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F 103.0 Example of Combination Indoor and Outdoor Combustion Air Opening Design
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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)
Solution:
- 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)
- 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: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.
- Determine ratio of the available volume to the required volume.
- 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
- 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.
- 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).
| 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
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