s extract each

WCs / Cloakrooms

6 L/s extract each

Kitchens

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 extract rates from all wet rooms (ADF Table 5.1b)
Calculate whole dwelling rate (ADF Table 5.1a)
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 Table 5.1a — whole dwelling rates

1 bedroom: 14 L/s
2 bedrooms: 21 L/s
3 bedrooms: 28 L/s
4 bedrooms: 35 L/s
5 bedrooms: 42 L/s

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.