UpCodes logo
Table of contentsContents

Appendix F Sizing of Venting Systems and Outdoor Combustion and Ventilation Opening Design

(The content of this Appendix is based on Annex F and Annex I of NFPA 54

F 101.0 General

F 101.1 Applicability

This appendix provides general guidelines for sizing venting systems serving appliances equipped with draft hoods, Category I appliances, and appliances listed for use with Type B vents.

F 101.2 Examples Using Single Appliance Venting Tables

See Figure F 101.2(1) through Figure F 101.2(14).
Table 803.1.2(1) is used where sizing a Type B double-wall gas vent connected directly to the appliance.
Note: The appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(1)
TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A TYPE B DOUBLE-WALL VENT
Table 803.1.2(2) is used where sizing a single-wall metal vent connector attached to a Type B double-wall gas vent.
Note: The appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(2)
TYPE B DOUBLE-WALL VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A SINGLE-WALL METAL VENT CONNECTOR
Table 803.1.2(3) is used where sizing a Type B double-wall gas vent connector attached to a tile-lined masonry chimney.
Notes:
  1. A is the equivalent cross-sectional area of the tile liner.
  2. The appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(3)
VENT SYSTEM SERVING A SINGLE APPLIANCE WITH A MASONRY CHIMNEY AND A TYPE B DOUBLE-WALL VENT CONNECTOR
Table 803.1.2(4) is used where sizing a single-wall vent connector attached to a tile-lined masonry chimney.
Notes:
  1. A is the equivalent cross-sectional area of the tile liner.
  2. The appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(4)
VENT SYSTEM SERVING A SINGLE APPLIANCE USING A MASONRY CHIMNEY AND A SINGLE-WALL METAL VENT CONNECTOR
Asbestos cement Type B or single-wall metal vent serving a single draft hood-equipped appliance. [See Table 803.1.2(5)]
FIGURE F 101.2(5)
ASBESTOS CEMENT TYPE B OR SINGLE-WALL METAL VENT SYSTEM SERVING A SINGLE DRAFT HOOD-EQUIPPED APPLIANCE
Table 803.2(1) is used where sizing Type B double-wall gas vent connectors attached to a Type B double-wall common vent.
Note: Each appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(6)
VENT SYSTEM SERVING TWO OR MORE APPLIANCES WITH TYPE B DOUBLE-WALL VENT AND TYPE B DOUBLE-WALL VENT CONNECTORS
Table 803.2(2) is used where sizing single-wall vent connectors attached to a Type B double-wall common vent.
Note: Each appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(7)
VENT SYSTEM SERVING TWO OR MORE APPLIANCES WITH TYPE B DOUBLE-WALL VENT AND SINGLE-WALL METAL VENT CONNECTORS
Table 803.2(3) is used where sizing Type B double-wall vent connectors attached to a tile-lined masonry chimney.
Notes:
  1. A is the equivalent cross-sectional area of the tile liner.
  2. The appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(8)
MASONRY CHIMNEY SERVING TWO OR MORE APPLIANCES WITH TYPE B DOUBLE-WALL VENT CONNECTORS
Table 803.2(4) is used where sizing single-wall metal vent connectors attached to a tile-lined masonry chimney.
Notes:
  1. A is the equivalent cross-sectional area of the tile liner.
  2. Each appliance is permitted to be either Category I draft hood-equipped or fan-assisted type.
FIGURE F 101.2(9)
MASONRY CHIMNEY SERVING TWO OR MORE APPLIANCES WITH SINGLE-WALL METAL VENT CONNECTORS
Asbestos cement Type B or single-wall metal pipe vent serving two or more draft hood-equipped appliances. [See Table 803.2(5)]
FIGURE F 101.2(10)
ASBESTOS CEMENT TYPE B OR SINGLE-WALL METAL VENT SYSTEMS SERVING TWO OR MORE DRAFT HOOD-EQUIPPED APPLIANCES
Example: Manifolded common vent connector LM can be no greater than 18 times the common vent connector manifold inside diameter; that is, a 4 inch (102 mm) inside diameter common vent connector manifold shall not exceed 72 inches (1829 mm) in length. [See Section 803.2.3]
Note: This is an illustration of a typical manifolded vent connector. Different appliance, vent connector, or common vent types are possible. [See Section 803.2]
FIGURE F 101.2(11)
USE OF MANIFOLDED COMMON VENT CONNECTORS
Example: Offset common vent
Note: This is an illustration of a typical offset vent. Different appliance, vent connector, or vent types are possible. [See Section 803.1 and Section 803.2]
FIGURE F 101.2(12)
USE OF OFFSET COMMON VENT
FIGURE F 101.2(13)
MULTISTORY GAS VENT DESIGN PROCEDURE FOR EACH SEGMENT OF SYSTEM
FIGURE F 101.2(14)
PRINCIPLES OF DESIGN OF MULTISTORY VENTS USING VENT CONNECTOR AND COMMON VENT DESIGN TABLES
[See Section 803.2.12 through Section 803.2.15]

F 101.3 Example 1: Single Draft Hood-Equipped Appliance

An installer has a 120 000 British thermal units per hour (Btu/h) (35 kW) input appliance with a 5 inch (127 mm) diameter draft hood outlet that needs to be vented into a 10 foot (3048 mm) high Type B vent system. What size vent should be used assuming: (1) a 5 foot (1524 mm) lateral single-wall metal vent connector is used with two 90 degree (1.57 rad) elbows or (2) a 5 foot (1524 mm) lateral single-wall metal vent connector is used with three 90 degree (1.57 rad) elbows in the vent system? (See Figure F 101.3)
Solution:
Table 803.1.2(2) shall be used to solve this problem because single-wall metal vent connectors are being used with a Type B vent, as follows:
  1. Read down the first column in Table 803.1.2(2) until the row associated with a 10 foot (3048 mm) height and 5 foot (1524 mm) lateral is found. Read across this row until a vent capacity exceeding 120 000 Btu/h (35 kW) is located in the shaded columns labeled NAT Max for draft hood-equipped appliances. In this case, a 5 inch (127 mm) diameter vent has a capacity of 122 000 Btu/h (35.7 kW) and shall be permitted to be used for this application.
  2. Where three 90 degree (1.57 rad) elbows are used in the vent system, the maximum vent capacity listed in the tables shall be reduced by 10 percent. This implies that the 5 inch (127 mm) diameter vent has an adjusted capacity of only 110 000 Btu/h (32 kW). In this case, the vent system shall be increased to 6 inches (152 mm) in diameter. See the following calculations:
122 000 Btu/h (35.7 kW) × 0.90 = 110 000 Btu/h (32 kW) for 5 inch (127 mm) vent
From Table 803.1.2(2), select 6 inches (152 mm) vent.
186 000 Btu/h (54.5 kW) × 0.90 = 167 000 Btu/h (49 kW)
This figure is exceeding the required 120 000 Btu/h (35 kW). Therefore, use a 6 inch (152 mm) vent and connector where three elbows are used.
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE F 101.3
SINGLE DRAFT HOOD-EQUIPPED APPLIANCE
EXAMPLE 1

F 101.4 Example 2: Single Fan-Assisted Appliance

An installer has an 80 000 Btu/h (23.4 kW) input fan-assisted appliance that shall be installed using 10 feet (3048 mm) of lateral connector attached to a 30 foot (9144 mm) high Type B vent. Two 90 degree (1.57 rad) elbows are needed for the installation. Is a single-wall metal vent connector permitted to be used for this application? (See Figure F 101.4)
Solution:
Table 803.1.2(2) refers to the use of single-wall metal vent connectors with Type B vent. In the first column find the row associated with a 30 foot (9144 mm) height and a 10 foot (3048 mm) lateral. Read across this row, looking at the FAN Min and FAN Max columns, to find that a 3 inch (76 mm) diameter single-wall metal vent connector is not recommended. Moving to the next larger size single-wall connector [4 inch (102 mm)] we find that a 4 inch (102 mm) diameter single-wall metal connector has a recommended maximum vent capacity of 144 000 Btu/h (42 kW). The 80 000 Btu/h (23.4 kW) fan-assisted appliance is outside this range, so the conclusion is that a single-wall metal connector shall not be used to vent the appliance using a 10 foot (3048 mm) of lateral for the connector. However, if the 80,000 Btu/hr (23.4 kW) input appliance is moved within 5 feet (1524 mm) of the vertical vent, a 4 inch (102 mm) single-wall metal connector shall be used to vent the appliance. Table 803.1.2(2) shows the acceptable range of vent capacities for a 4 inch (102 mm) vent with 5 feet (1524 mm) of lateral to be between 72 000 Btu/h (21.1 kW) and 157 000 Btu/h (46 kW).
Where the appliance cannot be moved closer to the vertical vent, then a Type B vent shall be used as the connector material. In this case, Table 803.1.2(1) shows that, for a 30 foot (9144 mm) high vent with 10 feet (3048 mm) of lateral, the acceptable range of vent capacities for a 4 inch (102 mm) diameter vent attached to a fan-assisted appliance is between 37 000 Btu/h (10.8 kW) and 150 000 Btu/h (44 kW).
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE F 101.4
SINGLE FAN-ASSISTED APPLIANCE
EXAMPLE 2

F 101.5 Example 3: Interpolating Between Table Values

An installer has an 80 000 Btu/h (23.4 kW) input appliance with a 4 inch (102 mm) diameter draft hood outlet that needs to be vented into a 12 foot (3658 mm) high Type B vent. The vent connector has a 5 foot (1524 mm) lateral length and is also Type B. Is this appliance permitted to be vented using a 4 inch (102 mm) diameter vent?
Solution:
Table 803.1.2(1) is used in the case of an all Type B Vent system. However, since there is no entry in Table 803.1.2(1) for a height of 12 feet (3658 mm), interpolation shall be used. Read down the 4 inch (102 mm) diameter NAT Max column to the row associated with a 10 foot (3048 mm) height and 5 foot (1524 mm) lateral to find the capacity value of 77 000 Btu/h (22.6 kW). Read further down to the 15 foot (4572 mm) height, 5 foot (1524 mm) lateral row to find the capacity value of 87 000 Btu/h (25.5 kW). The difference between the 15 foot (4572 mm) height capacity value and the 10 foot (3048 mm) height capacity value is 10 000 Btu/h (3 kW). The capacity for a vent system with a 12 foot (3658 mm) height is equal to the capacity for a 10 foot (3048 mm) height plus two-fifths of the difference between the 10 foot (3048 mm) and 15 foot (4572 mm) height values, or 77 000 Btu/h (22.6 kW) + 2/5 × 10 000 Btu/h (3 kW) = 81 000 Btu/h (23.7 kW). Therefore, a 4 inch (102 mm) diameter vent shall be used in the installation.

F 102.0 Examples Using Common Venting Tables

F 102.1 Example 4: Common Venting Two Draft Hood-Equipped Appliances

A 35 000 Btu/h (10.3 kW) water heater is to be common vented with a 150 000 Btu/h (44 kW) furnace, using a common vent with a total height of 30 feet (9144 mm). The connector rise is 2 feet (610 mm) for the water heater with a horizontal length of 4 feet (1219 mm). The connector rise for the furnace is 3 feet (914 mm) with a horizontal length of 8 feet (2438 mm). Assume single-wall metal connectors will be used with Type B vent. What size connectors and combined vent should be used in this installation? (See Figure F 102.1)
Solution:
Table 803.2(2) shall be used to size single-wall metal vent connectors attached to Type B vertical vents. In the vent connector capacity portion of Table 803.2(2), find the row associated with a 30 foot (9144 mm) vent height. For a 2 foot (610 mm) rise on the vent connector for the water heater, read the shaded columns for draft hood-equipped appliances to find that a 3 inch (76 mm) diameter vent connector has a capacity of 37 000 Btu/h (10.8 kW). Therefore, a 3 inch (76 mm) single-wall metal vent connector shall be used with the water heater. For a draft hood-equipped furnace with a 3 foot (914 mm) rise, read across the row to find that a 5 inch (127 mm) diameter vent connector has a maximum capacity of 120 000 Btu/h (35 kW) (which is too small for the furnace), and a 6 inch (152 mm) diameter vent connector has a maximum vent capacity of 172 000 Btu/h (50 kW). Therefore, a 6 inch (152 mm) diameter vent connector shall be used with the 150 000 Btu/h (44 kW) furnace. Since both vent connector horizontal lengths are less than the maximum lengths listed in Section 803.2.1, the table values shall be used without adjustments.
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE F 102.1
COMMON VENTING TWO DRAFT HOOD-EQUIPPED APPLIANCES
EXAMPLE 4
In the common vent capacity portion of Table 803.2(2), find the row associated with a 30 foot (9144 mm) vent height and read over to the NAT + NAT portion of the 6 inch (152 mm) diameter column to find a maximum combined capacity of 257 000 Btu/h (75 kW). Since the two appliances total 185 000 Btu/h (54 kW), a 6 inch (152 mm) common vent shall be used.

F 102.2 Example 5(a): Common Venting a Draft Hood-Equipped Water Heater With a Fan-Assisted Furnace Into a Type B Vent

In this case, a 35 000 Btu/h (10.3 kW) input draft hood-equipped water heater with a 4 inch (102 mm) diameter draft hood outlet, 2 feet (610 mm) of connector rise, and 4 feet (1219 mm) of horizontal length is to be common vented with a 100 000 Btu/h (29 kW) fan-assisted furnace with a 4 inch (102 mm) diameter flue collar, 3 feet (914 mm) of connector rise, and 6 feet (1829 mm) of horizontal length. The common vent consists of a 30 foot (9144 mm) 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 single-wall metal vent connector. (See Figure F 102.2)
Solution:
Water Heater Vent Connector Diameter. Since the water heater vent connector horizontal length of 4 feet (1219 mm) is less than the maximum value listed in Table 803.2(2), the venting table values shall be used without adjustment. Using the Vent Connector Capacity portion of Table 803.2(2), read down the Total Vent Height (H) column to 30 feet (9144 mm) and read across the 2 feet (610 mm) Connector Rise (R) row to the first Btu/h rating in the NAT Max column that is equal to or exceeding the water heater input rating. The table shows that a 3 inch (76 mm) vent connector has a maximum input rating of 37 000 Btu/h (10.8 kW). Although this rating is exceeding the water heater input rating, a 3 inch (76 mm) vent connector is prohibited by Section 803.2.18. A 4 inch (102 mm) vent connector has a maximum input rating of 67 000 Btu/h (19.6 kW) and is equal to the draft hood outlet diameter. A 4 inch (102 mm) 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 803.2(2), read down the Total Vent Height (H) column to 30 feet (9144 mm) and across the 3 feet (914 mm) Connector Rise (R) row. Since the furnace has a fan-assisted combustion system, find the first FAN Max column with a Btu/h rating exceeding the furnace input rating. The 4 inch (102 mm) vent connector has a maximum input rating of 119 000 Btu/h (34.9 kW) and a minimum input rating of 85 000 Btu/h (24.9 kW).
The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector shall be permitted. Since the furnace vent connector horizontal length of 6 feet (1829 mm) is less than the maximum value listed in Section 803.2.1, the venting table values shall be used without adjustment. Where the furnace had an input rating of 80 000 Btu/h (23.4 kW), then a Type B vent connector shall be needed in order to meet the minimum capacity limit.
Common Vent Diameter. The total input to the common vent is 135 000 Btu/h (40 kW). Using the Common Vent Capacity portion of Table 803.2(2), read down the Vent Height (H) column to 30 feet (9144 mm) and across this row to find the smallest vent diameter in the FAN + NAT column that has a Btu/h rating equal to or exceeding 135 000 Btu/h (40 kW). The 4 inch (102 mm) common vent has a capacity of 132 000 Btu/h (39 kW) and the 5 inch (127 mm) common vent has a capacity of 202 000 Btu/h (59 kW). Therefore, the 5 inch (127 mm) common vent shall be used in this example.
Summary: In this example, the installer shall use a 4 inch (102 mm) diameter, single-wall metal vent connector for the water heater and a 4 inch (102 mm) diameter, single-wall metal vent connector for the furnace. The common vent shall be a 5 inch (127 mm) diameter Type B vent.
For SI units: 1 foot = 304.8 mm, 1000 British thermal units per hour = 0.293 kW
FIGURE F 102.2
COMMON VENTING A DRAFT HOOD-EQUIPPED WATER HEATER WITH A FAN-ASSISTED FURNACE INTO A TYPE B DOUBLE-WALL COMMON VENT
EXAMPLE 5(a)

F 102.3 Example 5(b): Common Venting Into an Interior Masonry Chimney

In this case, the water heater and fan-assisted furnace of Example 5(a) are to be common-vented into a clay-tile-lined masonry chimney with a 30 foot (9144 mm) 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 (203 mm) by 12 inches (305 mm). Assuming the same vent connector heights, laterals, and materials found in Example 5(a), what are the recommended vent connector diameters, and is this an acceptable installation?
Solution:
Table 803.2(4) is used to size common venting installations involving single-wall connectors into masonry chimneys.
Water Heater Vent Connector Diameter. Using Table 803.2(4), Vent Connector Capacity, read down the Vent Height (H) column to 30 feet (9144 mm), and read across the 2 feet (610 mm) Connector Rise (R) row to the first Btu/h rating in the NAT Max column that is equal to or exceeding the water heater input rating. The table shows that a 3 inch (76 mm) vent connector has a maximum input of 31 000 Btu/h (9 kW), while a 4 inch (102 mm) vent connector has a maximum input of 57 000 Btu/h (16.7 kW). A 4 inch (102 mm) vent connector shall be used.
Furnace Vent Connector Diameter. Using the Vent Connector Capacity portion of Table 803.2(4), read down the total Vent Height (H) column to 30 feet (9144 mm) and across the 3 feet (914 mm) Connector Rise (R) row. Because the furnace has a fan-assisted combustion system, find the first FAN Max column with a Btu/h rating exceeding the furnace input rating. The 4 inch (102 mm) vent connector has a maximum input rating of 127 000 Btu/h (37 kW) and an input rating of not less than 95 000 Btu/h (27.8 kW). The 100 000 Btu/h (29 kW) furnace in this example falls within this range, so a 4 inch (102 mm) connector shall be permitted.
Masonry Chimney. From Table F 102.3, the equivalent area for a nominal liner size of 8 inches (203 mm) by 12 inches (305 mm) is 63.6 of a square inches (0.041 m2). Using Table 803.2(4), Common Vent Capacity, read down the FAN + NAT column under the Minimum Internal Area of Chimney value of 63 to the row for 30 foot (9144 mm) height to find a capacity value of 739 000 Btu/h (217 kW). The combined input rating of the furnace and water heater, 135 000 Btu/h (40 kW), is less than the table value so this is an acceptable installation.
Section 803.2.17 requires the common vent area to not exceed seven times the smallest listed appliance categorized vent area, flue collar area, or draft hood outlet area. Both appliances in this installation have 4 inch (102 mm) diameter outlets. From Table F 102.3, the equivalent area for an inside diameter of 4 inches (102 mm) is 12.2 of a square inches (0.008 m2). Seven times 12.2 equals 85.4, which is exceeding 63.6, so this configuration is acceptable.
TABLE F 102.3 MASONRY CHIMNEY LINER DIMENSIONS WITH CIRCULAR EQUIVALENTS*
NOMINAL LINER SIZE (Inches) INSIDE DIMENSIONS OF LINER (Inches) INSIDE DIAMETER OR EQUIVALENT DIAMETER (Inches) EQUIVALENT AREA (Square Inches)
4 × 8 21/2 × 61/2 4.0 12.2
5.0 19.6
6.0 28.3
7.0 38.3
8 × 8 63/4 × 63/4 7.4 42.7
8.0 50.3
8 × 12 61/2 × 101/2 9.0 63.6
10.0 78.5
12 × 12 93/4 × 93/4 10.4 83.3
11.0 95.0
12 × 16 91/2 × 131/2 11.8 107.5
12.0 113.0
14.0 153.9
16 × 16 131/4 × 131/4 14.5 162.9
15.0 176.7
16 × 20 13 × 17 16.2 206.1
18.0 254.4
20 × 20 163/4 × 163/4 18.2 260.2
20.0 314.1
20 × 24 161/2 × 201/2 20.1 314.2
22.0 380.1
24 × 24 201/4 × 201/4 22.1 380.1
24.0 452.3
24 × 28 201/4 × 241/4 24.1 456.2
28 × 28 241/4 × 241/4 26.4 543.3
27.0 572.5
30 × 30 251/2 × 251/2 27.9 607.0
30.0 706.8
30 × 36 251/2 × 311/2 30.9 749.9
33.0 855.3
36 × 36 311/2 × 311/2 34.4 929.4
36.0 1017.9
For SI units, 1 inch. = 25.4 mm, 1 square inch = 0.000645 m2
* Where liner sizes differ dimensionally from those shown in this table, equivalent diameters shall be permitted to be determined from published tables for square and rectangular ducts of equivalent carrying capacity or by other engineering methods.

F 102.4 Example 5(c): Common Venting Into an Exterior Masonry Chimney

Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a draft hood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces shall be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 Btu/h (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
  1. Determine the total available room volume.
    Appliance room volume.
    15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m3)
  2. Determine the total required volume.
    The standard method to determine combustion air shall be used to calculate the required volume.
    The combined input for the appliances located in the basement is calculated as follows:
    100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
    The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m3/kW).
    Using Table F 103.0 the required volume for a 140 000 Btu/h (41 kW) water heater is 7000 cubic feet (198.22 m3).
    Conclusion:
    Indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m3) does not meet the required volume of 7000 cubic feet (198.22 m3). Therefore, additional combustion air shall be provided from the outdoors.
  3. Determine ratio of the available volume to the required volume.
  4. Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on ratio of indoor spaces.
  5. Determine the single outdoor combustion air opening size as if combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors.
  6. Determine the minimum outdoor combustion air opening area.
    Outdoor opening area = 0.49 (from Step 4) × 47 square inches (0.03 m2)
      = 23 square inches (0.01 m2)
Section 701.7.3(3) requires the minimum dimension of the air opening shall be not less than 3 inches (76 mm).
TABLE F 103.0 STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES
[NFPA 54: TABLE A.9.3.2.1]
APPLIANCE INPUT (Btu/h) REQUIRED VOLUME (cubic feet)
5000 250
10 000 500
15 000 750
20 000 1000
25 000 1250
30 000 1500
35 000 1750
40 000 2000
45 000 2250
50 000 2500
55 000 2750
60 000 3000
65 000 3250
70 000 3500
75 000 3750
80 000 4000
85 000 4250
90 000 4500
95 000 4750
100 000 5000
105 000 5250
110 000 5500
115 000 5750
120 000 6000
125 000 6250
130 000 6500
135 000 6750
140 000 7000
145 000 7250
150 000 7500
160 000 8000
170 000 8500
180 000 9000
190 000 9500
200 000 10 000
210 000 10 500
220 000 11 000
230 000 11 500
240 000 12 000
250 000 12 500
260 000 13 000
270 000 13 500
280 000 14 000
290 000 14 500
300 000 15 000
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot = 0.0283 m3

F 103.0 Example of Combination Indoor and Outdoor Combustion Air Opening Design

Determine the required combination of indoor and outdoor combustion air opening sizes for the following appliance installation example.
Example Installation: A fan-assisted furnace and a draft hood-equipped water heater with the following inputs are located in a 15 foot by 30 foot (4572 mm by 9144 mm) basement with an 8 foot (2438 mm) ceiling. No additional indoor spaces shall be used to help meet the appliance combustion air needs.
Fan-Assisted Furnace Input: 100 000 Btu/h (29 kW)
Draft Hood-Equipped Water Heater Input: 40 000 Btu/h (11.7 kW)
Solution:
  1. Determine the total available room volume.
    Appliance room volume.
    15 feet by 30 feet (4572 mm by 9144 mm) with an 8 foot (2438 mm) ceiling = 3600 cubic feet (101.94 m3)
  2. Determine the total required volume.
    The standard method to determine combustion air shall be used to calculate the required volume.
    The combined input for the appliances located in the basement is calculated as follows:
    100 000 Btu/h (29 kW) + 40 000 Btu/h (11.7 kW) = 140 000 Btu/h (41 kW)
    The standard method requires that the required volume be determined based on 50 cubic feet per 1000 Btu/h (4.83 m3/kW).
    Using Table F 103.0 the required volume for a 140 000 Btu/h (41 kW) water heater is 7000 cubic feet (198.22 m3).
    Conclusion:
    Indoor volume is insufficient to supply combustion air since the total of 3600 cubic feet (101.94 m3) does not meet the required volume of 7000 cubic feet (198.22 m3). Therefore, additional combustion air shall be provided from the outdoors.
  3. Determine ratio of the available volume to the required volume.
  4. Determine the reduction factor to be used to reduce the full outdoor air opening size to the minimum required based on ratio of indoor spaces.
    1.00 — 0.51 (from Step 3) = 0.49
  5. Determine the single outdoor combustion air opening size as if combustion air is to come from outdoors. In this example, the combustion air opening directly communicates with the outdoors.
  6. Determine the minimum outdoor combustion air opening area.
    Outdoor opening area = 0.49 (from Step 4) × 47 square inches (0.03 m2)
      = 23 square inches (0.01 m2)
Section 701.7.3(3) requires the minimum dimension of the air opening shall be not less than 3 inches (76 mm).
TABLE F 103.0 STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES
[NFPA 54: TABLE A.9.3.2.1]
APPLIANCE INPUT (Btu/h) REQUIRED VOLUME (cubic feet)
5000 250
10 000 500
15 000 750
20 000 1000
25 000 1250
30 000 1500
35 000 1750
40 000 2000
45 000 2250
50 000 2500
55 000 2750
60 000 3000
65 000 3250
70 000 3500
75 000 3750
80 000 4000
85 000 4250
90 000 4500
95 000 4750
100 000 5000
105 000 5250
110 000 5500
115 000 5750
120 000 6000
125 000 6250
130 000 6500
135 000 6750
140 000 7000
145 000 7250
150 000 7500
160 000 8000
170 000 8500
180 000 9000
190 000 9500
200 000 10 000
210 000 10 500
220 000 11 000
230 000 11 500
240 000 12 000
250 000 12 500
260 000 13 000
270 000 13 500
280 000 14 000
290 000 14 500
300 000 15 000
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot = 0.0283 m3