Document YG283O1VXXB97zzEkYmDnp0XE

354 Chapter 19' 1945 Guide. largely eliminated through proper'care in the planning and installation of the control system6. Controls which are considered desirable for this system are: 1'. A thermostat located in a living room where maximum'fluctuation in temperature can be expected, in order, to secure frequent operation of fans,drafts, and burners. The thermostat location should not be on an outside wall, in a bed room, bath room or sun room, or in a location where it will be affected by direct radiant heat from the sun or from a fireplace, or by direct heat from any warm air duct, register or chimney. 2. A fan switch control located in the bonnet to start, blower operations at tempera tures between 120 and 150 F, and to stop the blower at about 25 to 30 F below the cut-in point. The lower settings are used for high side wall register installations, and the^higher settings for baseboard register installations. For most satisfactory results these settings should be as low as is feasible. 3. A protective high limit control located in the bonnet to stop the system independently of the thermostat if the bonnet temperature exceeds. 200 F. 4. On oil and gas burner installations, a protective control should be included which will stop the system if the fire is extinguished or if there is a failure of the ignition system. 5. On automatic stoker installations, a control is usually included which will start the operation regardless of thermostat settings whenever the bonnet temperature Table 2. Factors Corresponding to Register Temperature for Equation 2 . Register Temperature Dbg F . Factor 110 0.0221 120 0.0184 130 0.0158 140 0.0140 160 0.0125 160 0:0114 170 0.0105 indicates that the fire is dying, or a time interval contactor is used that will start the stoker to run a few minutes out of each hour. 6. A humidistat to regulate the moisture supplied to the rooms, located either in one -of the rooms or in the main return duct near the furnace. METHOD OF DESIGNING FORCED-AIR HEATING SYSTEMS8 1. Determine heat loss from each room in Btu per hour. (See Chapter 6.) .2. Locate warm air registers and return registers on plans of house, beginning with the upper story rooms. 3. Sketch in duct layout to connect all registers and grilles with the central unit. 4. Determine equivalent length of duct for each register, allowing at least 10 diameters of straight pipe as equivalent to each 90 deg elbow having an inner radius not less than the diameter of the round pipe or the depth of the rectangular pipe. 5. Select a value for temperature of the air at the furnace bonnet. It is customary to use some value between 145 to 165 F. 'Use lower value if larger number of air recircu lations is desired. The number of air recirculations should range from three to eight per hour.6. Determine approximate value of temperature reduction in each duct caused by heat loss from the ducts. A value of from 0.3 to 0.6 F per foot of duct has been obtained from tests conducted in the Research Residence installation for uninsulated duct lengths up to approximately 60 ft. `Automatic Controls for Forced-Air Heating Systems, by S. Konzo ahd A. F. Hubbard (A.S.H.V.E. Transactions. Vol. 40, 1934, p. 37). `Technical Code, Fourth Edition, January 1, 1942, published by the National Warm Air Healing and Atr Conditioning Association. 145 Public Square, Cleveland, Ohio. . Mechanical Warm Air Furnace Systems . . ___________ ,___________ 355. 7. Subtract this temperature reduction from the assumed bonnet air temperature to obtain ^approximate value of the register air temperature for each register. 8. Determine the required air volume for each room, from Equation 1, or from the values listed in Table 2: .v . H ' Q= 60 X 0.24 X d (fr - 65) (l) where Q = required air volume, cubic feet per minute. If = heat loss of room, Btu per, hour. . d -- density of air at register temperature, pounds per cubic foot. <r = register temperature, degrees Fahrenheit.. ' 0.24 = specific heat of air. 65 = return air temperature, degrees Fahrenheit. ' For any given register temperature the solution of this equation simplifies to: Q = H X Factor (2) in which the values of the Factor may be obtained from Table 2. 9. Determine register size from the air volume delivered to each room: - Table 3. Approximate Design Velocities through Ducts and Registers .Description - Low Velocity System (fpm) . Medium Velocity System : (ppm> High Velocity System (fpm) Main ducts.................... ...... Branch ducts............... ,, Wall RtArks Baseboard registers (Down deflecting} Wall registers above 5 ft (min.)____ 500 . 450 350. 300 500 750 , . 1000 600 750 500 600 400 500 . 550 600 Gross area of register, square feet = ^ . (3) where Q = required air volume, cubic feet per minute. , V = velocity at register face, feet per minute. R = ratio of free area to gross area of register. Allowable register velocities to be used in Equation 3 are given in Table 3. In residential applications it' is not advisable to handle more than 150 cfm through any single register. 10. Duct systems for forced-air installations may consist of either trunk systems or individual duct systems. . Trunk Systems. Determine duct sizes and friction losses as outlined in Chapter 31, *cept that for residence applications the velocities in the main duct and in the various Parts of the system should approximate the values recommended in Table 3. Individual Duct Systems. An individual duct system is one having'separate ducts attending from the heating unit to each register. In designing such asystem select first the duct having the greatest equivalent length. Select a reasonable velocity using Table o as a guide. From friction chart in Chapter 31 determine unit friction loss per 100 ft of run, and from this the total friction loss in the duct selected. If this total friction loss exceeds a reasonable value a lower velocity should be used. The remaining ducts are proportioned so that the total pressure in each duct is the same as that, calculated for the longest duct. The added resistance necessary in the shorter ducts is accomplished by increasing the velocity in these ducts. No duct should 19nrw t*lan *" 'n diameter, nor should the velocity in any duct exceed approximately 1400 fpm. The final adjustment in a duct system may be made by employing dampers.