12
afreeareaofonesquareinchper1000Btu/hr(22 cm2/kW)ofthe
totalinputofallappliancesintheenclosure,butnot lessthan100
squareinches(645cm2).
Iftheconnedspaceiswithinabuildingoftightconstruction, airfor
combustionandventilationmust beobtainedfromoutdoors. When
directlycommunicatingwiththeoutdoorsorcommunicatingthrough
verticalducts,twopermanentopenings,locatedintheabovemanner,
shouldbeprovided.Eachopeningshouldhaveafreeareaofnotless
thanonesquareinchper4000Btu/hr(5.5cm2/kW)oftotalinputofall
appliancesintheenclosure.Ifhorizontalductsareused,eachopening
shouldhaveafreeareaofnotlessthanonesquareinchper2000Btu/
hr(11cm2/kW)ofthetotalinputofallappliancesintheenclosure.
a. all aIr froM InsIde BuIldInGs: (SeeFigure5and6)
Theconnedspace should be provided with two permanent openings
communicatingdirectlywith an additional room(s) of sufcient volume 
sothatthe combined volume of all spaces meets the criteria for an
unconnedspace.Thetotalinputofallgasutilizationequipmentinstalled
inthecombinedspaceshouldbeconsideredinmakingthisdetermination.
Eachopeningshould have a minimum free area of one square inch
per1,000Btu per hour (22 cm2/kW) of the total input rating of all gas
utilizationequipmentintheconnedspace,butnotlessthan100square
inches(645cm2). Oneopeningshouldcommencewithin12inches
(30cm)ofthetopandonecommencingwithin12inches(30cm)ofthe
bottomoftheenclosures.
FIGURE 6.
B. all aIr froM outdoors:(SeeFigures7,8and9)
Theconfinedspace should be provided with two permanent
openings,onecommencingwithin12inches(30cm)ofthetopand
onecommencingwithin 12inches(30 cm)fromthe bottomofthe
enclosure.Theopeningsshouldcommunicatedirectly,or byducts,
withtheoutdoorsorspaces(crawlorattic)thatfreelycommunicate
with the outdoors.
1.When directlycommunica tingwiththeou tdoors,eachopeni ngshould
have
amini mumfr eeare aof1sq uarei nchpe r4,00 0Btup erho ur(5.5 cm2/
kW)
oftotalinputratingofallequipmentintheenclo sure,seeFigure7.
FIGURE 7.
2.Whencommunicating withtheoutdoorsthrough verticalducts,each 
openingshouldhaveaminimumfreeareaof1squareinchper4,000
Btuperhour(5.5cm2/kW)oftotalinputratingofallequipmentinthe
enclosure,seeFigure8.
3. Whencommunicatingwith��theoutdoorsthroughhorizontalducts,each
openingshouldhaveaminimumfreeareaof1squareinchper2,000
Btuperhour(11cm2/kW))oftotalinputratingofallequipmentinthe
enclosure,seeFigure9.
FIGURE 8.
4. Whenductsareused,theyshouldbeofthesamecross-sectionalarea
asthefreeareaoftheopeningstowhichtheyconnect.Theminimum
shortsidedimensionofrectangularairductsshouldnotbelessthan
3inches(7.6cm),seeFigure9.
FIGURE 9.
5. Alternatively a single permanent opening may be used when
communicatingdirectlywiththeoutdoors,orwithspacesthatfreely
communicatewiththeoutdoors.Theopeningshouldhaveaminimum
freeareaof1squareinchper3,000BTUperhour(8.3cm2/kW)oftotal
inputratingofallequipmentinenclosure.SeeFigure9A.
FIGURE 9.
6.LouversandGrilles:Incalculatingfreearea,considerationshouldbe
givento
theblockingeffectoflouvers,grillesorscreensprotectingopenings.
Screensusedshouldnotbesmallerthan1/4inch(6.4mm)mesh.Ifthefree
areathroughadesignoflouverorgrilleisknown,itshouldbeusedincalculating
thesizeopeningrequiredtoprovidethefreeareaspecied.Ifthedesignand
freeareaisnotknown, itmaybeassumed thatwoodlouverswill be20-25
percentfreeareaandmetal louversandgrilleswill have60-75percentfree
area.Louversandgrillesshouldbexedintheopenpositionorinterlockedwith
theequipment
sothattheyareopenedautomaticallyduringequipment
operation.
7.SpecialConditionsCreatedbyMechanicalExhaustingorFireplaces:
operationofexhaustfans,ventilationsystems,clothesdryersor
replacesmaycreateconditionsrequiringspecialattentiontoavoid
unsatisfactoryoperationofinstalledgasutilizationequipment.