CONVECTION 105 



Authorities differ somewhat, but experiments have shown some 

 valuable things regarding inlets and outlets for hot air-heating. 



The best location for air inlets has been found to be near the 

 ceiling of a room, since the warm air tends to rise and spread uni- 

 formly under the ceiling, gradually displacing other air, and filling 

 the room with pure air without causing drafts. The colder air will 

 of course pass out of the outlets near the bottom of the room. This 

 system requires a fan to force the air into the rooms and is mainly 

 used for supplying fresh air, as well as heat, to all parts of the build- 

 ing. We shall learn more of this under the subject of ventilation. 



The fan used for forcing the air through a building should be 

 carefully selected. Small fans must run very rapidly in order to 

 supply enough warm air to the rooms. They also set the air in 

 rapid vibration, causing a buzzing sound. Large fans may be run 

 at less speed, and will deliver the same amount of air without caus- 

 ing drafts or annoying sounds. The difference in cost will be 

 neutralized by less wear and tear on the large fan. 



The diagrams on page 179 give an idea of the movements of 

 air for different positions of the inlets and outlets for ventilation. 



Chimneys. A chimney is used for two purposes; first, to 

 create draft necessary to supply burning fuel with sufficient air; 

 second, to furnish a means for discharging noxious products of burn- 

 ing into the atmosphere at such a height from the ground that they 

 may not be a nuisance to people living in the vicinity of the chimney. 



The " draft " of a chimney depends upon the difference in weight 

 between the air inside and the same volume of air outside. The 

 chimney should be tall enough to enclose enough heated air to cause 

 the right amount of air pressure for a good " draft. 7 ' The chimney 

 should be provided with a " clean-out door " at the base, 3 or 4 feet 

 below the smoke-pipe entrance. Chimneys should ex end above the 

 highest surrounding roof, to prevent down drafts caused by eddies. 

 Where it is impossible or impracticable to build the chimney high 

 enough revolving chimney tops will often prevent down drafts. 

 Down drafts may also be prevented by covering the top of the 

 chimney with stone flag, leaving openings in two parallel sides of 

 the chimney, the sides parallel to the ridge of the adjoining roof or 

 building being closed. 



