Il8 PHYSIOLOGY. 



Direct Heating. In heating by steam or hot water, ii 

 the radiators are placed in the room they give direct 01 

 radiant heat. This system is called direct heating. In 

 itself it has no provision for renewing the air. It gives 

 direct heat, and produces air currents within the room ; 

 and any change in the air is wholly incidental, from escape 

 of heated air in the upper parts of the room and corre- 

 sponding suction of outside air through such openings as 

 the carpenters have left below. 



Indirect Heating. In indirect heating, coils of steam 

 or hot-water pipes are placed in air shafts which lead up 

 to the rooms above, and also have ducts to the outside. 

 As the air is heated by the heat of the pipes it rises into 

 the rooms above, and fresh, cold air presses in through the 

 ducts, to be, in turn, heated and sent up. If there is at 

 the same time a proper escape for the foul air, this makes 

 an excellent system. 



A Combination of Direct and Indirect Heating. In 



many situations the direct and indirect may be advan- 

 tageously combined. Where there is a grate in a room, it 

 serves very well as a foul-air shaft, especially when there 

 is a fire in the grate. It is well to have the flue from the 

 grate in the same chimney with that from the smoke pipe, 

 as then the heat from the smoke will cause a constant up- 

 draft in the grate flue, whether there is a fire going in the 

 grate or not. 



With a grate, in private houses, there is ordinarily no 

 need of other foul-air shaft for any room. But it is very 

 desirable to have at least some "indirect" heat, so that 

 the fresh air introduced will be sufficiently heated. 



If the introduction of air is thus provided for, it is then 

 safe to put on double windows and make the cracks around 



