STACKS AND BREECHING NOTE The Stack and Breeching shall be installed in conformance with national, regional and local codes and standards and/or appropriate legislation. The purpose of the stack and breeching is to conduct the products of combustion safely to the atmosphere. In addition a well designed stack will provide sufficient draft to overcome friction losses in the breeching and stack. Considerations On individual units, a direct vertical connection off the flue gas outlet of the boiler (it is preferable to support the stack/breeching independent from the boiler) or a horizontal offset connection may be utilized.
Figure 1 In the case of multiple boilers, it is always better to utilize a separate stack for each boiler. If this is not practical, See Figure 2 for a Typical Multiple Boiler Installation with Common Breeching.  Typical Multiple Boiler Breeching Installation
Figure 2 BREECHING Installation AND Construction of the Breeching shall be in conformance with national, regional and local codes and standards and/or appropriate legislation. Changes in breeching direction, shape and flow area must be accomplished separately and as gradually as possible to eliminate turbulence which may adversely affect burner performance.  Preferred Methods of Increasing Breeching Flow Area
Figure 3 The Breeching should be as short and straight as possible without excessive bends to minimize flow resistance. A round breeching is preferred over a square or rectangular breeching and stack. Insulating the breeching will help to minimize flue gas condensation which may result in excessive corrosion. In addition the higher resultant stack temperature will aid in increasing the available draft. STACKS Construction AND Installation of the Stack shall be in conformance with national, regional and local codes and standards and/or appropriate legislation. The following types of chimneys are suitable for use with boilers as supplied by Boilersmith Ltd. - Rectangular Brick Masonry Chimneys - Radial Brick Masonry Chimneys - Reinforced Concrete Chimneys - Metal Chimneys - Factory-Built, Type A Chimneys - Factory-Built Pressure Fired Chimneys Factory-Built, Type B, L or Class 1 BH for NATURAL GAS FIRED APPLIANCES ONLY may be used if the flue gas temperature is less than 500oF and a negative pressure exists at the flue gas outlet vent of the boiler. ... APPENDIX "B" The walls of any chimney, gas vent or flue pipe shall be constructed to be gas-, smoke- and flame-tight. A round stack is preferred over square stack. Every chimney shall be capable of providing sufficient draft to vent properly the appliances that it serves. The cross-sectional area of the flue pipe shall not be less than the area of the flue outlet of the appliance, except that a tapered reduction in the section of the flue adjacent to the chimney is permitted provided adequate draft is maintained. Insulating the stack will help to minimize flue gas condensation which may result in excessive corrosion. In addition the higher resultant stack temperature will aid in increasing the available draft. A rain cap or hood should be used at the top of the stack to minimize the entrance of rain or snow.  Rain Cap
Figure 4 The top of every chimney shall be at least 3 ft above the highest point at which it comes in contact with the roof, and 2 ft above a roof surface or structure with a horizontal distance of 10 ft from the chimney.  Stack Construction
Figure 5 Installation of the Stack shall be in conformance with national, regional and local codes and standards and/or appropriate legislation. The stack should be higher than nearby structures or geographical prominence since they could create down drafts or eddy current. SIZING BREECHING AND STACK A Breeching and stack should be sized on the basis of maintaining a flue gas flow of 30 ft/sec or less with the boiler operating at maximum capacity. In the case of a multiple boiler installation utilizing a common stack and breeching, the breeching should be sized to maintain a constant velocity of flow. The Equivalent Diameters of Breeching for Multiple Boiler Installations to maintain a constant velocity of flow are shown in Chart 1. For single boiler installations the Suggested Minimum Breeching Size and Boiler Outlet Diameters for Boilersmith Ltd. boilers is shown in Chart 2. It is generally recommended that the stack size selected be larger than the breeching diameter and should not be selected smaller than the flue gas outlet supplied with the boiler...
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