Vent or |
|
|
|
Chimney |
|
|
|
| Air Duct | Air Duct | |
| must be | ||
Furnace |
| at least | |
| 1 sq. in. | ||
|
|
| per 2000 |
|
|
| Btuh of |
| Air Duct | total input | |
| rating. | ||
|
|
| |
| Water Heater |
|
|
| Minimum |
|
|
Total Input | Free Area | Round Duct | |
Rating (Btuh) | (Each Opening) | Diameter | |
40,000 | 20 sq. in. |
| 5" |
60,000 | 30 sq. in. |
| 6" |
80,000 | 40 sq. in. |
| 7" |
100,000 | 50 sq. in. |
| 8" |
120,000 | 60 sq. in. |
| 9" |
140,000 | 70 sq. in. |
| 10" |
160,000 | 80 sq. in. |
| 10" |
Figure 13. Equipment in a Confined Space with all Combustion Air Drawn from the Outside through Horizontal Ducts
Air Requirements For One-Pipe Installation
When air for combustion is to be taken from around the furnace, a protective screen must be installed over the combustion air intake open- ing. This screen is provided with the furnace installation instructions and functions to prevent debris from entering the combustion system. It should be installed on the combustion air intake collar or inlet PVC. If furnace location is such that this opening might be unintentionally ob- structed, a 3" PVC elbow should be installed on the collar, and the screen placed inside the inlet of the elbow. See Figure 8.
Installation In An Unconfined Space
!CAUTION:
"Tight" buildings (with weather strip- ping and caulk to reduce infiltration), may require special provisions for in- troduction of outside air to ensure satisfactory combustion and venting, even though the furnace is located in an unconfined space.
An unconfined space is an area including all rooms not separated by doors with a volume greater than 50 cubic feet per 1,000 Btuh of the combined input rates of all appliances which draw combustion air from that space. For example, a space including a water heater rated at 45,000 Btuh and a furnace rated at 75,000 Btuh requires a volume of 6,000 cubic feet [50 x (45 + 75) = 6,000] to be considered unconfined. If the space has an 8 foot ceiling, the floor area of the space must be 750 square feet (6,000 / 8 = 750). In general, a furnace installed in an unconfined space will not require outside air for combustion.
!WARNING:
Furnaces installed with combustion air drawn from a heated space which includes exhaust fans, fireplaces, or other devices that may produce a negative pressure should be consid- ered confined space installations.
Installation In A Confined Space
A confined space is one which does not meet the unconfined space volume requirements, and typically involves installation in a small room. All such installations must have specific provisions for introduction of combustion and ventilation air. Codes require that two openings be pro- vided for this - one with bottom edge within 12" of the floor and one with top edge within 12" of the ceiling. The size and other criteria for these openings must be per the following sections.
Combustion air openings must not be restricted in any manner.
Furnaces installed in a confined space which supply circulating air to areas outside of the space must draw return air from outside the space and must have return air ducts tightly sealed to the furnace.
Air From Inside
Air for combustion and ventilation may be taken from inside the building through an interior wall if the building is not "tight" and if the total volume of the furnace space and the space from which air is drawn meets the volume requirements for an unconfined space. In such cases, the two openings in the wall must each have free area of at least one square inch per 1000 Btuh of total appliance input, but not less than 100 square
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