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Fresh Air for Combustion
WARNING
NOTE
Be sure to provide enough fresh airfor combustion. Enough air ensures
proper combustion and assures that no hazard will develop due to
the lackof oxygen.
If you use a fireplaceor a kitchenor bathroomexhaustfan, you
shouldinstallan outsideair intake.Thesedeviceswillrobthe boiler
andwaterheaterofcombustionair.
You must provide for enough fresh air to assure proper combustion. The fire
in the boiler uses oxygen. It must have a continuous supply. The air in a
house contains only enough oxygen to supply the burner for a short time.
Outside air mustenter the house to replace that used by the burner. Study
foIIowingexampIes 1and 2 to determineyour fresh air requirements.
EXAMPLE 1:Boiler Located in Unconfined Space
If your boiler isin an open area (unpartitioned basement) ina conventional
house, the air that leaks through the cracks around doors and windows will
usually be adequate to provide air for combustion. The doors should not fit
tightly. Do not caulkthecracks aroundthe windows.
An unconfinedspace isdefinedas a space whose volume isnot lessthan 50
cubic feet per 1,000 Btu per hour of the total inputrating of all appliances
installedin thatspace.
EXAMPLE 2: Boiler Located in Confined Space
A, All Air from Inside the Building: The confined space shallbe provided
with two permanent openings communicating directly with an additional
room(s) of sufficientvolume so that the combined volume of all spaces
meets the criteria for an unconfined space. The total input of all
combustion equipment installed in the combined space shall be
considered in making this determination. Each opening shall have a
minimumfree area of one square inch per 1,000Btu per hourof thetotal
inputratingofall combustionequipment inthe confinedspace, but not less
than 100squareinches. One openingshall be within 12 inches ofthe top
andone within 12 inchesof the bottomof theenclosure.See Figure4.
Example: Your boiler is rated at 100,000 Btu per hour. The water heater
is rated at 30,000 Btu per hour. The total is 130,000 Btu per hour. You
need two grilles, each with 130square inchesof FREE opening. Metal
grilles have about 60% FREE area. To find the Iouveredarea needed,
multiply the FREE area required by 1.7 (130 x 1.7 = 221.0 sq. in.
Iouvered area). In thisexample twogrilles, each having an 8"x 30" (240
sq.in,) Iouvered area would be used. Fig. 4 shows the grille locations.
B. All Air from Outdoors: The confined space shall be provided with two
permanent openings, one commencing within 12 inches of the top and
one commencing within 12 inches of the bottom of the enclosure. The
openings shall communicate directly, or by ducts, with the outdoors or
spaces (crawl or attic)thatfreelycommunicatewith the outdoors.
1.When directly communicatingwith the outdoors,each opening shall have
a minimumfree area of one squareinchper 4,000 Btu per hour of total
inputratingof all equipmentin theenclosure.
2. When communicatingwith the outdoors through ve_cal ducts, each
opening shall have a minimumfree area of one square inch per 4,000
Btu perhour of total input ratingof all equipmentin theenclosure.
3.Whencommunicatingwiththeoutdoorsthroughhorizontalducts,each
openingshall havea minimumfreearea of one squareinchper
2,000 Btu per hour of totalinput ratingof all equipmentin the
enclosure.
4. Whenductsareused,theyshallbeof thesamecross-sectionalarea
asthefreeareaoftheopeningstowhichtheyconnect.Theminimum
dimensionofrectangularairductsshal{benotlessthanthreeinches.
VENTILATING -'_--i
GRII.LE I
i I ' 1
i ,11 II ',
AIR GRILLE _
Fresh Air Duct Capacities for Ducts Supplying Fresh Air to
boiler in tightly constructed houses. British Thermal Units per
hour Input (Btuh)*.
Fresh Air
Duct Size
31/2"x 12"
8* x 8"
8" x 12"
8" x 16"
1/4"Mesh
Screen Btuh
144,000
256,000
384,000
512,0O0
Wood
Louvers Stuh
36,000
64,000
96,000
128,000
Metal
Louvers Btuh
108,000
!92,000
288,000
384,000
*Based on opening covered by IA" mesh screen, wood louvers, or mela{ louvers
FRESH
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