3. Whenonepermanentoutdooropeningisused,theopening
requires:
a. 1sq.inoffreeareaper3,000BTUH(700mm2/kW)for
combinedinputofallgasappliancesinthespace(see
Table1) and
b. notlessthanthesumoftheareasofallventconnectorsin
thespace.
Theopeningshallcommencewithin12"ofthetopoftheenclo-
sure.Appliancesshallhaveclearancesofatleast1"fromthesides
andbackand6"fromthefront.Theopeningshalldirectlycommu-
nicatewiththeoutdoorsorshallcommunicatethroughaverticalor
horizontalducttotheoutdoorsorspaces(crawlorattic)thatfreely
communicatewiththeoutdoors.
4. Combination of Indoor and Outdoor Air shall have:
a.
b.
C.
Indoor openings that comply with the Indoor Combus-
tion Air Method below and
Outdoor openings located as required in the Outdoor
Combustion Air Method above and
Outdoor openings sized as follows.
1) Calculate the Ratio of all Indoor Space volume divid-
ed by required volume for Indoor Combustion Air Meth-
od.
2) Outdoor opening size reduction Factor is I minus the
Ratio in 1) above.
3) Minimum size of Outdoor openings shall be the size
required in Outdoor Combustion Air Method above
multiplied by reduction Factor.
Area
BTUH MinimumFreeArea Requiredfor EachOpening or Ductto Outdoors
TwoVerticalDuctsor RoundDuct
Input TwoHorizontalDucts SingleOpening Openings (sq. in./4,000
Rating (sq. inJ2,000BTUH) (sq. in./3,000BTUH) (sq.in./4,000 BTUH) BTUH)
50,000 25 sq. in. 16.7 sq. in. 12.5 sq. in. 4"
75,000 37.5 sq. in. 25sq. in. 18.75 sq. in. 5"
100,000 50 sq. in. 33.3 sq. in. 25 sq. in. 6"
125,000 62.50sq. in. 41.7 sq. in. 31.25 sq. in. 7"
150,000 75sq. in. 50 sq. in. 37.5 sq. in. 7"
EXAMPLE: Determining Free Area
Furnace Water Heater Total Input
100,000 + 30,000 (130,000 + 4,000) 32.5 Sq. In. Vertical
Furnace Water Heater Total Input
100,000 + 30,000 (130,000 + 2,000) 65 Sq. In. Horizontal
Indoor Combustion Air The Standard Method may be used, if the space has no less vol-
ume than 50 cubic feet per 1,000 BTUH input rating for all gas ap-
pliances installed in the space. The standard method permits
indoor air to be used for combustion and ventilation air.
Standard and Known-Air-Infiltration Rate Methods
© NFPA&AGA
Indoor air is permitted for combustion and ventilation, if the Stan-
dard or Known-Air-Infiltration Rate Method is used.
CARBON MONOXIDE POISONING HAZARD.
Failure to supply adequate combustion air could
result in death and/or personal injury.
Most homes will require additional air from
outdoors for combustion and ventilation. A
space with at least 50 cubic feet per 1,000 BTUH
input rating or homes with tight construction
may need outdoor air, supplied through ducts, to
supplement air infiltration for proper
combustion and ventilation of flue gases.
The Known Air Infiltration Rate Method shall be used if the in-
filtration rate is known to be less than 0.40 air changes per hour
(ACH) and equal to or greater than 0.10 ACH. Infiltration rates
greater than 0.60 ACH shall not be used. The minimum required
volume of the space varies with the number of ACH and shall be
determined per Table 2 or Equations 1 and 2. Determine the
minimum required volume for each appliance in the space, and
add the volumes together to get the total minimum required vol-
ume for the space.
44001 102005 [_
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