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 Btu/h (29 kW)
Draft Hood-Equipped Water Heater Input: 40000 Btu/h (11. 7 kW)
Solution:
For SI units: 1000 British thermal units per hour = 0.293 kW, 1 cubic foot =
0.0283 m3
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 Btu/h (29 kW)
Draft Hood-Equipped Water Heater Input: 40000 Btu/h (11. 7 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:
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 F103.0 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. - 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.
Section 701.7.3(3) requires the minimum dimension of the air opening shall be not less than 3 inches (76 mm).Outdoor opening area = 0.49 (from Step 4) × 47 square
inches (0.03 m2)= 23 square inches (0.01 m2)
| 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 |
| 140000 | 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 |