A simple, quiet running engine is desirable for use in connection with a fan or blower. They may be either horizontal or vertical and for schoolhouse and similar work should be provided with large cylinders so that the required power may be developed without carrying a boiler pressure much above 30 pounds. In some cases cylinders of such size are used that a boiler pressure of 12 or 15 pounds is sufficient. The quantity of steam which an engine consumes is of minor importance as the exhaust can be turned into the coils and used for heating purposes. If space allows, the engine should always be belted to the fan. Where it is direct-connected, as in Fig. 14, there is likely to be trouble from noise, as any slight looseness or pounding in the engine will be communicated to the air ducts and the sound will be carried to the rooms above. Fig.. 15 and 16 show common forms of fan engines. The latter is especially adapted to this purpose as all bearings are enclosed and protected from dust and grit. A horizontal engine for fan use is shown in Fig. 17.