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Collapsing metalbestos insulated chimney (C) Daniel FriedmanMulti-Appliance BTUH Venting Capacity of Masonry Chimneys
Chimney Sizing Table C-9-F
Chimney/Vent Sizing Tables & Formulas

Chimney & flue sizing tables by type, BTUH, building codes for Multi-Appliances venting through masonry chimneys where a single-wall metal vent connector is used between the appliances and the chimney.

We include excerpts from UMC tables along with additional commentary to assist in sizing metal or masonry chimneys according to chimney diameter, height, materials, and the input BTUH of the appliance(s) to be vented. This article series describes the size requirements for chimney flues for different categories of heating appliances.

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Sizing of Metal & Masonry Chimney Flues, Flues, Vents Table C-9-F

Table C-9-F (Excerpts) Multi-Appliance BTUH Venting Capacity of Masonry Chimneys where a single-wall metal vent connector is used between the appliances and the chimney

This two-part table gives the chimney specifications for the total input BTUH of all of the appliances combined and joined into a single vent when the flue is a masonry chimney and when the flue vent connector (between the appliance and the chimney entry point) is a single wall metal component.

Multiple-Appliance Single-Wall Flue Vent Connector Capacity
Total Vent [Chimney] Height (feet)
Flue Vent Connector Total Rise (feet)
Flue Vent Connector Diameter in Inches
3 in.
4 in
5 in
6 in
7 in
8 in 
Maximum [Individual] Appliance Input BTUH Rating (Thousands of BTUs)
6 to 8 Ft
1 ft
21
   
100
  200
2 ft
28
   
123
 
231
3 ft
34
   
142
 
269
             
15 ft
1 ft
23
   
112
 
225
2 ft
30
   
132
 
256
3 ft
34
   
151
 
289
             
30 ft & up
1 ft
24
   
124
 
250
2 ft
31
   
142
 
282
3 ft
35
   
160
 
312
             

Watch out: Check with your local building code officials for local building code requirements when designing, building, or installing chimneys and vents.

Note: also see CHIMNEY FLUE SIZE SPECIFICATIONS TABLE 504.2 for a masonry chimney height, flue size, and venting capacity in input BTUH that considers the effects of outdoor temperature

Multiple-Appliance Common Vent [Chimney] Capacity - Type B-Vent 
Total Vent [Chimney] Height (feet)
Minimum Inernal Area [horizontal cross section] of the Chimney Flue
(square inches)
18 in
28 in
38 in
50 in
78 in 
113 in
Maximum Combined Input BTUH (thousands) for All of the Appliances
6
45
71
102
142
245
8
52
81
118
162
277
405
10
56
89
129
175
300
450
15
66
105
150
210
360
540
20
74
120
170
240
415
640
30
135
195
275
490
740
50
325
600
910

Notes to th3 Table Above

For accurate chimney and vent connector sizing additional factors are considered that are not shown in these table excerpts. For example using the Seattle WA Code, Table 504.2(6) Exterior Masonry Chimney data for venting a single appliance, the table is divided into different temperature ranges, assuming that the outdoor temperature affects the chimney draft, and where warmer temperatures produce weaker draft.

Also not all "square inches" of chimney cross section vent area are equal - where round cross sections give different venting perfomance than rectangular cross-sectional areas of the same total square inches. See CHIMNEY CROSS SECTION SHAPE EFFECTS for details.

Guide to Chimney Sizing Tables for Venting Heating Appliances: Which Chimney Sizing Table to Use

Here is a quick guide to the UMC's chimney sizing & BTUH venting capacity tables when sizing a metal or masonry chimney diameter to match the total input BTUH of the heating appliances (boiler, furnace, water heater) being vented. Note that different chimney sizes are required for venting fireplaces and woodstoves.

Masonry fireplace chimney size requirements are given separately at FIREPLACES & HEARTHS.

Chimney Type
Number of Appliances to be Vented

Metal B-Vent Chimney (Vent) 
Type of Flue Vent Connector:1

Single Appliance
Multiple Appliances
Which Chimney Flue / Vent Sizing Table to Use
Single Wall Flue Vent Connector

Table C-9-E
(two part table)

Double Wall B-vent Flue Vent Connector1
Table C-9-D
(two part table)
Masonry Chimney2    
Single Wall Flue Vent Connector
Table C-9-F
(two part table)
Minimum Input BTUH, Single appliance, natural gas, B-vent connector vs chimney height, size, & outdoor temperature  

Notes to the Table Above

1. A flue vent connector is the horizontal/sloped metal pipe connecting the heating appliance to the entry point of the chimney. Flue vent connectors and their materials, sizing requirements, & fire clearances are at FLUE VENT CONNECTORS, HEATING EQUIPMENT

At double-wall B-vent chimneys the flue vent connector between the appliance and the chimney entry point may be single-wall metal flue vent connector or it may be a double-wall Type-B flue vent material.

2. At most masonry chimneys the flue vent connector or "stackpipe" or "smokepipe" is single wall metal, though for fire clearance needs it may be amended to use sections of insulated metal chimneys.

See CHIMNEY CROSS SECTION SHAPE EFFECTS for the effects of square vs round chimney sectional areas

Types of metal chimneys and their properties and fire clearances are at METAL CHIMNEYS & FLUES.


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