277. The use of a fan to create an excess of air pressure, or plenum, in the interior of a building makes it practicable to secure perfect ventilation at all times, regardless of wind or weather. The certainty of its operation, and its ability to furnish more air if required, are features of great value.

Another important consideration is that the fan may be located at any point which may be most convenient, either at the bottom of the building, or at the top. It may be placed in another building, even on the opposite side of the street. The system of air supply may be operated from above, in a general downward direction, or in the reverse way, as may be best adapted to the situation.

The heating apparatus employed in conjunction with a fan may be of any variety desired, either steam, hot-water, or hot-air furnace. The use of a forced blast enables the heaters to operate with the maximum degree of efficiency, and to accomplish the work required with the minimum area of heating surface and of flues.

In selecting centrifugal fans, small wheels and very high speeds should be avoided, because they are very liable to be noisy. Large wheels waste less power, and have the advantage of possessing a large reserve capacity, which may be brought into use at any time by increasing the speed.

The use of belting or friction gears for driving fans should be avoided whenever possible. The engine or motor should, if convenient, be connected directly to the fan shaft.