Question 1

What are the four ventilation systems in Part F?

Accepted Answer

System 1 is intermittent extract fans with background ventilators (trickle vents). System 2 is passive stack ventilation (PSV). System 3 is continuous mechanical extract ventilation (MEV). System 4 is continuous mechanical ventilation with heat recovery (MVHR). AD-F Volume 1 (2021 edition) sets out the requirements for each system in its Table 1.1/1.2/1.3 series (Table 1.1 intermittent extract rates, Table 1.2 continuous extract rates, Table 1.3 whole-dwelling rates).

Question 2

What extract rate does a kitchen need?

Accepted Answer

For System 1 (intermittent extract fans), a kitchen requires a minimum of 30 l/s (litres per second) adjacent to the hob (ducted cooker hood over hob), or 60 l/s if located elsewhere in the kitchen (no hood, or a recirculating hood that does not extract outside). For Systems 3 and 4 (continuous extract), the minimum continuous rate is 13 l/s with a boost rate of 30 l/s. These values are from AD-F 2021 Vol 1 Table 1.1.

Question 3

What is purge ventilation and when is it needed?

Accepted Answer

Purge ventilation provides rapid air exchange to dilute pollutants from decorating, new furniture, or accidental releases. Every habitable room must have purge ventilation capability — typically an openable window with a free area of at least 1/20th of the room floor area. Purge ventilation is required in addition to the normal extract/background ventilation strategy.

Question 4

What size trickle vents are needed?

Accepted Answer

For System 1 (intermittent extract with background ventilators), the total equivalent area (EA) of background ventilators depends on the number of bedrooms: 1 bed = 25,000 mm² EA total, 2 bed = 35,000 mm² EA, 3 bed = 45,000 mm² EA, 4 bed = 55,000 mm² EA, 5+ bed = 65,000 mm² EA. These are distributed across habitable rooms and wet rooms per AD-F 2021 Vol 1 Table 1.4.

Question 5

Does Part F apply in Scotland?

Accepted Answer

Scotland has its own ventilation requirements under Technical Handbook Section 3 (Environment) which has broadly similar principles but different specific rates and requirements. Northern Ireland follows Technical Booklet K. The 2021 AD-F update is specific to England; Wales follows its own edition of Approved Document F.

Room	Minimum Extract Rate (l/s)	Notes
Kitchen (adjacent to hob)	30	Cooker hood or extract fan above hob
Kitchen (elsewhere)	60	Extract fan remote from hob
Utility room	30	Intermittent extract
Bathroom	15	With overrun timer (15 mins min)
Sanitary accommodation (WC only)	6	Intermittent or continuous

Room	Continuous (Low) Rate (l/s)	Boost Rate
Kitchen	13	30 l/s (adjacent to hob)
Utility room	8	30 l/s
Bathroom	8	15 l/s
WC (no bath/shower)	6	6 l/s

Number of Bedrooms	Whole Dwelling Min. Rate (l/s)
1 bedroom	19
2 bedrooms	25
3 bedrooms	31
4 bedrooms	37
5+ bedrooms	43

Part F Ventilation Rates Reference

Extract Rates — System 1: Intermittent Extract Fans (AD-F 2021 Vol 1, Table 1.1)

Extract Rates — Systems 3 & 4: Continuous Extract (AD-F 2021 Vol 1, Table 1.2)

Whole Dwelling Ventilation Rate — Systems 3 & 4

Background Ventilator Sizes — System 1 (AD-F 2021 Vol 1, Table 1.4)

Purge Ventilation

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Part F Extract Fan Requirements

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Part O Overheating Limits

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