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. American Society of Heating and Ventilating Engineers Guide, 1925-26 this successful the boiler, plant must be adapted to operate successfully on at least 50 per cent overload for short periods. In larger plants the boilers may be divided into two or three units, so that one unit may be operated during mild weather, one or two units during average weather and the entire plant during extreme weather which arrange ment is ideal for flexibility and economy. . Another important design problem is to bring the building up to its working tempera ture^ This is a general requirement for buildings that are not heated over night, or which are used periodically and not heated during the time that they are not used. Recent tests have indicated that from 10 to 20 per cent more^uel is required for main taining normal working temperatures in office buildings throughout the 24 hours than is ordinarily required for maintaining working temperatures throughout the day and allowing the heat to be shut off during the time that the building is not in use. It may be assumed that other classes of buildings would show similar results. In office buildings, factories and other work places,, it is not generally considered so . necessary to have the temperature up to normal at the beginning of the working period as it is in schools, churches, theatres and other places of assemblage where the occupants are sitting still, and for this reason the relative capacity of the heating system may be less. In school buildings and other buildings where the heating of the air for ventilation is a large part of the load, this factor may be greatly reduced by recirculating the air during the heating up period, so that while the entire normal average load on the building may be from two to three times as much for heating the air as for supplying the direct radiation the heating-up load for the air may be reduced to from one-half to one-third of its normal value, so that the total load may not be more, than twice the direct radiation load. By continuous recirculation for the. ventilation, arranged to vary the amount of. air taken in from the outside from 100 per cent at an outside temperature of 55 deg. to 25 per cent at an outside temperature of 0 deg. the boiler capacity required rfor heating the air may be held practically constant at about 25 per cent of that required for 100 per cent of 0 deg. air from the outside. The load factor of the average heating plant is such that the total heat required per month will vary according to the following, when measured in percentage of the total heat required for the season: October . November December January February March April 7% 12% 17% 20% 18% - 16% .1,0% Chapter I HEAT LOSSES FROM BUILDINGS By Arthur C. Willard^, Member 'HE procedure to be followed in determining the heat loss from any 1 Ibuilding can be divided into seven consecutive steps, as follows: 1. Determine on the inside air temperature, at the breathing line, which is to be maintained in the building during the coldest weather. 2. Determine on an outside air temperature for design purposes, based on the minimum temperatures recorded in the locality in question, which will provide for all but the most severe conditions. Such conditions as may exist for only a few con secutive hours are readily taken care of by the heat capacity of the building itself. 3. Select or compute the heat transmission coefficients for outside wall and glass, also for floor, or top-floor ceiling, if these are next to unheated space. Include roof if next to heated space. 4. Measure up net outside wall, glass and roof next to heated spaces, as well as any cold floor or ceiling next to unheated space. Such measurements are made from building plans. * 5. Compute the heat transmission losses for each kind of wall, glass, floor, ceiling and roof in the building by multiplying the heat transmisison coefficient in each case by the area of the surface in square feet and the temperature difference between the inside and outside air. See paragraphs 1 and 2 above. 6. Select unit values and compute the heat equivalent of the infiltration of cold air taking place around outside doors and windows. These unit values depend on kind or width of crack and wind velocity, and when multiplied by the length of crack and the temperature difference between the inside and outside air, the result expresses the heat required to warm up the cold air leaking into the building per hour. 7. The sum of the heat losses by transmission (paragraph 5) through the outside wall and glass, as well as through any cold floors, ceilings or roof, and the heat equivalent (paragraph 6) of the cold air entering by infiltration is the total heat required for warming any building. INSIDE TEMPERATURE The inside air temperature which must be maintained within a building, and which should always be stated in the heating specifications, is understood to be the temperature at the breathing line 5 ft. above the floor and hot less than 3 ft. from the outside walls. Inside air tempera tures usually specified vary in accordance with the use to which the building is to be put, and Table I presents values which are in conformity with good practice. .... 1 Professor of Heating and Ventilation and Head of Department of Mechanical Engineering. University of Illinois.