In social housing the correct location of extractor fans is absolutely critical to ensure the efficient extraction of moist air and so prevent any build-up of condensation. John Davison explains why and offers some guidance on where best to site domestic extraction units.

Social housing managers know that black mould is caused by high humidity and is an indicator of poor ventilation exacerbated by inadequate heating. The main source of this humidity is the kitchen where it is created by boiling water, cooking, washing, un-vented tumble driers and drying washing indoors, to name but a few.

Not quite so apparent is the fact that humidity generated in the kitchen causes mould growth upstairs and in any other room where it is cool. This is because as air heats it expands and will try and achieve equilibrium throughout a property. A kitchen door left open or removed, will allow any hot air to quickly move around a house, seeking out lower pressure areas in cooler rooms. As the warm moist air rushes to fill this void it con¬denses on contact with cooler surfaces such as walls and window panes.

Over time, spores of black mould will develop and thrive, often behind undisturbed furniture such as wardrobes or bedheads where the circulation of air is minimal.

The solution is to extract damp air before it can do any damage and this means extracting it at source, as and when it occurs. To do this effectively automatic extractor fans must be located in the right spot.

Adhering to the following basic rules will ensure condensation is efficiently dealt with:
Always install the fan at the source of the humidity in the furthest window, wall or ceiling from the door and at a high level. This will ensure the maximum flow or fresh air throughout the whole room.

As a rule do not install just a bathroom fan. Very often mould growth occurs in the bathroom and a common mistake is to install a fan only in this room. A bathroom fan has less than half the extract rate of a kitchen fan. On its own it will attempt to deal with any humidity throughout the whole property. As a result it may run continuously. In many cases it actually increases the problem in the bathroom by dragging in humid air from the rest of the dwelling. If you are only going to fit one fan then almost invariably it should be in the kitchen where the majority of humidity is created. Always remember: ‘Mould growth is a manifestation not a cause’.

Always ensure that there is sufficient provision to allow replacement fresh air into other rooms. This can be achieved by using internal grilles in the door, ceiling or walls. Air bricks, or passive vents as they are called, are a vital aid to air circulation especially in properties where tenants never open their windows. Modern passive vents have one way filters, and shutters to prevent draughts and although they help to equalise pressure they do not effectively tackle high humidity – the root cause of condensation.

Do not site extractor fans where temperatures are likely to exceed 50°C, or above a cooker hob or eye-level grill.

If installing a fan in a room containing a fuel burning device which has a non balanced flue there must be sufficient replacement air to prevent fumes being drawn down the flue when the fan is on maximum extract. Refer to BS 5440 and the Building Regulations for specific requirements.
Controlling air quality
Having determined the right location it makes sense to install an extractor fan that incorporates a microprocessor controlled sensor that continually monitors the environment and operates to extract moist air only when it occurs. Although this type of fan – such as the Energysaver Datamatic fan from Airtech Environmental – automatically detects rising levels of moisture (Relative Humidity –RH) and switches on the extractor, if it is fitted in a window the cold glass may not give a true reading of the atmosphere in the room. In this case a remote sensor may need to be fitted elsewhere – ideally between the main source of vapour production, for example a cooker, and the door.

A significant benefit of this type of product is that surveyors can download, view and print out data such as hours run, starts/stops to check if the fan has been tampered with – i.e. switched off. Equally, it reveals the temperature in the dwelling and the amount of moisture being generated by the occupant’s lifestyle.

Normal conditions
Generally fans should be set to activate at around 65%RH which allows 17g/m3 of water vapour at 20°C: the accepted comfortable living temperature for most people. This is below the %RH level that allows mould spores to reproduce. Anticipatory control and night time set back greatly enhance the effectiveness of the fans which need to run a lot when first installed. However, once equilibrium has been achieved they settle down and only operate when moisture producing activities occur.