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Cold draughts

Cold draughts are sometimes associated with air conditioning and indeed, this can be caused by incorrectly conceived systems. The likely effect on personnel resulting from the location of the indoor unit and its air distribution pattern must therefore be considered at the design stage.

The height of the ceiling is also relevant. Air conditioning equipment manufacturers generally assume the optimum ceiling height for a direct expansion system to be between 2.70 and 3.50 metres. Cold air at about 16°C supplied from this height is able to mix with warmer room air before reaching personnel level, thereby obviating any feeling or draught.

Notwithstanding this, in applications not meeting this general standard, a quality air conditioning system can invariably be "tuned" to compensate.

Clearly, unit location as well as ceiling height and shape have a major influence on draught or the lack of it. Explaining this requires a few comments concerning the properties of cold air. Cold air has a tendency to "stick" to the ceiling for a period prior to descending. This is known as the "coanda" effect and it helps the cold air and surrounding air to mix before dropping down into the room.

Unfortunately, the existance of any barrier such as a ceiling beam, has a detrimental effect on this by interrupting the air flow. In such a case, cold air colliding with the beam will sink immediately, to the certain discomfort of any individual unlucky enough to be seated directely below.

Similarly, two units located opposite to each other will generate cold air flow collision in much the same manner.