1、35.1CHAPTER 35 CHIMNEY, VENT, AND FIREPLACE SYSTEMSTerminology . 35.1Draft Operating Principles 35.1Chimney Functions 35.2Steady-State Chimney Design Equations. 35.3Steady-State Chimney Design Graphical Solutions. 35.12Vent and Chimney Capacity Calculation Examples. 35.14Gas Appliance Venting. 35.19
2、Oil-Fired Appliance Venting 35.21Fireplace Chimneys 35.23Air Supply to Fuel-Burning Appliances . 35.28Vent and Chimney Materials 35.28Vent and Chimney Accessories. 35.30Draft Fans 35.31Terminations: Caps and Wind Effects . 35.32Codes and Standards 35.34Conversion Factors 35.35Symbols 35.35PROPERLY d
3、esigned chimney or vent system provides andA controls draft to convey flue gas from an appliance to the out-doors. This chapter describes the design of chimneys and vent sys-tems that discharge flue gas from appliances and fireplace systems.Sustainability. Good chimney and vent design is not only as
4、afety issue, but also can enhance a buildings sustainability. Thischapter explains how to design vent systems to optimize and mini-mize the materials used to construct fuel-burning appliance ventsand chimneys for low cost and long reliability, reducing the need forvent or chimney replacement, thus s
5、aving natural resources. Also,systems designed to bring outdoor air directly into the appliancespace for combustion and vent gas dilution, instead of relying on airinfiltration into the building, reduce heat load and conserve fuel.TERMINOLOGYIn this chapter, appliance refers to any furnace, boiler,
6、or incin-erator (including the burner). Unless the context indicates other-wise, the term chimney includes specialized vent products such asmasonry, metal, and factory-built chimneys; single-wall metal pipe;type B gas vents; special gas vents; or masonry chimney liner sys-tems. (NFPA Standard 211).
7、Draft is negative static pressure, mea-sured relative to atmospheric pressure; thus, positive draft isnegative static pressure. Flue gas is the mixture of gases dischargedfrom the appliance and conveyed by the chimney or vent system.Appliances can be grouped by draft conditions at the applianceflue
8、gas outlet as follows (Stone 1971):1. Those that require draft applied at the appliance flue gas outlet toinduce air into the appliance2. Those that operate without draft applied at the appliance flue gasoutlet (e.g., a gas appliance with a draft hood in which the com-bustion process is isolated fro
9、m chimney draft variations)3. Those that produce positive pressure at the appliance flue gasoutlet collar so that no chimney draft is needed; appliances thatproduce some positive outlet pressure but also need some chim-ney draftIn the first two configurations, hot flue gas buoyancy, induced-draft ch
10、imney fans, or a combination of both produces draft. Thethird configuration may not require chimney draft, but it should beconsidered in the design if a chimney is used. If the chimney systemis undersized, draft inducers in the connector or chimney maysupply draft needs. If the connector or chimney
11、pressure requirescontrol for proper operation, draft control devices must be used.Vented gas-fired appliances have been grouped by draft and fluegas conditions as follows by installation codes in Canada (CSAB149.1) and in the United States (ANSI/NFPA 54/ANSI/AGAZ223.1):1. Category I appliances opera
12、te with nonpositive vent static pres-sure and a vent gas temperature that avoids excessive condensateproduction in the vent.2. Category II appliances operate with nonpositive vent static pres-sure and a vent gas temperature that may cause condensate pro-duction in the vent.3. Category III appliances
13、 operate with positive vent static pressureand a vent gas temperature that avoids excessive condensate pro-duction in the vent.4. Category IV appliances operate with positive vent static pressureand a vent gas temperature that may cause condensate produc-tion in the vent.Category I venting systems a
14、re typically sized using ventingtables for unobstructed vent systems, as listed in the installationcodes; they are provided for fan-assisted appliances and natural-draft appliances as well as multiappliance system vent arrange-ments. Although these categories are intended for gas-fired appli-ances,
15、they could apply to other appliances (e.g., oil- or coal-fired).DRAFT OPERATING PRINCIPLESAvailable draft Dais the draft supplied by the vent system, avail-able at the appliance flue gas outlet. It can be shown asDa= Dt p Dp+ Db(1)whereDa= available draft, in. of waterDt= theoretical draft, in. of w
16、aterp = flow losses, in. of waterDp= depressurization, in. of waterDb= boost (increase in static pressure by fan), in. of waterThis equation can account for a nonneutral (nonzero) pressuredifference between the space surrounding the appliance or fire-place and the atmosphere. If the surrounding spac
17、e is at a lowerpressure than the atmosphere (space depressurized), the pressuredifference Dpshould also be subtracted from Dtwhen calculatingavailable draft Da, and vice versa. This equation applies to allthree appliance draft conditions at the vent system inlets; forexample, in the second condition
18、 with zero draft requirement atthe appliance outlet, available draft required is zero, so theoreticaldraft of the chimney equals the flow resistance, if no depressuriza-tion or boost is present.The preparation of this chapter is assigned to TC 6.10, Fuels andCombustion.35.2 2012 ASHRAE HandbookHVAC
19、Systems and Equipment Operational consequences of various values of Daare describedas follows:Positive available draft (negative vent pressure); Dais posi-tive. Category I fan-assisted and draft hood-equipped appliancesand category II appliances can operate satisfactorily when servedby venting syste
20、ms having positive available draft at the applianceflue gas outlet, if the positive draft is sufficient to convey all fluegas from the appliance flue gas outlet to the outdoors and if thepositive available draft does not aspirate excessive excess air tocause flame lifting or other detriments to comb
21、ustion perfor-mance. Category III and IV appliances can operate satisfactorilywhen served by venting systems having positive available draft ifthe appliance flue gas discharge pressure plus positive draft is suf-ficient to convey all flue gas from the appliance flue gas outlet tothe outdoors. If pos
22、itive available draft and/or appliance flue gasdischarge pressure is insufficient to convey all flue gas from theappliance flue gas outlet outdoors, incomplete combustion,flame rollout, and/or flue gas spillage can occur at the appliance.Zero available draft (neutral vent pressure); Da= 0. CategoryI
23、 fan-assisted and draft hood-equipped appliances and CategoryII appliances can operate satisfactorily when served by ventingsystems having zero available draft (neutral draft) at the appli-ance flue gas outlet, if the venting system creates sufficient the-oretical draft to convey all flue gas from t
24、he appliance flue gasoutlet to the outdoors. If the venting system creates insufficienttheoretical draft to convey all flue gas from the appliance fluegas outlet to the outdoors, incomplete combustion, flame rollout,and/or flue gas spillage can occur at the appliance. Category IIIand IV appliances c
25、an operate satisfactorily when served byventing systems having zero available draft, if appliance fluegas discharge pressure is great enough to overcome the ventflow pressure-drop loss.Negative available draft (positive vent pressure); Dais nega-tive. Category I and II appliances cannot operate sati
26、sfactorilywhen served by venting systems having negative available draft.Category III and IV appliances can operate satisfactorily whenserved by venting systems having negative draft, if appliance fluegas discharge pressure plus the theoretical draft created by theventing system is sufficient to ove
27、rcome the vent negative draft atthe vent inlet and the vent flow pressure-drop loss.If chimney height and flue gas temperatures provide surplusavailable draft Da(excessive excess air), draft control is required.Theoretical draft Dtis the natural draft produced by thebuoyancy of hot gases in the chim
28、ney relative to cooler gases inthe surrounding atmosphere. It depends on chimney height, localbarometric pressure, and the mean chimney flue gas temperaturedifference tm, which is the difference in temperature between theflue gas and atmospheric gases. Therefore, cooling by heat transferthrough the
29、chimney wall is a key variable in chimney design. Pre-cise evaluation of theoretical draft is not necessary for most designcalculations because of the availability of chimney design charts,computer programs, capacity tables in the references, buildingcodes, and vent and appliance manufacturers data
30、sheets.Chimney temperatures and acceptable combustible materialtemperatures must be known in order to determine safe clearancesbetween the chimney and combustible materials. Safe clearancesfor some chimney systems, such as type B gas vents, are determinedby standard tests and/or specified in buildin
31、g codes.Losses from Flow p represent the friction losses imposed onthe flue gas by flow resistance through the chimney.Depressurization Dpis negative pressure in the space surround-ing the appliance with respect to the atmosphere into which thechimney discharges. Dpcan be caused by other appliances
32、and fansoperating in the building that remove or add air to the space sur-rounding the appliance, by building stack effect, by outdoor atmo-spheric effects such as wind impacting the chimney exit or the sideof the building facing or leeward to the wind, and other buildingphenomena.Boost Dbis the pre
33、ssure boost from a mechanical-draft fan. Achimney with an forced-draft fan at the inlet of the chimney wouldhave positive boost (increased static pressure). A chimney with aninduced-draft fan at the outlet of the chimney would have negativeboost (decreased static pressure).The following sections cov
34、er the basis of chimney design foraverage steady-state category I and III appliance operating condi-tions. For other appliance operating conditions, a rigorous cyclicevaluation of the flue gas and material surface temperatures in thechimney vent system can be obtained using the VENT-II computerprogr
35、am (Rutz and Paul 1991). For oil-fired appliances, chimneyflue gas and material surface temperature evaluations can be ob-tained using the OHVAP computer program (Krajewski 1996).CHIMNEY FUNCTIONSThe proper chimney can be selected by evaluating factors such asdraft, configuration, size, and operatin
36、g conditions of the appliance;construction of surroundings; appliance usage classification; resi-dential, low, medium, or high heat (NFPA Standard 211); and build-ing height. The chimney designer should know the applicable codesand standards to ensure acceptable construction.In addition to chimney d
37、raft, the following factors must be con-sidered for safe and reliable operation: adequate air supply for com-bustion; building depressurization effects; draft control devices;chimney materials (corrosion and temperature resistance); flue gastemperatures, composition, and dew point; wind eddy zones;
38、and par-ticulate dispersion. Chimney materials must resist oxidation and con-densation at both high and low fire levels at all design temperatures.Start-UpThe equations and design charts in this chapter may be used todetermine vent or chimney size for average category I and III ventsystem operating
39、conditions based on steady-state operating con-ditions. The equations and charts do not consider modulation,cycling, or time to achieve equilibrium flow conditions from a coldstart. Whereas mechanical draft systems can start gas flow, gravitysystems rely on the buoyancy of hot flue gases as the sole
40、 force todisplace the cold air in the chimney. Priming follows Newtonslaws of motion. The time to fill a system with hot flue gases, dis-place the cold air, and start flow is reasonably predictable and isusually a minute or less; however, unfavorable thermal differen-tials, building/chimney interact
41、ion, mechanical equipment (e.g.,exhaust fans), or wind forces that oppose the normal flow of ventgases can overwhelm the buoyancy force. Then, rapid priming can-not be obtained solely from correct system design. The VENT-IIcomputer program contains detailed analysis of gas vent and chim-ney priming
42、and other cold-start considerations and allows forappliance cycling and pressure differentials that affect performance(Rutz and Paul 1991). A copy of the solution methodology (Rutz1991) for VENT-II, including equations, may be ordered fromASHRAE Customer Service.Air IntakesAll rooms or spaces contai
43、ning fuel-burning appliances musthave a constant supply of combustion air from outdoors (eitherdirectly or indirectly) at adequate static pressure to ensure propercombustion. In addition, air (either directly or indirectly) is requiredto replace the air entering chimney systems through draft hoods a
44、ndbarometric draft regulators and to ventilate closely confined boilerand furnace rooms.The U.S. National Fuel Gas Code (ANSI/NFPA 54/ANSI/AGAZ223.1) and the Canadian Natural Gas and Propane InstallationCode (CSA Standard B149.1), along with appliance manufacturers,provide requirements for air openi
45、ngs. Any design must considerChimney, Vent, and Fireplace Systems 35.3flow resistance of the air supply, including register-louver resistance,air duct resistance, and air inlet terminations. Compliance with thesecodes or the appliance manufacturers instructions accounts for theair supply flow resist
46、ance.Vent SizeSmall residential and commercial natural-draft gas appliancesneed vent diameters of 3 to 12 in. U.S. and Canadian codesrecommend sizes or input capacities for most acceptable gas appli-ance venting materials. These sizes also apply to gas applianceswith integral automatic vent dampers,
47、 as well as to appliances withfield-installed automatic vent dampers. Field-installed automaticvent dampers should be listed for use with a specific appliance by arecognized testing agency and installed by qualified installers.Draft ControlPressure, temperature, and other draft controls have replace
48、ddraft hoods in many residential furnaces and boilers to attain highersteady-state and seasonal efficiencies. Appliances that use pulsecombustion or forced- or induced-draft fans, as well as thosedesigned for sealed combustion or direct venting, do not have drafthoods but may require special venting
49、 and special vent terminals. Iffan-assisted burners deliver fuel and air to the combustion chamberand also overcome the appliance flow resistance, draft hoods orother control devices may be installed, depending on the design ofthe appliance. Vent category II, III, and IV and some category Iappliances do not use draft hoods; in such cases, the listed appliancemanufacturers vent system design requirements should be fol-lowed. The section on Vent and Chimney Accessories has informa-tion on draft hoods, barometric regulators, draft fans, and other draftcontrol devices.Frequentl
