(The content of this Appendix is based on Annex I of NFPA 54)
This appendix provides general guidelines for the sizing of combination indoor and outdoor combustion and ventilation air openings.
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: 100000 British thermal units per hour (Btu/h) (29 kW)
Draft Hood-Equipped Water Heater Input: 40000 Btu/h (11.7kW)

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:
    100000 Btu/h (29 kW) + 40000 Btu/h (11.7 kW) = 140000 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 J 101.2, the required volume for a 140000 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 where 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) x 47 square inches (0.03 m2) = 23 square inches (0.01 m2)
    Section 506.5.3(3) requires the minimum dimension of the air opening shall be not less than 3 inches (76 mm).

TABLE J 101.2
STANDARD METHOD: REQUIRED VOLUME, ALL APPLIANCES* [NFPA 54: TABLE A.9.3.2.1]
APPLIANCE INPUT
(Btu/h)
REQUIRED VOLUME
(cubic feet)
5000 250
10000 500
15000 750
20000 1000
25000 1250
30000 1500
35000 1750
40000 2000
45000
2250
50000 2500
55000 2750
60000 3000
65000 3250
70000 3500
75000 3750
80000 4000
85000 4250
90000 4500
95000 4750
100000 5000
105000 5250
110000 5500
115000 5750
120000 6000
125000 6250
130000 6500
135000 6750
14000 7000
145000 7250
150000 7500
160000 8000
170000 8500
180000 9000
190000 9500
200000 10000
210000 10500
220000 11000
230000 11500
240000 12000
250000 12500
260000
13000
270000 13500
280000 14000
290000 14500
300000 15000
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot = 0.0283 m3
*   See Section 506.4.1, Section 506.4.1(1), Figure 506.4(1), Figure 506.4(2), Section 506.4.1(2), Figure 506.4(3), Section 506.4.2, and Figure 506.4.2.
Resources