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C. A. Dunham Company Specialties, Heating the steam supplied to the radiators is varied to suit the changing weather con ditions. This is the basic principle of the Dunham DifferentialVacuum Heat ing System. (Fig. 1083A). The heating profession has long realized, that there is a large amount of heat wasted in mild weath er because buildings are overheated. The cause of this waste in steam heated systems is that most all of them are oper ated on and limited in their operation to pres sures above atmospheric, even in mild weather. Steam pressures ranging Fiff, 107SA. Relation ofHeal Loos and Heat Emission to Temperature Difference ^This iulluuusutraautiovun sUh1UoWws9 hUUoWw tUhie rIWooImfl tICeCmperature is maintained uniformly under changing weather conditions, by varying tmhee sgtleeaamm pprretssure so that the heat given oft by the radiator equals the heat lost by the building. from a few ounces to 2 or 3 lb. afford but a small operating range of steam tempera tures to meet changes in weather condi tions. That is steam at a relatively con stant pressure is usually supplied to the radiators regardless of what the outside Weather may be. The heat output of the radiators will therefore, be entirely too high in mild weather, when only a fraction of the radia tion installed would actually be required to balance the heat loss of the building and maintain the desired temperature. The building temperature will increase above that desired, resulting in excessive heat loss from the building structure. This is gener ally termed overheating and represents a direct waste of fuel. Government weather reports show that extreme weather prevails during about 5 per cent of the average heating season when the system must be operated at full^rating. This leaves 95 per C. A. Dunham Company Specialties, Heating cent of the heating season when the heat output of the system must be reduced if uniform room temperature is to be main tained. Daily weather reports show that the temperature rises and falls quite rapidly over the greater portions of the country. Temperature changes of 40 deg. per day occur each winter in the Chicago area, while daily changes of .20 deg. are but slightly higher than normal. To this varia tion in heat demand must be added the cooling effect of wind, if a true picture of the great fluctuations in heating require ments is wanted. The graph, Fig. 1288, shows the official maximum and minimum temperatures at Chicago for each day of the heating season of 1928-29. It clearly shows.the tempera ture fluctuations for that season and also the extreme limits of temperature recorded in the previous 54 years. During that year, there was a total of 131 hours when the temperature was 10 deg. or lower, with but 35 hours of this at zero or below. The heat ing season was but 0.4 deg. warmer than the average since the year 1900. Fuel Saving of 25 Per Cent or More Compared with Ordinary Types of Steam Heating Systems On buildings changed over to the Dun ham Differential Vacuum System the per formance records show actual fuel savings of 25 per cent or more compared with pre vious system.' The charts and illustrations on this and the following page tell this story in a non technical way. Fig- 1066C, Steam Table Applied-- This drawing gives a comparison of the properties of steam as used in the ordinary type heating system and the Dunham Dif ferential Vacuum System. The Ordinary System Has-- (1) Operating range zero to 5 lb. gauge pressure or higher. (2) Temperature range 212 deg. fahr. to 227.1 deg. fahr. (3) Volume per pound steam 26.79 to 20.42 cu. ft. The Dunham Differential Vacuum System Has-- (1) Operating range 25 in. vacuuny to 5 lb. gauge pressure or higher. (2) Temperature range 133.22 deg. fahr. to 227.1 deg. fahr. (3) Volume per pound steam 145 to 20.42 cu. ft. By comparing the steam volumes and temperatures used in the two types of systems an index to the flexibility of each is obtained. The extreme flexibility of the Differential System eliminates overheating and reduces the heat lost through open windows. Fig. If88. Average Temperature for Chicago Heating Seaton 1998-1999 *= 40.t deg. (1) Maximum and minimum daily temperatures for 1928-29. (2) Normal daily temperature from 54-year record. (3) Hightest temperature on record. (4) Lowest temperature on record. . (5) 65 deg. temperature base for degreeday calculation. 786 787