It is not because the house is dirty

15 litres per second for an intermittent extract fan, from Table 1.1 of Approved Document F (2021, Volume 1). Where the bathroom is served by a continuous system instead, there is no separate per-room continuous figure to quote: Table 1.2 gives a high rate for boost, and the requirement is that the total continuous extract across the whole dwelling meets the whole-dwelling rate in Table 1.3. The figure is a minimum for the system as installed and measured, not the rating printed on the fan — duct length, bends and terminal type all take something off what actually leaves the room.

60 litres per second, unless there is a cooker hood that extracts to the outside, in which case it is 30. Table 1.1 splits on that and not on where the extract is sited, which is the detail that catches people out: a recirculating hood sits directly over the hob and does not qualify, because it filters the grease and the smell and returns the steam to the room. A kitchen with a recirculating hood needs 60. So does a kitchen with nothing but a wall extract fan, because an extract fan is not a cooker hood.

30 litres per second for a utility room and 6 litres per second for a WC, both intermittent, from Table 1.1 of Approved Document F (2021, Volume 1). The utility figure is the one most often missed on a refit, because a utility room is usually treated as a cupboard with a machine in it rather than as a wet room — and a tumble dryer or an airer in there puts around two litres of water into the house per load.

Table 1.3 of Approved Document F sets it by the number of bedrooms: 19 litres per second for one bedroom, then 25, 31, 37 and 43 for two, three, four and five, plus 6 litres per second for each bedroom beyond the fifth. There is a second test as well — 0.3 litres per second for every square metre of internal floor area — and the higher of the two figures applies, so the floor area has to be measured rather than assumed. One footnote is worth knowing: a dwelling with only one habitable room, which means a studio, takes 13 litres per second rather than the one-bedroom figure.

Not an extract rate, no, and that is the point rather than an omission. Extract rates in Table 1.1 apply to wet rooms — kitchen, utility, bathroom, WC. A living room, dining room, study or conservatory carries no extract requirement at all. What those rooms do need is their share of the whole-dwelling supply from Table 1.3, and a background ventilator so replacement air can get in. There is one trap worth naming: a conservatory with an openable door between it and the rest of the house sits outside the calculation entirely, and permanently knocking that door through is one of the commonest ways a house quietly stops complying.

Only by measuring it at the terminal, with a meter, once it is installed. The rating on the box is what the fan achieves on a test rig with nothing attached to it, and every metre of duct, every bend, every grille and every insect screen reduces it — a fan sold as 15 l/s on four metres of sagging flexible duct with two bends can be delivering half that while sounding exactly the same. Noise is not airflow. A ceiling valve is measured with a capture hood rather than a vane meter waved near it, because a vane reads the air going past the vane and not the air going through the terminal. We give the readings for every terminal in writing, which is also what goes on a commissioning certificate.

In most cases, yes. Building Regulations Part F sets minimum ventilation requirements whenever you're altering a wet room or kitchen, replacing windows, or changing the layout of extract points. The exact requirement depends on the room type, extract rate, and how the space is used, so it's always worth having a survey before work starts rather than after.

MVHR (Mechanical Ventilation with Heat Recovery) continuously extracts stale, humid air from wet rooms and supplies filtered fresh air to living spaces, recovering most of the heat in the process. It's especially valuable in well-insulated or airtight homes where natural background ventilation has been reduced. Whether you need it depends on your property's airtightness, layout, and how it's currently ventilated.

Common warning signs include condensation on windows, a musty smell in enclosed spaces, and dark spotting around cold-bridge areas such as loft/joist junctions, ceiling breaks, and the wall behind wardrobes pushed against external walls. Our humidity simulator further up this page gives an illustrative sense of risk, but a site survey is the only way to get an accurate picture.

As a general guide, filters are checked every six months and changed on the manufacturer's schedule, and the condensate trap and drain are checked with them. This is the part of a ventilation system almost everybody leaves alone, and it is the part that decides whether it still does what it was installed to do: filters load up gradually, the airflow falls with them, and a unit can be well down on the rate it was commissioned at before anyone notices anything except that the bathroom is taking longer to clear. Nothing warns you — there is no light on the wall, and the fan sounds the same. The only way to know is to measure it again, which is what a service visit is for. We run service plans for exactly this reason.

We cover most of South East London and the Bromley borough — Beckenham, Bromley, West Wickham, Chislehurst, Dulwich, Penge, Sydenham, Forest Hill, Catford, Crystal Palace, Petts Wood, Orpington, Blackheath, Lee, Eltham and the areas around them. If you're just outside, get in touch anyway: we travel further for larger installations and commercial fit-outs.