Chimney size rules of thumb & sizing factors:

this article describes both some simple rules of thumb that used to be used for guesstimating the required chimney size in diameter, height etc. and continues with a list of the considerations taken by engineers who calculate the tables of chimney flue sizing requirements given in our separate article linked at Continue Reading at the end of this one. Page top sketch of masonry chimney flue separation is provided courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Chimney Sizing Rules of Thumb & Factors Affecting Proper Chimney & Vent Sizing

Before taking a look at the various metal and masonry chimney and vent sizing guidelines listed below, it is useful to understand that there are a number of critical factors affecting the permitted (that is safe and functional) chimney and vent sizes including at least those listed below.

Chimney Sizing Rules of Thumb

An old sheet metal workers rule of thumb was to size the chimney flue by taking the

square root of the sum of the squares of the appliance draft hood diameters

i.e.: a 3" plus a 4" appliance vent will require a 5" flue. ( 32 + 42 = 25. The square root of 25 = 5.)

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 following calculation works out to be the same answer as the "Rule of Thumb" sizing, i.e.: (4/22 x 3.14 = 12.56) + (3/22 x 3.14/2 = 3.53) = 16.09 / 3.14 = 5.1251/2 = 2.26 x 2 = 4.52" or 5".

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.

These tables also indicate that a 3" vent is not capable of venting the 38,000 BTU water heater.

A 4" would be required. Also the flow area of the chimney must not be more that 7 times the area of the smallest vent area. Therefore, 7 x 7.065 = 49.455 vs 28, a 6 inch flue pipe is ok. The maximum size flue that could be used is 8 inch at 50.3.

These Factors Affect the Required Chimney Size

As with most building inspection and safety topics, this material may be incomplete. Check with your local building code officials for local building code requirements when designing, building, or installing chimneys and vents.

Watch out: you cannot safely choose a chimney flue diameter based onlyOn the BTU input of the appliance(s) being vented. The required size or diameter of a metal chimney such as a B-vent depends on at least the following factors:

1. Chimney vent height: The total vent ("smoke pipe" or "chimney") height from the point of connection of the appliance (or its flue vent connector pipe) to the top of the chimney opening (not the top of the chimney cap).
2. Flue vent connector size: in a chimney& vent system in which the chimney is larger in diameter (or cross sectional area) than the flue vent connector that connects the heating appliance to the chimneyh flue, the smaller diameter flue vent connector limits the draft that can be produced by the chimney - it's acting as a restriction. With restricted input to the larger chimney flue condensation becomes a problem as may the establishment of draft.
   
   Rule of thumb: the cross-sectional flow area of the vent (the chimney interior) cannot be more than 7 x the flow are of the appliance outlet (where it connects to the flue vent connector) or of the appliance draft hood. - NFPA 54: National Fuel Gas Code Handbook (2012) 13.1.9
3. Flue vent connector horizontal run: The total lateral or horizontal distance that the vent runs before turning to its vertical rise. The longer the horizontal run the greater chimney height is needed to develop adequate draft. For that reason, in the table below you won't see any long horiontal run data for short chimneys. For example in our first chimney sizing table below, if a chimney's total rise is just 6 ft., then the longest horiztontal run that chimney can support is 5 ft.
   
   In some chimney sizing tables we consider not the horizontal run distance of the flue vent connector but its total rise over its run.
4. Flue vent connector type: The use of single-wall (TABLE C-9-A) vs. double-walll B-Vent (TABLE C-9-B) of horizontal flue vent connectors ("smoke pipe" between the appliance and its connection to the vertical chimney).
   
   The table below is for appliances connected through a single wall flue vent connector to a Type B Vent chimney. BTU capacities are increased if the flue vent connector is also a type B Double-wall system. [Table C-9-B in the same resource]. [10]
5. Fuel: different rules pertain to gas fired heating equipment venting than oil-fired or wood-fired equipment. Generally the higher operating temperatures of heating oil or other fuels (compared with natural gas or LP gas) means differences flue gas temperatures & thus in in chimney drafts produced.
6. Input BTUH capacity: The maximum total BTUs of the appliance(s) that will be connected to the chimney
7. The number of appliances being vented. The first table below (Table C-9-A) gives chimney sizing data for chimneys serving a single heating appliance. Do not Table C-9-A if multiple heating devices are to be connected to a single vent (chimney).
   
   When multiple heating appliances are connected to a single chimney, a different, more restricted table of chimney size requirements is used (TABLE C-9-D or TABLE C-9-E, depending on the flue vent connector material) and the total input BTUH capacity of a given flue diameter and height will be less than that where only a single appliance is used.
   
   An additional restriction applies when a vent (chimney) is venting multiple heating appliances: there is a maximum input BTUH specified not only for the total of all appliances to be connected, but also a maximum input BTUH for any of the individual appliances.
   
   So when selecting a chimney diameter for venting multiple heating appliances through a single vent, for any particular chimney type (Masonry vs. B-vent, for example) you will use two BTUH tables: a table of the maximum individual appliance BTUH and a table of the maximum combined appliances input BTUH.
8. The chimney type used. The first two tables illustrated below are for B-Vent double wall metal chimneys, generally used for gas fired heating appliances.
   
   Separate tables of chimney size requirements (TABLE C-9-C) is needed for masonry chimneys as these have different heating and draft properties. The cross sectional shape of the chimney flue (for example rectangular vs. round) in masonry chimneys also affects its BTU capacity. Insulated metal chimneys require still another table of BTU capacities.
9. Outdoor temperatures: colder outdoor temperatures particularly affect the draft or initial draft formed in exterior masonry chimneys. While warmer outdoor air and temperatures can have a negative effect on chimney draft, very cold outdoor temperatures can produce a tall column of cold air whose weight cannot be overcome by the heat produced by gas fired appliances. See Table 504.2(6) for details.

Generally, taller chimneys, larger diameter chimneys, and chimneys that are larger in diameter can support a greater total BTU input load. The UMC table includes data for chimneys up to 12" in diameter and 50 ft. in height but we have restricted our version to typical residential chimney diameters and heights.

Watch out: venting a small, low-BTUH appliance into a chimney of any type (masonry, metal, insulated metal, B-vent, etc) that is too large can be dangerous - the small appliance may be unable to heat the chimney to sufficient temperature to generate sufficient draft to vent the appliance safely.

We find this problem in older homes where a residential gas-fired water heater is the sole appliance vented into a large, tall masonry chimney. You will see this restriction illustrated by the blank squares in the lower left corner of the table below.

For typical residential heating appliance BTU capacities, the chimney sizing tables given below provides examples of B-Vent Chimney (or "smoke pipe") sizing for common single-appliance BTU capacity and is adapted from the 1991 Uniform Mechanical Code.[10]

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2018-11-20 - by (mod) -

The need for a chimney liner was going to depend upon a variety of factors such as change of use of fuel type, and of course the condition of the chimney itself. To make that determination you need to have such a chimney inspected by a certified Chimney Sweep. So I'm sorry but I can't give a simple yes or no answer. I just don't know enough about your situation.

On 2018-11-18 by jim

how do you know when a chimney liner is required when a new furnace is installed.

On 2018-01-07 - by (mod) -

Jack

Nowhere that I have been able to find.

But you will see smaller flue vent connectors such as for a water heater that are cojoined and vented into a larger flue sized, for example, for an oil fired heating boiler .

On 2018-01-07 by Jack

Where in the code does it state flue size can be downsized no more than one inch when venting one appliance?

On 2017-05-04 - by (mod) -

Peter:

CHeck with your local building department: most likely you'll need to see their requirements for firestops between floors as well as use of noncombustible or fire-rated materials.

On 2017-05-04 by Peter

Are there requirements regarding the drywall box built around chimneys on floors above the fireplace?
How much is the minimum distance between the chimney flue and the dry walls?
Does the inside of this drywall box need to be lined with a particular fire resistant material?
Thank you!

On 2016-12-09 - by (mod) -

Yes Jim, the flue vent connector between stove and chimney and the chimney diameter are important for draft and need to be matched to the stove. The flue vent connector needs to be same size (or one up) from the point at which it connects tothe stove.

In most locations you can get a free fire safety inspection from your fire department or from a certified chimney sweep (for fee). Be sure to do that, and to have working smoke detectors and a high-rated Class A fire extinguisher at a nearby exit door.

On 2016-12-09 by Jim

Woo D's stove about 12" HTand width ,about 18" in depth. About 6' from stove to safety pipe, about the same to the roof exit hole then 2' to vent cap. Question. Does the diameter the pipe matter as far as draft etc.

Question:

(Mar 9, 2015) Kevin said:
HI,
Can anyone tell me what size terra-cotta chimney I need in order to properly vent 273,000 BTUs. The total chimney is roughly 40 ft. tall, four stories counting the basement. I need to know ASAP. It look,s like a 8" sq. chimney would be sufficient.

Reply:

Kevin it depends.

See Table C-9-C as a good place to start. Height, number of appliances, and other variables may apply.

...

Continue reading at CHIMNEY FLUE SIZE SPECIFICATIONS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CHIMNEY DRAFT & PERFORMANCE
• CHIMNEY FLUE SEPARATION REQUIREMENTS
• CHIMNEY FLUE SIZE SPECIFICATIONS - home
• CHIMNEY FLUE SIZE SPECIFICATIONS TABLE 504.2 - Minimum Allowable Chimney Input BTUH for Heating Appliances
• CHIMNEY FLUE SIZE SPECIFICATIONS_TABLE_C_9_B - B-Vent Chimney Capacities Venting a Single Heating Appliance
• CHIMNEY FLUE SIZE SPECIFICATIONS_TABLES C9_C - Masonry Chimney Venting Capacities Venting a Single Appliance, BTU Capacity
• CHIMNEY FLUE SIZE SPECIFICATIONS_TABLE_C_9_E - Multi-Appliance BTUH Venting Capacity of Double-Wall B-Vent Chimneys
• CHIMNEY FLUE SIZE SPECIFICATIONS TABLE C-9-F - Multi-Appliance BTUH Venting Capacity of Masonry Chimneys
• CHIMNEY HEIGHT & CLEARANCE CODE
• CHIMNEY INSPECTION, FLUE INTERIOR
• CHIMNEY SIZE FACTORS that go into calculating the size of a chimney or flue
• FLUE TILE DAMAGE in CHIMNEY

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CHIMNEY SIZE FACTORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to CHIMNEYS & FLUES

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