Chimney Size Factors
What factors go into tables of chimney & flue sizes, diameters, heights, & BTUH capacities?
CHIMNEY SIZE FACTORS - CONTENTS: Factors that determine the required chimney/vent diameter for a given BTUH capacity. Why is a rectangular or square chimney flue clay tile liner translated into "round flue" equivalent square inches when determing chimney BTUH capacity? Chimney codes.
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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 More Reading at the end of this one. Page top sketch of masonry chimney flue separation is provided courtesy of Carson Dunlop Associates.
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
square root of the sum of the squares of the appliance draft hood
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
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
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 only on 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:
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).
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
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.
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]. 
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.
Input BTUH capacity: The maximum total BTUs of the appliance(s) that will be connected to the chimney
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.
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.
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.
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