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Collapsing metalbestos insulated chimney (C) Daniel FriedmanMulti-Appliance BTUH Venting Capacity of Double-Wall B-Vent Chimneys where a single-wall metal vent connector is used
Table C-9-E Chimney/Vent Sizing Tables & Formulas

Chimney & flue size specifications when venting multiple heating appliances through a B-vent type metal chimney.

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 Table C-9-E

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
             

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

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
               

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.

 

References for Venting Multiple Appliances into a Common Flue or Chimney

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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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