In certain circumstances the available space between the underside of the raised floor panels and the sub floor surface (known as the plenum) will be used as part of the building’s heating and ventilation system. In these instances a pressure differential will be created between the air within the plenum chamber i.e. under the floor, and the air above the floor in the office environment.
In these situations there are certain attributes of the raised access floor that become important. The leakage of air through panel joints and at perimeters will need to be kept within specific parameters for the required air pressure differential. The air leakage rate through the panel joint line is dependent on the size of gap between the panels, which is dependent upon the manufactured quality of the panel edge detail and also the quality of the raised floor installation. The air leakage at perimeters is also dependent upon the specific detail design and the quality of the installation.
Air leakage rates for under floor air systems
Some HVAC systems operate at high pressure, 25-30pa and in these instances the air leakage rate for a raised floor system without a finish, i.e. carpet tiles, should be considered and advice sought from the manufacturer.
With an HVAC system operating at low pressure, e.g. 7-15pa the air leakage through a raised floor will be of a lesser importance, but advice should be sought from the manufacturer.
Where the permitted air leakage rate through the raised access floor complete with the required floor covering is very low, or the specified air pressure differential is high, the raised floor in its standard form may not suffice. In these instances, the raised floor panels may be fitted with neoprene or similar gaskets to their edges in order to form a seal when installed against other similar panels. Alternatively stringers fitted with gasket seals may be installed to seal the panel joint lines.
The air leakage rate through a raised access floor can be determined by laboratory testing in order to give indicative air leakage rates through both the panel joint lines and perimeter detail. This can be undertaken at a project specific pressure differential or across a range of pressures.
Alternatively the raised floor can be tested in its entirety on site, as part of the installation process. This on site testing can determine not only the air leakage through the raised floor but also the leakage through the floor void via other means, e.g. incorrectly sealed service penetrations through the sub floor slab. Obviously the cost of testing on site can be prohibitive and in this case indicative results may be acceptable.
Air outlets and distribution
The conditioned air within the floor plenum is delivered into the area above the floor via a range of outlets. These are usually aluminium grilles of the same module size as the floor panels. Dampers may be fitted to allow control of the airflow through individual grilles in order to achieve localised control to suit specific local requirements. Perforated floor panels can be used again fitted with dampers if required as an alternative to grilles. A further alternative is use of smaller circular air diffusers fitted into the floor panels, again fitted with dampers to allow adjustment locally to suit specific requirements. All of these air outlets are readily relocated within the raised floor area as requirements change.