What are the Part F ventilation rates?

Part F 2021 (MVHR/System 4 rates per Table 5.1b) requires continuous extract rates of: kitchen 13 L/s (boost at 30 L/s), bathroom and en-suite 8 L/s, WC 6 L/s, utility room 8 L/s. Supply rates for habitable rooms should be at least 10 L/s per room. The MVHR unit must provide the higher of total extract or total supply as the design flow rate.

What size duct do I need for MVHR?

Duct size depends on the air flow rate and maximum velocity. At 3 m/s (recommended maximum for noise), typical sizes are: 75mm semi-rigid for single rooms (up to 10 L/s), 100mm for two rooms, 125mm for small mains, 150mm for main trunk ducts. Oversizing ducts reduces noise and pressure drop. Semi-rigid ducts (like Zehnder ComfoFlex) are popular for their ease of installation.

How do bends affect MVHR performance?

Each 90° bend adds approximately 1.5 metres of equivalent straight duct length to the pressure drop calculation. Tight bends are worse than gentle sweeps. Minimise bends to keep pressure drop low — high pressure drop means the MVHR fan works harder, uses more energy, and creates more noise. Aim for fewer than 4 bends per duct run where possible.

What MVHR unit size do I need?

The MVHR unit must provide the design flow rate (higher of total extract or total supply) at the system pressure drop. For a typical 3-bed house, this is usually 40-60 L/s. Common units: Vent-Axia Sentinel Kinetic Plus (up to 71 L/s), Zehnder ComfoAir Q350 (up to 350 m³/h), Blauberg MVHR (up to 90 L/s). Always select a unit with capacity above your design flow rate.

Where should the MVHR unit be located?

The MVHR unit should be centrally located to minimise duct runs — usually in a cupboard, utility room, or loft space. It needs access for filter changes (every 3-6 months) and annual servicing. The unit should be on a vibration-isolating mount and the duct connections should have flexible sections to prevent noise transmission. Allow 600mm clearance around the unit.

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MVHR Ductwork Calculator — Extract, Supply & Duct Sizing

Calculate MVHR ductwork requirements from Part F extract and supply rates. Sizes main and branch ducts by air velocity.

Kitchens

13 L/s each

Bathrooms

8 L/s each

En-Suites

8 L/s each

WCs

6 L/s each

Utility Rooms

8 L/s each

Habitable Rooms

10 L/s supply each

Total Duct Run

Total straight duct length

Number of 90° Bends

Each adds ~1.5m equivalent

Price Per Metre Duct (optional)

How We Calculate This

This calculator determines MVHR ductwork requirements based on Building Regulations Part F extract and supply rates, then sizes ducts for appropriate air velocity.

The formula

Design flow = Max(Total extract, Total supply)

Duct area = Flow rate ÷ Max velocity

Duct diameter = √(4 × Area ÷ π)

Effective length = Straight run + (Bends × 1.5m each)

Pressure drop ≈ Effective length × 1 Pa/m

For a 3-bed house with kitchen (13), bathroom (8), en-suite (8), WC (6) = 35 L/s extract. 4 habitable rooms × 10 = 40 L/s supply. Design flow = 40 L/s. Main duct at 3 m/s: area = 0.0133 m², diameter = 130mm → use 150mm.