What is a trickle vent and why is it needed?

A trickle vent is a small opening built into a window frame that provides background ventilation. Under Part F, they are required in all new and replacement windows to ensure adequate air quality. The required equivalent area (EA) depends on room type: 5,000 mm² for habitable rooms and 2,500 mm² for wet rooms (kitchens, bathrooms, WCs).

What extract rate is needed for a kitchen?

Under Part F, a kitchen requires an intermittent extract rate of 60 litres per second (l/s) when cooking, or a continuous rate of 13 l/s if running permanently. Intermittent fans are typically controlled by a switch or humidistat. The extract point should be positioned near the hob/cooker for maximum effectiveness.

What is purge ventilation?

Purge ventilation is rapid ventilation to dilute pollutants (e.g., after painting or a smoke event). Part F requires the ability to achieve 4 air changes per hour (ACH) via openable windows. The openable window area should be at least 1/20th of the floor area. For a 4m × 3m room, that's 0.6 m² of openable window.

What extract fan is needed for a bathroom?

A bathroom requires an intermittent extract rate of 15 l/s or a continuous rate of 8 l/s under Part F. The fan should be connected to the light switch with a 15-minute overrun timer. In bathrooms without windows (internal), mechanical extract is mandatory. Fans must be ducted to outside, not into the loft.

Do I need ventilation in a WC?

Yes. A WC requires intermittent extract of 6 l/s or continuous at 6 l/s. If the WC has an openable window, a trickle vent of 2,500 mm² EA is also needed. Internal WCs without windows must have mechanical extract. The extract should have an overrun of at least 15 minutes to clear the air after use.

TradeCalculator

Room Ventilation Calculator

Calculate Part F ventilation requirements: trickle vent equivalent area, extract rates for wet rooms, and purge ventilation for 4 air changes per hour.

Room Type

Determines extract rate and vent size

Number of Windows

Each window needs a trickle vent

Room Length

Room Width

Mechanical extract fan installed

Intermittent fan vs continuous MVHR

How We Calculate This

This calculator determines the ventilation requirements for rooms under Building Regulations Part F (Ventilation). It covers trickle vent sizing, mechanical extract rates, and purge ventilation capacity.

The formula

Purge rate = Room volume × 4 ACH
Purge opening = Floor area ÷ 20
Extract rate = Part F table by room type
Trickle vent EA = 5,000 mm² (habitable) or 2,500 mm² (wet rooms)

Part F extract rates

Kitchen: 60 l/s intermittent or 13 l/s continuous
Bathroom: 15 l/s intermittent or 8 l/s continuous
Utility: 30 l/s intermittent or 8 l/s continuous
WC: 6 l/s intermittent or 6 l/s continuous

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.