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  • FLUE SIZE SPECIFICATIONS - CONTENTS: Tables of Metal & Masonry Chimney flue & vent sizes vs. BTU input maximum ratings, height, dimensions & other factors. Chimney 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
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Chimney & flue sizing tables by type, BTUH, building codes, and other factor:

This article series 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

See FLUE SIZE SPECIFICATIONS_TABLE_C_9_B

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

See FLUE SIZE SPECIFICATIONS_TABLE_C_9_C

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

See CHIMNEY FLUE SIZE SPECIFICATIONS_TABLE_C_9_D

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. An additional table, the Continuation of C-9-D gives the chimney specifications for the total input BTUH of all of the appliances, combined.

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.

See CHIMNEY FLUE SIZE SPECIFICATIONS_TABLE_C_9_E

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.

See CHIMNEY FLUE SIZE SPECIFICATIONS TABLE C-9-F

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

See CHIMNEY FLUE SIZE SPECIFICATIONS TABLE 504.2

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.]

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

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Continue reading at CHIMNEY FLUE SIZE SPECIFICATIONS_TABLE_C_9_B or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CHIMNEY FLUE SEPARATION REQUIREMENTS

Or see CHIMNEY SIZE FACTORS that go into calculating the size of a chimney or flue

Or see FIREPLACES & HEARTHS for masonry fireplace chimney size requirements

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