What TGD F requires
For a dwelling with central continuous mechanical extract, TGD F gives a general ventilation rate and minimum boost rates for the wet rooms. The page follows the 2019 edition, which applies to work from 1 November 2019. A consultation version, TGD F 2025, has been published; this page will follow it when it takes effect.
The general ventilation rate
The rate is the higher of two values: 5 l/s plus 4 l/s per person, or 0.3 l/s per m² of internal floor area. The standard case is a three-bedroom home with five people, which gives 25 l/s: two people in each of the two largest bedrooms and one in the third.
The system must be able to deliver at least 25 per cent more than the calculated general rate and more than the total of the boost rates.
Boost extract rates
Minimum boost extract rates for central extract (Table 1) and for balanced ventilation with heat recovery (Table 2) are the same:
| Room | Minimum boost rate (l/s) |
|---|---|
| Kitchen | 13 |
| Utility room | 8 |
| Bathroom | 8 |
| Toilet without bath or shower | 6 |
Worked example
A three-bedroom home of 100 m² with five people. The general rate is the higher of 5 + 4 × 5 = 25 l/s and 0.3 × 100 = 30 l/s, so 30 l/s. The system must be able to deliver at least 25 per cent more than that: 37.5 l/s.
How Aira applies it
Aira works out the general rate from the floor area and the number of occupants, which it derives from the bedrooms on the plan (two in each of the first two bedrooms and one in each further bedroom) and shows as an assumption you can change. Supply and extract both get that total, rounded up: supply is divided over the habitable rooms by floor area, extract over the wet rooms in proportion to their boost rates. Each wet room must be able to reach at least its boost rate, and the system must be able to deliver at least 25 per cent more than the general rate; both are shown as capacity. You can adjust all figures in the table before you download the plan.
Source
The figures are taken from the official 2019 text, checked on 14 October 2026.