222 



The Principles of Ventilation. 



The natural philosophy of the water pan in the hot-air furnace is 

 not understood by many people; aud the necessity of adding moisture 

 to the air that has passed through a heater is actually denied by men 

 whose habits of thought and occupation should make them zealous 

 adv-ocates of the practice. The capacity of the air for moisture depends 

 upon its temperature, being greater as the temperature is higher. If, 

 then, air bo saturated at, say 25° Fah., when heated to 70° Fah., it 

 will no longer be saturated, but its capacity for water vapor will be so 

 much increased, that the air is properly described as dry. It is not 

 anhydrous, of course, for there is just as much water, by weight, in 

 the air as before the heating. 



The use of steam-coils in place of the furnace is a decided advantage, 

 for, so far as I have observed, no one goes to the expense of putting in 

 steam apparatus without providing for the escape of the foul air; and 

 indeed without these, indirect heating by steam is almost sure to fail 

 — fortunately. Steam-pipes never attain the temperature of hot-air 

 furnaces and the resulting hot air is generally of a better quality. A 

 small steam jet supplies the moisture needed. 



Heating by direct radiation is most commonly effected by stoves. 

 With these in general there is practically no ventilation. The air 

 next the stove is heated, rises to the ceiling and flows off toward tlie 

 Avails, then down to the floor, and so on. A small amount of air is 

 constantly withdrawn to supply the stove, and a corresj)ondingly small 

 amount is drawn under doors and around windows to make good the 

 loss. Good ventilation is not possible with stoves, since there is no 

 way of heating the air before it enters the room. One point in favor 

 of stoves is their economy in fuel. A stove will ordinarily give out 

 nearly ninety per cent, of the surplus heat in the fuel, after deducting 

 the amount necessary to make a draft in the chimney. 



Heating by direct radiation from steam-pipes and hot-water pipes 

 is not necessarily accompanied by any change of air. In this connec- 

 tion I ought to say that the heat radiated from surfaces which, like 

 steam and hot-water pipes, are usually not more than 213°, amounts 

 to but little and is not perceptible at a distance of from ten to twelve 

 feet. These appliances really heat principally by conduction and con- 

 vection. 



The grate fire is an excellent ventilator, so far as removing foul air 

 is concerned, but it tends to produce drafts near doors and windows, 

 It heats solely by radiation and is very wasteful of heat, not more than 

 about 10 to 15 per cent, being utilized. As I shall presently show, the 

 open grate is a highly useful adjunct to other systems of heating, but 

 alone and simple it is a poor source of heat to rely on, save for small 

 rooms. 



