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Collapsing metalbestos insulated chimney (C) Daniel FriedmanB-Vent Chimney Capacities

Venting a Single Heating Appliance Table C-9-B
Masonry Chimney Venting Capacities
Venting a Single Appliance, BTU Capacity Table C-9-C

Chimney & flue sizing tables for venting a single appliance through a B-vent and for venting a single appliance through a masonry chimney.

These two tables give required flue sizes vs input BTUH for heating appliances vented through a metal B-vent chimney (Table C-9-B) or through a masonry chimney (Table C-9-C).

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 Tables C-9-B & C-9-C

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.
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
             
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

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

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.
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
  
             

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 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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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: chimney size vs. btuh ratings

(Aug 30, 2011) david said:
what if you had a 4" furnace outlet and 2 3" outlets. is it the same formula or do you go off of actual btus

Reply:

David I don't quite understand the question. But in general the chimney venting requirements are set by the fuel type and the BTU rating of the appliance.

Question: standard chimney heights

(Sept 9, 2011) Graham said:
What is the standard height above a tiled roof to the top of chimney ?

Reply:

Graham please see CHIMNEY HEIGHT & CLEARANCE CODE

Question: reduce furnace flue vent connector size below furnace outlet size?

(Oct 13, 2012) Brent said:
I have a single 2650 sq ft home that was once subdivided, one side had a 133,000 btu oil boiler and I have a 115,000 gas boiler. I want to purchase a new boiler to heat both sides which I have zoned.

My question is that my looad calculation calls for a 136,000 btu furnace, in this range I'm looking at a 142,000 furnace.

My chimney (30 ft tall) has a stainless 6 " liner but the new furnace outlet is 7"

Can I safely reduce the outlet from 7" to 6" ? I also have a gas hot water heater on the same flue in a "y" connection.

Brent

Reply:

No

Question: ok to share chimney flue if heaters don't run at the same time?

(Jan 27, 2014) (mod) said:
Bill, I am re-posting your question without the imbedded hyperlink - our system doesn't permit links in comments for reasons of security.

"My coal boiler and oil boiler vent into the same chimney. I never use both at the same time. The chimney is about 30 feet high. The stove pipes are 6" single wall. The coal boiler enters below the oil boiler and on the opposite side of the chimney. There is no rise to the coal boiler's stove pipe, it enters straight into the chimney and runs slightly down hill. I'm burning bituminous coal get smoke billowing out the door when I open it even though I have lots of draft. I'm thinking it may be due to the way it has been piped causing a limitation in flow.
It looks to me like the coal boiler's input was changed when the 6" square clay liner was installed. There is a knock out cemented over that I can use, I would just have to cut an opening into the clay liner. The coal boiler would then enter about 6" above the oil boiler. I don't have any room to go higher. This would put about 16" of rise on the coal boiler's smoke pipe. I stuck 2 old elbows in to prove to myself that I have room to rearrange the pipe, you can see them in the pic.
Do you think this alternate piping might make an improvement? "

Before thinking about sharing a chimney flue - generally prohibited as a safety concern but permitted in some jurisdictions, particularly with gas and oil fuels provided proper venting arrangement - I'd check with your local building department about what they permit. Permission may also depend on the chimney type and design.

Safety issues aside for a moment there can be draft control and operating difficulties - any solution has to prevent automatic draft from one appliance from fouling up the other.

Reply:

You might find draft conflicts (can't optimize for both appliances), fire hazards (one heater igniting deposits from the other), code violations, and fire spread safety hazards.

Question: shared Y connectionfor chimney connection?

(Apr 2, 2014) sal avanzato said:
my chimeney [30ft tall] has8"x13"terracotta flue liner with 7" round terracotta crock going into the flue liner. can I safely hook up gas boiler 175000 btu and water heater 75000 btu into 7" round crock on the same flue in a"Y"connection

Reply:

Sal,

You have 175,000 + 75,000 BTUH = 250,000 BTUH venting required

You don't give the horizontal distances of your 7" flue vent connector(s) between the appliances and the chimney, nor do you say if the appliances are natural draft or forced draft, so we don't have the total answer to your question, nor am I confident commenting on "safety" for an unknown system since a mistake can be fatal.

At the start of the article above we give some help choosing which tables to use to answer chimney questions.

If we are venting multiple appliances into a single chimney flue we use Table C-9-F
(two part table) in the article above.

Your chimney cross section area is 8 x 13" = 104 sq.in. Your chimney height is 30 ft. (which gives good draft).

But an important limit here is the input 7" flue diameter. And some jurisdictions (see Seattle WA code for example) limit the horizontal run distance (to about 10 ft. for 7" flue vent connectors OR you have to reduce the assumed venting capacity of the system)

There are also assumptions about outdoor temperatures that affect the chimney draft conditions.

Not shown anyway in our table (I'll add some data) is that per the AGA NFPA code, if I assume the horizontal flue vent connector distance is 10 feet or less and the chimney height is 30 feet,

assuming an outdoor temperature range of 27 to 36 degF or higher (colder ranges give better draft once the chimney has heated-up so this is a cautious assumption) your 104 sq. in. chimney (assuming that's *internal* area, falls between two standards:

a 78 sq. in. 30 foot masonry chimney in this outdoor temperature range can vent 445,000 input BTUH
a 113 sq. in. 30 foot masonry chimney in this temperature range can vent 485,000 input BTUH.

So as your chimney is in between those two and even the smaller internal size chimney can vent more BTUH than you are connecting the chimney itself meets the standards as I read them. (Check with your local building code officials for safety and local code compliance and of course check that the chimney is in fact intact, not leaking, damaged, blocked, that there is safe combustion air etc. or ALL of this discussion would be nonsense since there could be very unsafe conditions, even fatal ones, present).

Adding about outdoor chimney temperatures vs. working chimney height, 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.

Question: moving a wood heater insert into a shop

13 Jan 2015 Steve Said:

I took a 20 year old wood heater insert out of my fire place. Im putting it in my shop.The heater does not have a pipe collar .It has a 7 x 8 inch opening where i can put a collar.I dont know what size to put 6 inch or 8 inch? Inside measurements of heater are 24 inch wide by 19 inch deep by 18 inch height 24x19x18 inside

Height from proposed collar to ceiling 13feet 4inches through metal roof and insulation 2inches roof has a 1 by 12 pitch.. What size stove pipe do i use and why

Reply:

Steve,

Generally a larger vent is functional and safe as long as the chimney size and height are not so large that the heating appliance won't develop adequate draft. It's not just stove pipe size you need to consider, but proper fire-rating of the flue or chimney materials, clearances from combustibles, cleanout access and code.

Watch out: But keep in mind that most likely you need to obtain a building permit and inspections for installing any new heater anywhere - don't view that as a stumbling block but rather as an extra inspection that might help avoid a fire.


...

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