This appendix is informative and is not part of the code. This appendix is an excerpt from the 2018 International Fuel Gas Code, coordinated with the section numbering of the International Residential Code.
User note:
About this Appendix: Appendix B provides commentary, guidance and examples for the design of venting systems for the types of appliances that vent by natural draft and have draft hoods or are listed as Category I or are listed for use with Type B vents.
An installer has a 120,000 British thermal unit (Btu) per hour input appliance with a 5inchdiameter draft hood outlet that needs to be vented into a 10foothigh Type B vent system. What size vent should be used assuming (a) a 5foot lateral singlewall metal vent connector is used with two 90degree elbows, or (b) a 5foot lateral singlewall metal vent connector is used with three 90degree elbows in the vent system?
Solution:
Table 504.2(2) should be used to solve this problem, because singlewall metal vent connectors are being used with a Type B vent.
 Read down the first column in Table 504.2(2) until the row associated with a 10foot height and 5foot lateral is found. Read across this row until a vent capacity greater than 120,000 Btu per hour is located in the shaded columns labeled "NAT Max" for drafthoodequipped appliances. In this case, a 5inch diameter vent has a capacity of 122,000 Btu per hour and can be used for this application.

If three 90degree elbows are used in the vent system, then the maximum vent capacity listed in the tables must be reduced by 10 percent (see Section 504.2.3 for single appliance vents). This implies that the 5inchdiameter vent has an adjusted capacity of only 110,000 Btu per hour. In this case, the vent system must be increased to 6 inches in diameter (see the following calculations).
122,000 (0.90) = 110,000 for 5inch vent From Table 504.2(2), Select 6inch vent 186,000 (0.90) = 167,000; This is greater than the required 120,000. Therefore, use a 6inch vent and connector where three elbows are used.
An installer has an 80,000 Btu per hour input fanassisted appliance that must be installed using 10 feet of lateral connector attached to a 30foothigh Type B vent. Two 90degree elbows are needed for the installation. Can a singlewall metal vent connector be used for this application?
Solution:
Table 504.2(2) refers to the use of singlewall metal vent connectors with Type B vent. In the first column find the row associated with a 30foot height and a 10foot lateral. Read across this row, looking at the FAN Min and FAN Max columns, to find that a 3inchdiameter singlewall metal vent connector is not recommended. Moving to the next larger size single wall connector (4 inches), note that a 4inchdiameter singlewall metal connector has a recommended minimum vent capacity of 91,000 Btu per hour and a recommended maximum vent capacity of 144,000 Btu per hour. The 80,000 Btu per hour fanassisted appliance is outside this range, so the conclusion is that a singlewall metal vent connector cannot be used to vent this appliance using 10 feet of lateral for the connector.
However, if the 80,000 Btu per hour input appliance could be moved to within 5 feet of the vertical vent, then a 4inch singlewall metal connector could be used to vent the appliance. Table 504.2(2) shows the acceptable range of vent capacities for a 4inch vent with 5 feet of lateral to be between 72,000 Btu per hour and 157,000 Btu per hour.
If the appliance cannot be moved closer to the vertical vent, then Type B vent could be used as the connector material. In this case, Table 504.2(1) shows that for a 30foothigh vent with 10 feet of lateral, the acceptable range of vent capacities for a 4inchdiameter vent attached to a fanassisted appliance is between 37,000 Btu per hour and 150,000 Btu per hour.
An installer has an 80,000 Btu per hour input appliance with a 4inchdiameter draft hood outlet that needs to be vented into a 12foothigh Type B vent. The vent connector has a 5foot lateral length and is also Type B. Can this appliance be vented using a 4inchdiameter vent?
Solution:
Table 504.2(1) is used in the case of an all Type B vent system. However, since there is no entry in Table 504.2(1) for a height of 12 feet, interpolation must be used. Read down the 4inch diameter NAT Max column to the row associated with 10foot height and 5foot lateral to find the capacity value of 77,000 Btu per hour. Read further down to the 15foot height, 5foot lateral row to find the capacity value of 87,000 Btu per hour. The difference between the 15foot height capacity value and the 10foot height capacity value is 10,000 Btu per hour. The capacity for a vent system with a 12foot height is equal to the capacity for a 10foot height plus ^{2}/_{5} of the difference between the 10foot and 15foot height values, or 77,000 + ^{2}/_{5} (10,000) = 81,000 Btu per hour. Therefore, a 4inchdiameter vent can be used in the installation.
A 35,000 Btu per hour water heater is to be common vented with a 150,000 Btu per hour furnace using a common vent with a total height of 30 feet. The connector rise is 2 feet for the water heater with a horizontal length of 4 feet. The connector rise for the furnace is 3 feet with a horizontal length of 8 feet. Assume singlewall metal connectors will be used with Type B vent. What size connectors and combined vent should be used in this installation?
Solution:
Table 504.3(2) should be used to size singlewall metal vent connectors attached to Type B vertical vents. In the vent connector capacity portion of Table 504.3(2), find the row associated with a 30foot vent height. For a 2foot rise on the vent connector for the water heater, read the shaded columns for drafthoodequipped appliances to find that a 3inchdiameter vent connector has a capacity of 37,000 Btu per hour. Therefore, a 3inch singlewall metal vent connector can be used with the water heater. For a drafthoodequipped furnace with a 3foot rise, read across the appropriate row to find that a 5inchdiameter vent connector has a maximum capacity of 120,000 Btu per hour (which is too small for the furnace) and a 6inchdiameter vent connector has a maximum vent capacity of 172,000 Btu per hour. Therefore, a 6inchdiameter vent connector should be used with the 150,000 Btu per hour furnace. Since both vent connector horizontal lengths are less than the maximum lengths listed in Section 504.3.2, the table values can be used without adjustments.
In the common vent capacity portion of Table 504.3(2), find the row associated with a 30foot vent height and read over to the NAT + NAT portion of the 6inchdiameter column to find a maximum combined capacity of 257,000 Btu per hour. Since the two appliances total only 185,000 Btu per hour, a 6inch common vent can be used.
In this case, a 35,000 Btu per hour input drafthoodequipped water heater with a 4inchdiameter draft hood outlet, 2 feet of connector rise, and 4 feet of horizontal length is to be common vented with a 100,000 Btu per hour fanassisted furnace with a 4inchdiameter flue collar, 3 feet of connector rise, and 6 feet of horizontal length. The common vent consists of a 30foot height of Type B vent. What are the recommended vent diameters for each connector and the common vent? The installer would like to use a singlewall metal vent connector.
Solution: [Table 504.3(2)].
Water Heater Vent Connector Diameter. Since the water heater vent connector horizontal length of 4 feet is less than the maximum value listed in Section 504.3.2, the venting table values can be used without adjustments. Using the Vent Connector Capacity portion of Table 504.3(2), read down the Total Vent Height (H) column to 30 feet and read across the 2foot Connector Rise (R) row to the first Btu per hour rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3inch vent connector has a maximum input rating of 37,000 Btu per hour. Although this is greater than the water heater input rating, a 3inch vent connector is prohibited by Section 504.3.21. A 4inch vent connector has a maximum input rating of not more than 67,000 Btu per hour and is equal to the draft hood outlet diameter. A 4inch vent connector is selected. Since the water heater is equipped with a draft hood, there are no minimum input rating restrictions.
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 504.3(2), read down the Total Vent Height (H) column to 30 feet and across the 3foot Connector Rise (R) row. Since the furnace has a fanassisted combustion system, find the first FAN Max column with a Btu per hour rating greater than the furnace input rating. The 4inch vent connector has a maximum input rating of 119,000 Btu per hour and a minimum input rating of 85,000 Btu per hour. The 100,000 Btu per hour furnace in this example falls within this range, so a 4inch connector is adequate. Since the furnace vent connector horizontal length of 6 feet does not exceed the maximum value listed in Section 504.3.2, the venting table values can be used without adjustment. If the furnace had an input rating of 80,000 Btu per hour, then a Type B vent connector [see Table 504.3(1)] would be needed in order to meet the minimum capacity limit.
Common Vent Diameter. The total input to the common vent is 135,000 Btu per hour. Using the Common Vent Capacity portion of Table 504.3(2), read down the Total Vent Height (H) column to 30 feet and across this row to find the smallest vent diameter in the FAN + NAT column that has a Btu per hour rating equal to or greater than 135,000 Btu per hour. The 4inch common vent has a capacity of 132,000 Btu per hour and the 5inch common vent has a capacity of 202,000 Btu per hour. Therefore, the 5inch common vent should be used in this example.
Summary. In this example, the installer can use a 4inchdiameter, singlewall metal vent connector for the water heater and a 4inchdiameter, singlewall metal vent connector for the furnace. The common vent should be a 5inchdiameter Type B vent.
In this case, the water heater and fanassisted furnace of Example 5a are to be common vented into a clay tilelined masonry chimney with a 30foot height. The chimney is not exposed to the outdoors below the roof line. The internal dimensions of the clay tile liner are nominally 8 inches by 12 inches. Assuming the same vent connector heights, laterals, and materials found in Example 5a, what are the recommended vent connector diameters, and is this an acceptable installation?
Solution:
Table 504.3(4) is used to size common venting installations involving singlewall connectors into masonry chimneys.
Water Heater Vent Connector Diameter. Using Table 504.3(4), Vent Connector Capacity, read down the Total Vent Height (H) column to 30 feet, and read across the 2foot Connector Rise (R) row to the first Btu per hour rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3inch vent connector has a maximum input of only 31,000 Btu per hour while a 4inch vent connector has a maximum input of 57,000 Btu per hour. A 4inch vent connector must therefore be used.
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 504.3(4), read down the Total Vent Height (H) column to 30 feet and across the 3foot Connector Rise (R) row. Since the furnace has a fanassisted combustion system, find the first FAN Max column with a Btu per hour rating greater than the furnace input rating. The 4inch vent connector has a maximum input rating of 127,000 Btu per hour and a minimum input rating of 95,000 Btu per hour. The 100,000 Btu per hour furnace in this example falls within this range, so a 4inch connector is adequate.
Masonry Chimney. From Table B1, the equivalent area for a nominal liner size of 8 inches by 12 inches is 63.6 square inches. Using Table 504.3(4), Common Vent Capacity, read down the FAN + NAT column under the Minimum Internal Area of Chimney value of 63 to the row for 30foot height to find a capacity value of 739,000 Btu per hour. The combined input rating of the furnace and water heater, 135,000 Btu per hour, is less than the table value, so this is an acceptable installation.
Section 504.3.17 requires the common vent area to be not greater than seven times the smallest listed appliance categorized vent area, flue collar area, or draft hood outlet area. Both appliances in this installation have 4inchdiameter outlets. From Table B1, the equivalent area for an inside diameter of 4 inches is 12.2 square inches. Seven times 12.2 equals 85.4, which is greater than 63.6, so this configuration is acceptable.
In this case, the water heater and fanassisted furnace of Examples 5a and 5b are to be common vented into an exterior masonry chimney. The chimney height, clay tile liner dimensions, and vent connector heights and laterals are the same as in Example 5b. This system is being installed in Charlotte, North Carolina. Does this exterior masonry chimney need to be relined? If so, what corrugated metallic liner size is recommended? What vent connector diameters are recommended?
Solution:
In accordance with Section 504.3.20, Type B vent connectors are required to be used with exterior masonry chimneys. Use Tables 504.3(7a), (7b) to size FAN+NAT common venting installations involving TypeB double wall connectors into exterior masonry chimneys.
The local 99percent winter design temperature needed to use Table 504.3(7b) can be found in the ASHRAE Handbook of Fundamentals. For Charlotte, North Carolina, this design temperature is 19°F.
Chimney Liner Requirement. As in Example 5b, use the 63 square inch Internal Area columns for this size clay tile liner. Read down the 63 square inch column of Table 504.3(7a) to the 30foot height row to find that the combined appliance maximum input is 747,000 Btu per hour. The combined input rating of the appliances in this installation, 135,000 Btu per hour, is less than the maximum value, so this criterion is satisfied. Table 504.3(7b), at a 19°F design temperature, and at the same vent height and internal area used above, shows that the minimum allowable input rating of a spaceheating appliance is 470,000 Btu per hour. The furnace input rating of 100,000 Btu per hour is less than this minimum value. So this criterion is not satisfied, and an alternative venting design needs to be used, such as a Type B vent shown in Example 5a or a listed chimney liner system shown in the remainder of the example.
In accordance with Section 504.3.19, Table 504.3(1) or 504.3(2) is used for sizing corrugated metallic liners in masonry chimneys, with the maximum common vent capacities reduced by 20 percent. This example will be continued assuming Type B vent connectors.
Water Heater Vent Connector Diameter. Using Table 504.3(1), Vent Connector Capacity, read down the Total Vent Height (H) column to 30 feet, and read across the 2foot Connector Rise (R) row to the first Btu/h rating in the NAT Max column that is equal to or greater than the water heater input rating. The table shows that a 3inch vent connector has a maximum capacity of 39,000 Btu/h. Although this rating is greater than the water heater input rating, a 3inch vent connector is prohibited by Section 504.3.21. A 4inch vent connector has a maximum input rating of 70,000 Btu/h and is equal to the draft hood outlet diameter. A 4inch vent connector is selected.
Furnace Vent Connector Diameter. Using Table 504.3(1), Vent Connector Capacity, read down the Vent Height (H) column to 30 feet, and read across the 3foot Connector Rise (R) row to the first Btu per hour rating in the FAN Max column that is equal to or greater than the furnace input rating. The 100,000 Btu per hour furnace in this example falls within this range, so a 4inch connector is adequate.
Chimney Liner Diameter. The total input to the common vent is 135,000 Btu per hour. Using the Common Vent Capacity Portion of Table 504.3(1), read down the Vent Height (H) column to 30 feet and across this row to find the smallest vent diameter in the FAN+NAT column that has a Btu per hour rating greater than 135,000 Btu per hour. The 4inch common vent has a capacity of 138,000 Btu per hour. Reducing the maximum capacity by 20 percent (Section 504.3.19) results in a maximum capacity for a 4inch corrugated liner of 110,000 Btu per hour, less than the total input of 135,000 Btu per hour. So a larger liner is needed. The 5inch common vent capacity listed in Table 504.3(1) is 210,000 Btu per hour, and after reducing by 20 percent is 168,000 Btu per hour. Therefore, a 5inch corrugated metal liner should be used in this example.
SingleWall Connectors. Once it has been established that relining the chimney is necessary, Type B doublewall vent connectors are not specifically required. This example could be redone using Table 504.3(2) for singlewall vent connectors. For this case, the vent connector and liner diameters would be the same as found above with Type B doublewall connectors.
Vent connector size depends on:

Common vent size depends on:

FIGURE B13
MULTISTORY GAS VENT DESIGN PROCEDURE FOR EACH SEGMENT OF SYSTEM
TABLE B1
MASONRY CHIMNEY LINER DIMENSIONS WITH CIRCULAR EQUIVALENTS^{a}
NOMINAL LINER SIZE (inches) 
INSIDE DIMENSIONS OF LINER (inches) 
INSIDE DIAMETER OR EQUIVALENT DIAMETER (inches) 
EQUIVALENT AREA (square inches) 
4 × 8  2^{1}/_{2} × 6^{1}/_{2}  4  12.2 
5  19.6  
6  28.3  
7  38.3  
8 × 8  6^{3}/_{4} × 6^{3}/_{4}  7.4  42.7 
8  50.3  
8 × 12  6^{1}/_{2} × 10^{1}/_{2}  9  63.6 
10  78.5  
12 × 12  9^{3}/_{4} × 9^{3}/_{4}  10.4  83.3 
11  95  
12 × 16  9^{1}/_{2} × 13^{1}/_{2}  11.8  107.5 
12  113.0  
14  153.9  
16 × 16  13^{1}/_{4} × 13^{1}/_{4}  14.5  162.9 
15  176.7  
16 × 20  13 × 17  16.2  206.1 
18  254.4  
20 × 20  16^{3}/_{4} × 16^{3}/_{4}  18.2  260.2 
20  314.1  
20 × 24  16^{1}/_{2} × 20^{1}/_{2}  20.1  314.2 
22 ×  380.1  
24 × 24  20^{1}/_{4} × 20^{1}/_{4}  22.1  380.1 
24  452.3  
24 × 28  20^{1}/_{4} × 20^{1}/_{4}  24.1  456.2 
28 × 28  24^{1}/_{4} × 24^{1}/_{4}  26.4  543.3 
27  572.5  
30 × 30  25^{1}/_{2} × 25^{1}/_{2}  27.9  607 
30  706.8  
30 × 36  25^{1}/_{2} × 31^{1}/_{2}  30.9  749.9 
33  855.3  
36 × 36  31^{1}/_{2} × 31^{1}/_{2}  34.4  929.4 
36  1017.9 
For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 m^{2}.
 Where liner sizes differ dimensionally from those shown in Table B1, equivalent diameters can be determined from published tables for square and rectangular ducts of equivalent carrying capacity or by other engineering methods.