Appendix J Combination of Indoor and Outdoor Combustion and Ventilation Opening Design

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.