What Size MVHR Unit Do I Need?
Quick Answer
Sum all room extract rates. Typical 3-bed house: 60–80 l/s MVHR unit.
Based on: Approved Document F continuous extract rates for all wet rooms, with boost capacity
How to Size an MVHR Unit
MVHR (Mechanical Ventilation with Heat Recovery) sizing is based on:
- Calculate the whole-dwelling ventilation rate (based on floor area and occupants)
- Sum the individual room extract rates for all wet rooms
- Take the higher of the two values as your design flow rate
- Choose a unit that delivers this at mid-speed (not maximum)
Example: 3-Bed Semi (90m²)
- Whole-dwelling rate: 90 × 0.3 = 27 l/s
- Room extract rates: kitchen 13 + bathroom 8 + en-suite 8 + WC 6 = 35 l/s
- Design rate: 35 l/s (higher value)
- Boost capacity needed: ~60–80 l/s
- Choose unit: Rated at 80–100 l/s maximum (so 35 l/s is mid-speed)
Sizing by House Type
- 2-bed flat (60m²): 40–60 l/s unit
- 3-bed semi (90m²): 60–80 l/s unit
- 4-bed detached (140m²): 80–120 l/s unit
- 5-bed detached (200m²): 120–180 l/s unit
Cost Guide (2026)
- MVHR unit (60–80 l/s): £1,500–3,000
- Ductwork and fittings: £1,000–2,000
- Installation: £1,500–3,000
- Total installed: £4,000–8,000
Key Considerations
- Airtightness: MVHR is only worthwhile in airtight homes (<5 m³/hr/m² at 50Pa)
- Heat recovery efficiency: Look for 90%+ rated units
- Noise: Check dB ratings at design flow rate, especially for bedroom supply
- Summer bypass: Essential to prevent overheating in summer
- Filter maintenance: Clean/replace filters every 3–6 months
Last updated: April 2026