Document zoNpBVV7LBeqG8YrgnG69jYwB

446 CHAPTER 22 .. . : -1949 Guide,; Table 2. Return-Air Duct System ,Combinations- of Parts Selected as ..11!/MSTA?tllARD .. 1- Return-Air Intake * Riser,Size, Combi : Size, In,. , In..Where Branch Pipe Size, In'- ..WhenJoist Lining IS U8EDb nation No. Stack is ' Used in Number op Joist " Spaces Lined and Base-. -Board Floor* Stud Space ' Round - - Rec Minimum Depth op Space Required tangular Required Increasein Width op Trunk Duct (fob 8 In. Depth op Duct), In. 1 23 4 6 :8 7 8 51 10 x 6 6 x 10 10 x or 4 x 14 6 4x8 I space of 3 in. ix depth l; t. 52 10 x 6 6 x 10 10 x 3p or 4 x 14 6 .4x8 1 space of 3 in1, depth 2 53 12x6 6 x 12 12 x 3}d -- ,7 5x8 1 space of 4 in. . -or':',; depth . j:` 6 x 14 3 54 14 x 6 6x14 14 x 3id 55 24 x 6 6 x 30 Two;, or stacks. 30 x 6 each 10 x 31". 8 6 x 8 1 space of 5 in. - 4 idepth 9 8x8 1 space of 6 in. depth or 2 spaces of 3 in. depth. 5 56 30x6 6 x 30 Two ' 10' 10x8 1 space of 7 in. stacks depth or 2 spaces 12 x 3Jd of 4 in. depth r .57 8 x 30 . 12 15 x 8 .1 space, of. 9 in. . .12 depth or 2 spaces of 5 ih. depth J. : .. . * Use these items only when building construction,.or capacities,'require the use oi floor intakes. The sues-listed correspond to standard sizes for gniviiy'uistallatidna.'oxccpt floor`box ooUan. The' use of- ` standard-blind, boxea is suggested. , - - j. . sPdce^between ;joista., .Use full depth.of joist, except .when joist depth is less than - minimum depth required, in which case a drop' pan mutt be used' This may occur when two or more re^ : turn.ducts, are connected to the/aamei joist,space.,. - > . ,.r.,, T.. .ins keys ordensoirte. d to,use 14 in. x 3f in..stud ' space,-itm" &kes.no difference whether - thTM space has protrud* If it is desired,.to use;I4-in/x`3i ini stud spadB,:the plaster base must be smooth.- without any pro- '1 trading plaster keys.to,interfere with the flow of air.. .- ' STANDARD COMBlNATlbllS OF PARTS f "'V The combinations of parts selected as standard by the National ivhrm ' Air- Heating and Air ConditioningyAssociation arc shown in Tables l and 2. A method.for selecting these combinations is indicatedinthe;foilowing,- section Simplified Method of Design,,' Vi, ; ; ; '' "'X.SIMPLIFIED METHOD OF DESIGN . /, V`; A simplified method for. selecting .the .combinations of, branches,,boots, stacks, and registers, is.given.in.Manual: No..7, pf ,tke .National^Warm, Air, Heating ,and Air Conditioning Association,,, In this method the sizes of ,theV bi^chducte are. obtained from two tables;giying their Btu capacities:; The.V proper combination of parts for each-branch can'be determined,if,the'fpUow>. [ ing information is available. ;,/ a. Location of room, that is, whether on first or second story. ^Mechanical Warm Air Systems : 447 b. Actual length of duct from bonnet to boot, in feet. c. 'Btu loss' from room to be heated. d. Equivalent lengths in feet of all fittings and of the register. Fig. 3 shows the values of equivalent lengths of fittings commonly used for domestic systems. This simplified method is applicable to structures having heat losses not in excess of approximately 120,000 Btu per hour. The capacities, shown in Tables 3 and 4 are based upon the most reliable data pertaining to friction losses and temperature drops in ducts. They are also based upon a 100 deg temperature rise of the air, and a static pressure`available for over coming friction losses in the external duct system alone of 0.20. in. water gage. The use of this method assumes that the ;fan in, the fan-fumace assembly will be . capable not only of overcomuig; the! resistance, of the external duct system alone, but also the resistances imposed by the. blower inlet, the filter, and the furnace casing. The.combination numbers shewn in the right hand column of Tables 3 and 4 correspond;,to; those given in Tables ! and 2: "Tables 3 and 4 are also applicable'for the'selection of the return air branches. A depth of 8 in. has,been adopted, as the standard for the trunk ducts.' The width of a trunk duct serving two branches is deter mined by adding to the width of the.remote branch the value shown in column_7_of Tablb-1, or column 8 of Table 21' .\ ' AUTOMATIC CONTROLS Air stratification, high bonnet temperatures, excessive flue gas tempera tures, and. heat overrun or lag in a properly designed system can be largely eliminated through proper care in the planning and installation of the con trol system*; desirable .controls usually employed are: " ' - 1. A thermostat located in a living room where maximum fluctuation in tempera ture can be expected, in order to secure frequent operation of fansy.drafts, and burn ers. The thermostat location should not be on an outside wally iri: a bedroom, 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. Afan switch control located in the bonnet to start blower operations at tempera tures between HO and 130 F, and'to stop the blower at about 25 to 30 deg 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 Bettings should be as low as is feasible. ; 3. A prolective high limit switch located in the bonnet to stop the system inde pendently of the thermostat if the bonnet temperature exceeds 175 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 afailure of the:igni- tion.system. . , .-.-i-v:.-;' : 5. On automatic stoker installations, a control is usually included which will start the operation regardless of thermostat settings whenever the-bonnet temperature 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 qf; the rooms or in the main return duct near the furnace. .. DESIGN PROCEDURE FOR LARGE SYSTEMS41 For buildings haying a heat loss in. excess, of 120,000 Btu per hour thedesign procedure given in Manual No. 9 of the NWAH.&ACA, may.be uted except, where ventilation air volume exceeds volume required to supply calculated heat loss. This procedure consists of 1. Calculation of design heat losses from individual spaces in the structure. (See Chapter 14.)