Document gDm4JmZgMB6rvz8RgwXE8jJVQ

542 CHAPTER 36 1960 Guide rents for gas appliances, shall not be used unless listed by Underwriters' Laboratories, Inc., installed in full compliance with the listing and the manufacturer's instructions, and ap proved for such use by tbe building official. 3. Smoke Pipe Connections. a. No flue shall have smoke pipe connections in more than one story of a building, nnlw provision is made for effectively closing smoke pipe openings with devices made of noncombusuble materials whenever their use is discontinued temporarily, and completely closing them with masonry when discontinued permanenUy. b. Two or more smoke pipes shall not be joined for a mngle connection, unless the smoke pipes and flue are of sufficient size to serve all the appliances thus connected. c. The smoke pipe of a heating appliance shall not be con nected into the flue of an incinerator which has the rubbish chute identical with toe smoke flue. 4. Construction of Chimneys. a. Chimneys hereafter erected within or attached to a struc ture -shall be constructed in compliance with the provisions of this section. b. Chimneys shall extend at least 3 ft above the highest point where they pass through the roof of the building, and at least 3 ft higher than any ridge within 10 ft of such chimney. c. Chimneys shall be wholly supported on masonry or selfsupporting fireproof construction. d. No chimney shall be corbeled from & wall more than 6 in.; nor shall a chimney be corbeled from a wall which is less than 12 in. in thickness, unless it projects equally on **ch side of the wall; provided that in the second story of 2-story dwellings cor beling of chimneys on the exterior of the enclosing walls may equal the wall thickness. In every case the corbeling rKh.11 not exceed 1 in. projection for each course of brick, projected. e. No change in the size or shape of a chimney, where the chimney pages through tbe roof, shall be made within a dis tance of 6 in. above or below toe roof joists or rafters. 5. Chimneys for Heating Appliances, Low Heat Industrial Appliances and Portable Type Incinerators. a. Chimneys for stoves, cooking ranges, warm air, hot water and low pressure steam heating furnaces, fireplaces, low heat industrial appliances, other than chimneys for incinerators of nonportable type, shall be constructed of solid masonry units or of reinforced concrete. The walls shall be properly bonded or tied with corrosion-resistant metal anchors. In dwellings buildings of like heating requirements, the thickness of the chimney shall be not less than 4 in. Iq other buildings the thick ness shall be not less than 8 in., except that rubble stone masonry shall be not lea than 12 in. thick. b. Every such chimney hereafter erected or altered shall be lined with a flue lining conforming to the requirements below. c. Flue linings shall be made of fire clay or other refractory clay which will withstand the action of flue resist, without softening or cracking, the temperatures to which they will be subjected, but not lea than 2000 F. Flue lining* may be of east iron of approved quality, form and construction. d. Required clay flue linings shall be not lea than % in. thick for toe smaller flues, and shall increase in thirlmoi for the larger flues. e. Flue linings shall be installed ahead of the construction of the chimney as it is carried up, carefully bedded one on the other in Type A, Type B, or fire clay mortar with close fitting joints left smooth on the inside. f. Flue linings shall start from a point not lea than 8 in. be low the intake, or, in the case of fireplaces, from the throat of the fireplace. They shall extend, as nearly vertically as possible, for the entire height of the chimney, and be extended 4 in. above the top of cap of the chimney. g. Cleanouts for flues or fireplaces shall be equipped with cast-iron doors and frames arranged to remain tightly closed when not in use. h. When two or more flues are contained in the same chim ney, at least every third flue shall be separated by masonry at least 4 in. thick bonded into the masonry wall of the chimney. Where flue lining* are not so separated, the joints of adjacent flue linings shall be staggered at least 7 in. 6. Sizes of Flues. a. The cross-sectional area of smoke flues shall be designed and proportioned to meet toe conditions of temperature, within and without flue, thickness of masonry, exposure, shape and material of flue, and other influences. Tire National Building Code specifies lined chimneys and metal smoke stacks for all gas appliances which may be con verted readily to the use of solid or liquid fuel, and also for all boilers and furnaces, except those having a flue-gas tem perature not exceeding 550 F at the outlet of the draft hood when bunting gas at the manufacturer's rating and which may, therefore, be connected to Type B vent piping. Ap proved Type B vent piping is noncombustible, corrosion-re sistant piping of adequate strength and heat-insulating value, and having bell and spigot or other acceptable joints. Fig. 10 may be used for selection of vent-pipe size. Important points to be considered in the use of Type B vent piping are: 1. Type B flues must be plainly and permanently marked at the point where the vent connection enters the flue: For Use on Gas Appliances Only. 2. Type B gas vents shall be installed with a clearance to combustible material or construction, whether plastered or un plastered, of not less than one inch, provided that for vents of floor furnaces, such clearance "hall be not lees than 3 ft from the outlet of the draft hood, measured along toe center line of the vent piping. Other important points that should be considered for flues and vents for gas appliances are as follows: 1. Clearances from combustible material to gas appliance vent piping other than approved Type B gas vents shall be in accordance with the Building Code Standards of the National Board of Fire Underwriters for the Installation of Heat Produc ing Appliances, Heating, Ventilating, Air-Conditioning, Blower, and Exhaust Systems. 2. Every flue-connected appliance, except an incinerator, un less its construction serves the same purpose, shall be equipped with an effective draft hood which either (a) has been approved as part of the appliance or (6) complies with nationally recog nised standards for draft hoods. The draft hood shall be at tached to the flue collar of toe appliance as conditions permit, and in a position for which it is designed with reference to hori zontal and vertical planes. Hie draft hood shall be so located that the relief opening is not obstructed by any part of toe appliance or adjacent construction. 3. No vent pipe from a gas appliance shall be interconnected with any other vent pipe, smoke pipe, or flue, unless such gas appliance is equipped with an automatic device to prevent the escape of unbumed gas at the main burner or burners. Where a gas appliance vent pipe is joined with a smoke pipe from an appliance burning some other type of fuel, for connection into a single flue opening, they shall be joined by a Y fitting located as close as practicable to the chimney. With liquefied petroleum gases, the automatic device to prevent the escape of unburned gas shall shut off the pilot light, as well as the main burner or Burners. Recent tests of masonry chimneys" made of a variety of materials have developed additional recommendations re garding the construction of masonry chimneys that will de crease the hazard to surrounding combustible materials. GENERAL CONSIDERATIONS FOR CHIMNEYS The draft of domestic chimneys may be subject to a variety of influences not usually encountered in power chimneys" Chimneys and Draft Calculations 543 because of the low available draft often supplied by a short chimney. Horizontal winds have an aspirating effect as they ext the chimney and are an aid to draft. However, sur rounding objects, such as trees or other buildings, may affect the direction of the wind at the chimney top, and may even direct it down the chimney, tending to reduce the draft or even to cause it to change to a positive pressure. It is not to be assumed that increasing the cross-sectional area of a chimney will always effect a cure for poor draft. The opposite result may occur because of the cooling effect of the larger area, and the effect of recirculation of the flue gases. The flow of gases into the chimney top has been observed at low rates, and recirculation in small residential chimneys has been noted throughout the entire length of a chimney and smoke pipe, with the greater amount of recirculation occur ring at the thimble. The effect of recirculation decreases with chimney height and the increase in flue-gas velocity. It is also important to consider the course of the air supply for proper combustion. The boiler or furnace is usually lo cated in the basement. In the majority of cases, the furnace room has windows and doors opening to the outdoors on two or more sides of the house. Usually enough air leaks into the furnace room through these openings to sustain combustion. In some cases, however, windows and doors are so tight as to restrict the flow of combustion air, and thereby affect the correct operation of the chimney. If the boiler room is fairly tight and is open to the outdoors on only one side of the house, the draft will be affected in windy weather even with windows or doors open. If the wind is blowing toward the boiler room, the draft will be increased, but if blowing in the opposite direction, the draft may be decreased. The surrounding of a heating appliance with a restrictive enclosure that will limit entrance of combustion air is more likely to occur in a small utility closet installation on the first floor than in a basement installation. An opening with a free area approximately twice the area of the smokepipe should therefore, be provided between the utility closet and the living space or the outdoors. If the utility closet is con nected to the outdoors, greater difficulty with wind pressures will be encountered. Where a draft regulator is used it should have ample communication with the same space from which the combustion air is taken. Where forced warm air furnaces are enclosed in utility closets, care should be used to make the return air connection inside the utility closet airtight so that the blower cannot reduce the pressure in the closet and cause a downdraft in the chimney. Two or more chimneys, either large or small, should never be connected. If connected at the bottom, hot gases in the U-tube thus formed would be in unstable equilibrium. Cold air from the top would descend through one such chimney and drive the hot gases out of the other, thus annulling the draft. * More than one device can be served by one chimney. Bat teries of boilers are commonly connected to a single chimney in power plants. However, if two or more chimneys are used, each chimney should be used separately for part of the boil ers; and not connected in manifold with another chimney, in order to avoid the difficulty described previously. ' In domestic installations it is sometimes necessary to serve a space heater or cooking stove and a water heater with the same chimney flue. This is not desirable, especially for low chimneys, since doors left open on one device, while it is un fired, will tend to annul the draft on another device. Gasburning devices, with their draft hoods and lack of draft dampers, are especially bad in this respect. The traditional method of avoiding this with brick chimneys has been to construct multiple-flue chimneys, so that each fuel-burning device could be served by a separate opening. If two devices must be served by one flue in a chimney, their connections to the chimney should not be located opposite each other. The connection from the larger device should be reasonably low, and that from the smaller should be up near the ceiling, so that each device can be serviced as well as possible, re gardless of the treatment of the other. Even where such precautions are taken, there is, under unusual conditions, some possibility of flow of combustible from one appliance into another. Reverse flow of cooled gases has been demonstrated in a chimney at very low rates of flow.1 Under certain similar quiescent conditions, acci dental discharge of combustible gases from a defective device into a chimney could result in flow of these gases counter current into the combustion chamber of another device at tached to the amt chimney. If ignition occurred in this sec ond device, an explosion could result. Excessive height in a chimney does no harm, but means for controlling the draft are more than ordinarily essential if the chimney is too tall. Coal-burning devices often have air leaks around the firebox, and tbe draft doors sometimes fit so poorly that tbe fire cannot be controlled at a low rate. The simplest remedy for such cases is the barometric damper which admits air into the flue pipe and thus reduces draft. Where a chimney serves a fireplace, it is important that no other heating device be connected to it unless the fire place is effectively sealed. REFERENCES 1R. H. Sherlock and B. A. Stalker: A Study of Flow Phe nomena in the Wake of Smokestacks (University of Michigan, Department 0} Engineering Research Bulletin No. 29, 1941). * E. F. Miller and James Holt: Notes on Power Plant Design (Massachusetts Institute of Technology, 1930). *L. F. Moody: Friction factors for pipe flow (ASME Trans actions, Vol. 66, 1944, p. 671). `R. T. Kent (ed.): Mechanical Engineers' Handbook (John Wiley & Sons, New York). 'G. A. Hool and N. C. Johnson: Handbook 0} Building Con struction (McGraw-Hill Book Co, New York, 1929). * R. S. Dill, P. R. Achenbach, and J. T. Duck: Observed per formance of some experimental chimneys (ASHVE Trans actions, Vol. 48, 1942, p. 351). * L. B. Schmitt and R. B. Engdahl: Performance of residential chimneys (ASHVE Transactions, Vol. 55, 1949, p. 241). * P. R. Achenbach and S. D. Cole: Performance of fourteen masonry chimneys under steady state conditions (ASHVE Thansactions, Vol. 55, 1949, p. 129). R. D. Thulman and W. H. Shenkle: A theoretical and ex-/ periment&l investigation of the performance of some short flues under steady-state conditions (Thesis at Masachuaetts Insti tute of Technology, June 1951). " P. R. Achenbach: Physics of chimneys (Physics Today, Vol. 2, December 1949). UR. K. Thulman- Performance of Masonry Chimneys for Houses (Housing and Home Finance Agency Technical Paper No. 13, August 1949). "CSlOl-43 Oil-Burning Space Heaters Equipped with Vo-. porizing Pot-Type Burners, CS75-42 Automatic Mechanical Oil Burners Designed for Domestic Installations GS(E)104-43