Document 752rmLQdw2daOrQKrZkXjv9a

430 CHAPTER 17 1955 Guide temperature drop and friction loss in the flue pipe cannot be calculated accurately,7 and the resistance of various chimney connections is uncertain, the minimum calculated heights should have a generous factor of safety applied to allow for these and other uncertainties. - Where ample height cannot be provided, forced or induced draft should be provided. Extreme care should be used in the application of forced draft to avoid producing a pressure in the combustion chambers, because they are not usually de signed to avoid the leakage of toxic gases which might result. Induced draft by means of motor-driven fans or blowers will solve lowdraft problems, but would usually be employed for residential systems only as a last resort. Operation of a blower entails some cost, introduces main tenance requirements, and may be dangerous in event of power failure unless suitab'e protective devices are used. Induced draft fans designed Table 3. Drafts Required by Typical Residential Heating Devices ok Appliances Device . Draft, Inches Water Stack Tempera ture F Deo 0.06 to 0.08 0.06 0.06b 0.06 0.03* 0.05* or less 0.04b 0.06b * Draft in firebox. b For cheatnut sixed anthracite. 0 18 in. from heater. 1000 860 900 860 -- -- 400 900 to resist effects of high temperature and corrosion are available. Fans not carefully designed or modified to meet these conditions should not be ap plied to this service. CHIMNEYS FOR GAS HEATING Since a gas-designed appliance must be able to operate at rated input (plus 10 or 15 percent) without chimney connection, and without produc ing carbon monoxide, the only function of the chimney is to remove the products of combustion from the room. The chimney provides draft to overcome the friction in the flue pipe and chimney, but does not draw air into the appliance. Chimneys for venting appliances designed for burning gas, can therefore be low in height, but must have adequate area. The height is usually established by the building height. Chimney sizes are usually selected on the basis of heat input of the appliance. The chimney sizes adopted by the American Standards Association and the National Board of Fire Un derwriters in 1950 for gas appliances18 are shown in Fig. 10. Additional provisions of ASA Standard Z21.30-1950 relating to chimney size are: 1. In no case shall the vent area be less than the area of a 3-in. diameter pipe2. When more than one appliance vents into a flue or vent, the flue or vent area shall be not less than the area of the largest flue or vent connector pins au per cent of the areas of the additional flue or vent connectors. 3. An elliptical flue or vent may be used provided its flue gas venting capacity is equal to the capacity of the round pipe for which it is substituted. Chimneys and Draft Calculations 431 Since Fig. 10. has been prepared for circular flues, relative capacities for rectangular and semi-elliptical flues14 are shown in Fig. 11. Heating appliances designed to bum gas, as well as appliances converted to gas burning, except those' equipped with power type burners and ex cepting conversion burner installations in excess of 400,000 Btu per hour input in large steel boilers, are always equipped with a draft hood attached to the flue outlet of the appliance. This draft hood is required if the appliance is to meet the approval requirements of the American Gas As sociation and the American Standards Association, and is essential for safe Fig. 10. Capacity in Btu per Hour fob Gas Appliance Flues or Vents operation. It is designed to prevent excessive chimney draft which would lower appliance efficiency, to prevent a blocked flue or a down-draft in the chimney from impairing combustion, to provide a relief opening for the products of combustion during down-draft or blocked flue conditions, and prevent spillage of the products of combustion to the space surrounding the appliance, if there is a chimney draft equivalent to that provided by a 3-ft chimney. As the draft hood is designed without moving parts, the relief opening is always open, and consequently some air is drawn into the chimney. This air lowers the gas temperature in the chimney, but it a*so 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.