Collapsing metalbestos insulated chimney (C) Daniel Friedman Chimney/Vent Sizing Tables & Formulas
     

  • FLUE SIZE SPECIFICATIONS - CONTENTS: Table of B-vent sizes vs. BTU input maximum ratings. Size specification requirements for chimney flues and vents - diameter, height, etc .Flue diameter and height requirements for Category I Draft Hood and Fan Assisted Appliances. "Smoke pipe" diameter sizes vs. BTUs or BTU venting requirements. Chimney codes. Chimney flue size requirements for fireplaces, boilers, furnaces, water heaters, coal stoves
  • POST a QUESTION or READ FAQs about size requirements for chimney flues
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Chimney & flue sizing tables by type, BTUH, building codes, and other factor: this article describes the size requirements for chimney flues for different categories of heating appliances.

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.

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Sizing of Metal & Masonry Chimney Flues, Flues, Vents - Category I Draft Hood and Fan Assisted Appliances

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

NFPA 1992 (7.5.3) states that draft hood appliances, Category I appliances and other appliances that use Type B vents must have a

chimney flue area not less than the area of the largest vent outlet plus 50% of the area of the additional flue outlet.

The 1992 GAMA vent sizing tables for single-wall metal vent connectors attached to a tile lined masonry chimney uses Table 8. The result of those calculation using a 38,000 BTU water heater with a 3" draft hood and a 37,500 BTU boiler with a 4" draft hood connected to a 20' high chimney is to use a common flue with an area of 28 square inches or a 6" flue vs a 5" flue in the previous examples.

Details about factors that go into calculating the size of a chimney or flue are discussed separately at CHIMNEY SIZE FACTORS.

Tables of Chimney, Flue, or Vent Size Requirements vs. BTUs of Appliances being Vented

A 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 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:

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.

Table C-9-A (Excerpts): Single Appliance Venting, B-Vent Chimney Capacities for Venting a Single Heating Appliance where a Single Wall Metal Flue Vent Connector is Used Between the Appliance and the Chimney

BTU Color Coding Note: in the tables below we use colors to indicate where an example-approximate appliance BTUH vent capacity rating falls across various chimney parameter combinations. The Gray color indicates that this chimney height and diameter combination cannot be used.

Total Chimney/Vent Height (ft)
Lateral (horizontal) Run (ft)
Table of B-Vent Diameter (inches) for a Single Appliance Chimney
3 in.
4 in.
5 in.
6 in.
7 in.
8 in.
Comments
Maximum (single) Appliance BTU Input Rating (thousands of BTUs)
6 ft.
0
39
70
116
170
232
312
 
2
31
55
94
141
194
260
 
5
28
51
88
128
177
242
 
8 ft
0
42
76
126
185
252
340
 
2
32
61
102
154
210
284
 
5
29
56
95
141
194
264
 
10
49
86
131
180
250
 
10 ft
0
45
84
138
202
279
372
 
2
35
67
111
168
233
311
 
5
32
61
104
153
215
289
 
10
54
94
143
200
274
 
15
84
130
186
258
 
15 ft.
0
49
91
151
223
312
420
 
2
39
72
122
186
260
350
 
5
67
110
170
240
325
 
10
103
158
223
308
 
15
144
207
291
 
20
195
273
 
20 ft.
101
163
252
342
470
770
 
80
136
210
286
392
641
 
123
192
264
364
610
 
178
246
345
571
 
163
228
326
550
 
306
525
 
30 ft.
183
276
384
529
878
 
230
320
441
730
 
210
296
410
694
 
274
388
656
 
366
625
 
344
596
 
540
 

Table C-9-B (Excerpts): Single Appliance Venting, B-Vent Chimney Capacities for Venting a Single Heating Appliance where a Double-Wall B-Vent Flue Vent Connector is Used Between the Appliance and the Chimney

Total Chimney/Vent Height (ft)
Lateral (horizontal) Run (ft)
Table of B-Vent Diameter (inches) for a Single Appliance Chimney
3 in.
4 in.
5 in.
6 in.
7 in.
8 in.
Comments
Maximum (single) Appliance BTU Input Rating (thousands of BTUs)
6 ft.
0
46
86
141
205
285
370
 
2
36
67
105
157
217
285
 
6
32
61
100
149
205
273
12
28
55
91
137
190
255
 
               
8 ft
0
50
94
155
235
320
415
 
2
40
75
120
180
247
322
 
8
35
66
109
165
227
303
 
16
28
58
96
148
206
281
 
               
10 ft
0
53
100
166
255
345
450
 
2
42
81
129
195
273
355
 
10
36
70
115
175
245
330
 
20
60
100
154
217
300
 
               
15 ft.
0
58
112
187
285
390
525
 
2
48
93
150
225
316
414
 
15
37
76
128
198
275
373
 
30
60
107
169
243
328
 
               
20 ft.
0
61
119
202
307
430
575
 
2
51
100
166
249
346
470
 
10
44
89
150
228
321
443
 
20
35
78
134
206
295
410
 
30
68
120
186
273
380
 
               
30 ft.
0
64
128
220
336
475
650
 
2
56
112
185
280
394
535
 
20
90
154
237
343
473
 
40
200
298
415
 

Table C-9-C (Excerpts): Single Appliance Venting, BTU Capacity of Masonry Chimneys where a Single Wall Metal Flue Vent Connector is Used Between the Appliance and the Chimney

Total Chimney/Vent Height (ft)
Lateral (horizontal) Run (ft)
Table of B-Vent Diameter (inches) for a Single Appliance Chimney
3 in.
4 in.
5 in.
6 in.
7 in.
8 in.
Comments
Maximum (single) Appliance BTU Input Rating (thousands of BTUs)
6 ft.
2
28
52
86
130
180
247
 
5
48
81
118
164
230
 
               
8 ft
2
29
55
93
145
197
265
 
5
51
87
133
182
246
 
10
79
123
169
233
 
               
10 ft
2
31
61
102
161
220
297
 
5
56
95
147
203
276
 
10
86
137
189
261
 
15
125
175
246
 
               
15 ft.
2
67
113
178
249
335
 
5
61
106
163
230
312
 
10
96
151
214
294
 
15
138
198
278
 
20
184
261
 
               
20 ft.
2
73
123
200
273
374
 
5
115
183
252
348
 
10
170
235
330
 
15
156
217
311
 
20
202
292
 
               
30 ft.
2
136
215
302
420
 
5
196
279
391
 
10
260
370
 
15
349
 
20
327
 
               

Table C-9-D (Excerpts): Multiple Appliance BTU Vent Capacity of Type B Double-Wall Vents where a Double Wall Metal B-Flue Vent Connector is Used Between the Appliance and the Chimney

Watch out: the first portion of Table C-9-D specifies the maximum input BTUH capacity of any individual heating appliance that is to be vented along with other appliances out through a single chimney. The continuation of C-9-D below gives the chimney specifications for the total input BTUH of all of the appliances, combined.

Total Chimney/Vent Height (ft)
Connector Rise (ft) over its run
Table of B-Vent Diameter (inches) for a Multiple Appliance Chimney
3 in.
4 in.
5 in.
6 in.
7 in.
8 in.
Comments

Maximum [Individual] Appliance BTU Input Rating (thousands of BTUs)

6 ft.
1
26
46
72
104
142
185
 
2
31
55
86
124
168
220
 
3
35
62
96
139
189
248
 
               
8 ft
1
27
48
76
109
148
194
 
2
32
57
90
129
175
230
 
3
36
64
101
145
198
258
 
               
10 ft
1
28
50
78
113
154
200
 
2
33
59
93
134
182
238
 
3
37
67
104
150
205
268
 
               
15 ft.
1
30
53
83
120
163
214
 
2
35
63
99
142
193
253
 
3
40
71
111
160
218
286
 
               
20 ft.
1
31
56
87
125
171
224
 
2
37
66
104
149
202
265
 
3
42
74
116
168
228
300
 
               
30 ft.
1
33
59
93
134
182
238
 
2
39
70
110
158
215
282
 
3
44
79
124
178
242
317
 
               
               

Table C-9-D (Excerpts Continued): Multi-Appliance BTUH Venting Capacity of Type-B Double-Wall Vents [Chimneys] with Type-B flue vent connectors

This 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 Type-B double wall metal chimney and when the flue vent connector (between the appliance and the chimney entry point) is also a Type-B double wall unit.

Common [2 or more heating appliance] Vent Capacity in BTUH
Total Vent [Chimney] Height (feet)
Diameter (inches) of the Commmon Vent [Chimney]
             
3
4 in
5 in
6 in
7 in
8 in
10 in
6
65
103
147
200
260
410
8
73
114
163
223
290
465
10
79
124
178
242
315
495
15
91
144
206
280
365
565
20
102
160
229
310
405
640
30
118
185
266
360
470
440
40
131
203
295
405
525
820
             

Table C-9-E (Excerpts) Multi-Appliance BTUH Venting Capacity of Double-Wall B-Vent 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 Type-B double wall metal 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
   
102
 
205
2 ft
28
   
124
 
235
3 ft
34
   
147
 
275
             
15 ft
1 ft
23
   
117
 
240
2 ft
30
   
134
 
265
3 ft
35
   
155
 
298
             
30 ft & up
1 ft
25
   
129
 
270
2 ft
31
   
145
 
295
3 ft
36
   
164
 
321
             
Multiple-Appliance Common Vent [Chimney] Capacity - Type B-Vent
Total Vent [Chimney] Height (feet)
Diameter (inches) of the Commmon Vent [Chimney]
4 in
5 in
6 in
7 in
8 in
10 in
12 in
Maximum Combined Input BTUH (thousands) for All of the Appliances
6
48
78
111
155
205
320
8
55
89
128
175
234
365
505
10
59
95
136
190
250
395
560
15
71
115
168
228
305
480
690
20
80
129
186
260
340
550
790
30
147
215
300
40
650
940
50
360
490
810
1190
               

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
             
Note: also see Table 504.2(6) 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:

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.

  • "Chapter 5, Chimneys and Vents, Section 501 (IFGC)", Seattle building code, retrived 4/2/2014, original source http://www.seattle.gov/dpd/cs/groups/pan/@pan/documents/web_informational/s047809.pdf See table 504.2(6) Exterior Masonry Chimney for details.

 

Table 504.2(6) Exterior Masonry Chimneys Venting Natural Gas Appliances

Table of MINIMUM Allowable Input BTUH for Heating Appliances for Masonry Chimneys by Height, Size, & Outdoor Temperature

Chimney Vent
Height (Feet)
Square Inches of Internal Vent Area Inside the Chimney 1
28
sq. in.
38
sq. in.
50
sq. in.
63
sq. in.
78
sq. in.
113
sq. in.
Outdoor Temperature: 37°F or Greater - Local 99% Winter Design Temperature of 37°F or Higher
20 ft. high 123 KBTUH 190 KBTUH 249 KBTUH 184 KBTUH    
30 Note 2     393 334  
50           579 K BTUH

Outdoor Temperature: 27-36°F

15 Note 2   233 253 274 305
20       307 330 362
30       419 445 485
Outdoor Temperature: 17-26°F
15 Note 2     296 331 398
20 Note 3     352 387 457
30 Note 4       507 581
Outdoor Temperature: 5-16°F
10       289 331 430
15         393 485
20         450 547
30           682
Outdoor Temperature: -10 - 4°F
15 Note 2         586
20           650
30           805

Notes:

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

2. Blank areas in the table indicate not allowed, not acceptable, or not applicable combination of measurements & conditions.

3. Not all allowable height and size combinations are given in this table. E.G. we for most examples we omit chimneys under 15 ft. in height or over 30 ft. in height while in fact there are allowable combinations for some lower and taller chimney heights. See the original source below - and from which this table is excerpted.

4. The reason for expressing a MINIMUM input BTUH for these cases is the concern about the ability of the gas fird appliance to establish an updraft: this is about outdoor chimney temperatures vs. working chimney height: while warm conditions can make the establishment of draft more difficult, there is a second concern that in cold conditions it is more difficult to get draft working in a taller exterior masonry chimney since the flue gas from the appliance has to overcome the weight of a tall column of cold air.

Source: "Chapter 5, Chimneys and Vents, Section 501 (IFGC)", Seattle building code, retrived 4/2/2014, original source http://www.seattle.gov/dpd/cs/groups/pan/@pan/documents/web_informational/s047809.pdf See table 504.2(6) Exterior Masonry Chimney for details.

Details About Chimney Cross Sectional Areas and Effects on Venting Capacity of Masonry Chimneys - Translating Rectangular Chimney Flues to Round Flue Equivalents

The UMC Appendix C Table C9-G, "Masonry Chimney Liner Dimensions with Circular Equivalents" translates square or rectangular chimney flue inside dimensions into an equivalent "round" flue diameter and into equivalent square inches of venting capacity.

Why does this translation matter? The venting capacity of a rectangular or square masonry flue, if translated directly into actual or measured square inches of cross section, is less than a round chimney flue of the same square inches of venting capacity.

For example an 8" x 8" square nominal clay chimney flue tile liner has inside dimensions of 6 3/4" x 6 3/4". Just muliplying these dimensions together (6.75 x 6.75) = 45.56 sq.in. But chimney experts and draft measurements show that in a rectangular flue not all of the cross-sectional area is equally effective in developing draft. The effective square inches of venting area, translated to an equivalent round flue, would be less - or aboutd 42.7 square inches.

So when calculating the venting capacity of a square or rectangular clay chimney flue tile or liner by looking at its cross section, we use the UMC's Table C9-G of "round chimney flue equivalents" to get the effective chimney flue vent cross sectional area before we enter the masonry chimney BTUH venting capacity tables 6-9-C (single wall flue vent connectors and single heating appliances) or 6-9-F (single wall flue vent connectors and multiple appliances).

[Note: pending clarification, we have not provided excerpts of Table C-9-G because, frankly, some of its data is confusing. - Ed.]

 

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