Document KGOjOMdKe5MJdMr2vVYG5zyo

424 Chapter 17: . V 1953 Giiid'e able temperature drop of the flue gases-between heater and chimney, data from Fig. 8 and Tabic 1 shov* -that an effective chimney height of 8 to 12 ft -.above the center line of the thimble,-depending on the type of . chimney connection, is the minimum height that should-be. employed for such equipment. Goal-burning heaters and furnaces that must attain rated output at a design outdoor temperature'of 0 F would probably perform satisfactorily with an effective chimney height of about 8 ft. Application of adequate insulation to the smokepipe: and breeching might reduce these minimum values by -1 to 2 ft.: When these minimum chimney heights cannot be provided for architectural reasons or where flue gas temperatures lower than those shown in Table, 3 exist, forced draft or induced draft should be employed. Since most .residential heating plants are not de signed for pressurized combustion chambers, forced draft should be used Table 3. Drafts Required by Typical Residential. Heating Devices or Appliances Deytce Space Heater, Oil Burning. Pot Burner Warm Air Furnace, Oil Burning, Pot Burner Warm Air Furnace. Hand Fired... Floor Furnace, Oil Burning. Pot Burner Mechanical Oil Burner. Less than 5 gph Mechanical Oil Burner, More than 5 goh Cooking Stove, Solid Fuel.... Space Heater. Coal Burning a Draft in firebox. b For chestnut sized anthracite. Draft, Stack Tempera- Inches Water tube F- Deo 0.06 to 0.08 0 06 0.06b 0 06 0 03 0.05* or less 0.04b 0.06b e 18 in. from heater. 1000 . 860 900. 860 400 900 only to overcome the fuel bed or burner friction unless it is known that the plant has been constructed with no possibility of leakage in the combustion chamber and flue gas passages. 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 appliances13 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 pipe. 2. 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 plus 50 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 tne round pipe for. which'it is substituted. Churiheys'and Draft Calculations '<'425 ;-.> SinceiFig. 10 has been prepared -for circular flues,' relative-capaeities for rectangular' anil semi-elliptical flues11 are shown in Fig.il.- Heating appliances designed to burn gas, as weil as appliances'coriverted tongas burning,- except- those Equipped with power- type burners'and 'ex-' cepting'-coriversion- burner-installations ini excess of-400,000-Btu -penhbur input in large steel boilers, are always equipped with a draft hood-attached toIthe'flue-ioutlet of the-appliance: This draft-hood is.required-if1 t2h3e appliance-is to meet the approval requirements`oif-the'- American Gas'Association and the American Standards Association, and is essential for safe Fig, 10. Capacity in Btu per Hour for 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 to 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 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.