Provision for Air Replacement

While in many cases the normal crevices around doors and windows are sufficient for this purpose, it is often necessary and advisable to make special provision for replacement fresh air to be brought into the room through grilles of a suitable size and design fitted in doors or walls to ensure draughtfree ventilation, and the minimum of restriction to extract fans.

Special provision for air replacement should be considered if:

(a)	windows and doors are draught proofed
(b)	the location of fans is such that satisfactory coverage of the space by cross-ventilation cannotbe made with air pulled in from the available doors and windows
(c)	smoke from solid fuel fires, fumes from gas fires or boilers are pulled back down the flue into the room.

Minimum Free Areas

If special provision has to be made, then the minimum free areas to allow for air should be based on 1,300cm² for every 1,500 m³/h performance.

Fan Impeller Ø (mm)	Grille Free Area
150	300cm²
230	650cm²
300	1300cm²

Note that the free area of a grille or louvered panel is the open area through which air can pass freely, and may be as low as 20% of the face area for fly-proof gauze, or as high as 90% for eggcrate type grille. The above minimum figures give an air velocity through the grille of between 2.6 to 3.3m/s and would cause a 10% drop in fan output; they should be doubled or trebled if:

(a)	more comfortable air velocities are required in the room
(b)	maximum fan output is required

Location of Inlets

When considering air replacement, the location of suitable air intake points is as important as the location of the extract units, because the distribution of the air flow through the space being ventilated can give success or failure to the scheme.

The main points are:

1.	Aim for full cross-ventilation of the space.
2.	Eliminate “dead” spots by preventing short-circuiting of air flow straight from inlets to extract units without “sweeping” the room.
3.	Locate units at high level, and inlet grilles usually just above head level to avoid uncomfortable draughts to the occupants. The natural upward convection currents and the secondary entrained air movements caused by the “jet” of air, more or less horizontal, from the inlet grilles will give sufficient gentle air turbulence around breathing level to maintain a feeling of freshness. It is, however, sometimes convenient to locate the inlet grilles in the walls behind existing radiators, in which case the incoming air can be warmed slightly in cold weather.
4.	Use sufficient well-spaced inlet grilles to keep incoming air velocities below 1.5 m/s if possible (that is double the figures given earlier).
5.	If the room is very wide, say over 25m, it may be necessary to extract centrally and bring in replacement air along each side. If central extraction is difficult to arrange, an alternative method of ensuring a reasonable velocity of air movement over the full width of the space and prevention of a near-stagnant area down the centre, is to use intake units along one side, extract units along the other, and a number of ceiling sweep fans as air circulators, fixed centrally between 3.0m and 7m above floor level to any convenient structural beams or roof truss members. This method is usually considerably cheaper and less unsightly than a central extrac duct, has proved very acceptable and successful in large stores, particularly basement, and open plan offices.