The air in the enclosure must be replaced regularly, so the ventilator must allow the movement of air without also causing cold draughts. The simplest ventilation systems are grills made of metal or plastic which can be built into the top or sides of the enclosure and closed when desired. Electrically powered extractor fans are better for large enclosures which have a greater volume of air to move. Regardless of the method used, the air which is drawn into the enclosure must not be so cold that it causes a drop of more than a few degrees in the ambient temperature, nor should it alter the humidity to any great extent. For best heat efficiency the ventilators should be situated as far from the secondary heat source as possible. It is very simple to set an electric fan on a time switch, so that it comes on for just a few minutes each hour.
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