142 THE PRODUCTION AND USE OF HEAT 



165. The Pressure on a Vacuum System. There is always 

 some vapor pressure on a vacuum system. By turning to our 

 temperature-pressure table, Art. 155, we can readily tell what 

 that pressure is. A fire is not likely to be needed unless the 

 temperature of the rooms falls below 68F., or 20C. Turning 

 to our table we see that the absolute vapor pressure at 20C., 

 or 68F., is 0.34 Ib. per sq. in. and that the vacuum gauge 

 would read 14.36 Ib. Another way of stating the same fact 

 would be to say that the boiling point of the water in such a 

 vacuum system of heating is 20C., or 68F., when the vacuum 

 gauge reads 14.36 Ib. vacuum, or the absolute pressure is 0.34 

 Ib. In fact, in a vacuum system of steam heating, if it works 

 perfectly, the water always begins to boil as soon as the fire is 

 built. 



166. Advantages of the Vacuum System over the Pressure 

 System of Steam Heating. It is evident that in mild weather, 

 when but little heat is needed, it is quite unnecessary to 

 heat the radiators to a point above 100C., or 212F., in order 

 to warm the rooms sufficiently. Yet that is exactly what is 

 necessary in any pressure system of steam heating, because 

 the radiators receive no heat at all till the water in the boiler 

 begins -to boil. With the vacuum system in good working 

 order, the heat is at once transferred to the radiators as soon 

 as the fire is lighted. Since the radiators begin to warm at 

 once, it is necessary to heat them only to the temperature 

 required to warm the rooms. Therefore, on a mild day when 

 little heat is required, the vacuum system is much superior to 

 the pressure system. In its quick response to the fire the 

 vacuum system resembles hot water and furnace heating and 

 is in marked contrast to the pressure system of steam heating. 



THE OPEN GRATE 



167. The Low Efficiency of the Old Fireplace. The old 



colonial fireplace was of immense size, often 6 or 8 ft. in 

 width, and consumed immense quantities of fuel. Still the 

 room was but poorly heated. The burning logs resembled a 



