Question 1

What are the four ventilation systems in Part F?

Accepted Answer

System 1: Background ventilation (trickle vents) + intermittent extract fans in wet rooms. System 2: Passive stack ventilation (natural draught ducts). System 3: Continuous mechanical extract ventilation (MEV). System 4: Continuous supply and extract with heat recovery (MVHR). Any system can be used for any dwelling.

Question 2

How do I calculate the whole building ventilation rate?

Accepted Answer

The whole building rate is the greater of: 0.3 l/s per m2 of floor area, or the number of occupants x 8 l/s (with a minimum based on number of bedrooms + 1). For a 100m2 3-bed house, this would be max(30 l/s, 32 l/s) = 32 l/s.

Question 3

What extract rate does a kitchen need?

Accepted Answer

Per ADF 2021 Vol 1 Table 5.1a: 30 l/s intermittent adjacent to the hob (a ducted cooker hood sited over the hob meets this) OR 60 l/s intermittent elsewhere in the kitchen (no hood, or a recirculating hood that does not extract outside). The continuous-extract alternative is 13 l/s. A recirculating hood does not satisfy Part F on its own — the room still needs a separate 60 l/s extract path.

Question 4

What is MVHR and when should I use it?

Accepted Answer

MVHR (Mechanical Ventilation with Heat Recovery) supplies fresh air and extracts stale air through a heat exchanger recovering up to 90% of heat. Best for airtight buildings (air permeability below 3 m3/hr/m2). Costs £2,500-£5,000 installed but saves £100-£200/year on heating.

Question 5

Do I still need trickle vents with MVHR?

Accepted Answer

With MVHR (System 4), trickle vents are not required for background ventilation as the MVHR provides continuous supply air. However, openable windows are still needed for purge ventilation (1/20th of floor area). With Systems 1, 2 and 3, trickle vents are mandatory.

Part F Ventilation Calculator — Extract Rates & Trickle Vents

How We Calculate This

Extract rates (ADF 2021 Vol 1 Table 5.1a)

Whole building rate

Frequently Asked Questions

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