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524 .CHAPTER 36 1959 Guide the gas temperature in the ehimney, but it also lowers the dew point of the gases and tends to prevent condensation. The installation'of conversion burner equipment in large boilers is usually made in accordance with regulations of the local gas company. In such installations a definite chimney draft may be required for proper combustion, and conse quently the foregoing reference to the use of draft hoods would not apply. The products of complete combustion of gas are water va por and carbon dioxide. In the case of manufactured gas, the presence of organic sulfur compounds, generally between 3 and 15 grains^per hundred cubic feet, gives rise -to minute percentages of sulfur dioxide and sulfur trioxide. The volume of water vapor in the flue products from natural or coke oven gas is about twice the volume of carbon dioxide. It is ex tremely important that the chimney be tight and resistant to corrosion, not only from moisture, but also from dilute sulfur trioxide. Clay linings with joints which prevent retention of mois ture, and linings made of noncorrosive materials, are advan- Table 4.... Results of Reid Survey on Chimney Damage Typm of CofufiucliiNi Number of Chiaaeyt Number Reported Pend Damaged Oomogod Chimneys without liners............ Chimneys with sewer tile con- struction. .............................. Wilder metal coated with as- phalt chromate emulsion... Steel coated with plastic........... Vitreous enamel on steel........... Aluminum...................................... 4720 8410 5185 9200 614 50,000 562 104 291 1928 No record 228 11.9 1.2 5.6 21.0 -- 0.45 tageous. The interior protection of unlined chimneys by ap plying an asphalt-chromate emulsion has been investigated by some gas companies but the material is not of current in terest because insurance companies do not favor potentially combustible coatings." An investigation of damage occurring in 78,129 chimneys used for venting gas appliances in a severe climate has been made by the Minneapolis Gas Company with the results shown in Table 4 taken from Reference 15. - Advice regarding recommended practice and materials for flue connections and chimney linings can usually be obtained from the local gas company, and should be given careful con sideration. RECOMMENDATIONS OF THE NATIONAL BOARD OF FIRE UNDERWRITERS For general data on the construction of chimneys, reference should be made to the Notional Building Code, recommended by the National Board of Fire Underwriters, Article X, Sec tion 1001 to 1006, in which the following are some of the important provisions listed in the 1951 edition: 1. Flue Connections Required. Every heating apparatus or heat producing appliance requiring a flue connection shall be connected with a flue conforming to the provisions of this arti cle. This shall not include electric appliances; gas applianees, except as specifically required in this article; or oil fired appli ances especially designed for use without flue connection. 2. Use of Nonconforming Flues. Flues not conforming to the requirements of this article for chimneys, metal smokestacks or vents for gas appliances, shall not be used unless listed by Underwriters' Laboratories, Inc., installed.in full compliance with tiie h'gring and the manufacturer's instructions, and ap proved for such use by the building official. 3. Smoke Pipe Connections. a. No flue shall have smoke pipe connections in more than one story of a building, unless provision is made for effectively closing smoke pipe openings with devices made of noncombustible materials whenever their use is discontinued temporarily, and completely closing them with masonry when discontinued per manently. b. Two or more smoke pipes shall not be joined for a angle connection, unices 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 the 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 2 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 a wall more than 6 in.; nor shall a chimney be corbeled from a wall which is less than 12 in. in thicVnowi, unless it projects equally on each 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 tiuefcnea. In every case the corbeling shall 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 passes through the roof, shall be made within a dis tance of 6 in. above or below the 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 beating furnaces, fireplaces, and 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 and buildings of like heating requirements, the thickness of the chimney shall be not leas than 4 in. In other buildings the thick ness shall be not less than S in., except that rubble stone masonry shall be not less 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 gases and resist, without softening or cracking, the temperatures to which they will be subjected, but not less than 2000 F. Flue linings may fce of cast iron of approved quality, form and construction. d. Required clay flue lining* shall be not les than H in. thick . for the smaller flues, and shall increase in thickness 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 less 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 doom and frames arranged to remain tightly closed when not in use. b. When two or more flues are contained in the same chim ney, at least every third Sue shall be separated by masonry at least 4 in. thick bonded into the masonry wall of the chimney. Where flue linings are not so separated, the joints of adjacent flue lininp shall be staggered at least 7 in. Chimneys and Draft Calculations 525 6. Sizes of Flue*. a. The cross-sectional area of smoke flues shall be designed and proportioned to meet the conditions of temperature, within flpH without flue, thirlmesg of masonry, exposure, shape and material of flue, and other influences. The National Budding 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 burning 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-resistant 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 clearanpp 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 shall be-.not less than 3 ft from the outlet of the draft hood, measured along the 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 ofHeat 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 (b) complies with nationally recog nized standards for draft hoaids. The draft hood shall be at tached to the flue collar of the appliance as conditions permit, and in a position for which it is designed with reference -to hori zontal and vertical planes. The draft hood shall be so located that the relief opening is not obstructed by any part of the appliance or adjacent construction. " 3. No vent pipe from a gas appliance whall be interconnected with any other vent pipe, smoke pipe, or flue, nntexs such gas appliance is equipped with an automatic device to prevent the escape of unhurried 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 unbunted 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" because of the low available draft often supplied by a short chunney. Horizontal winds have an aspirating effect as they cross 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. Hnwn 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, aztd 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 some 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 inride 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 Berve a space heater or cooking stove and a water beater 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. Gas burning 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-opening in a chimney, their con nections to the chimney should not be located opposite each