W415-0919 / D / 03.27.13
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In this circumstance, the free area of each of the two combustion and ventilation air openings is based on
a minimum of 1 square inch per 4000 btu/hr. In this configuration, one opening can originate from the floor
drawing combustion and ventilation air from the ventilated crawl space.
The other opening may communicate freely with a ventilated attic. If using the attic air, ensure that the open-
ing is ducted from the ceiling high enough to be above the insulation. The attic must be adequately vented
with soffit vents or gable vents
(Figure 5)
As an alternative to creating an opening in the floor to draw air from a crawl space, a duct may be dropped from
the attic terminating 12” above the floor.
The following table shows minimum free areas
and round pipe sizes when drawing combustion
air vertically from the attic or crawl space for the
furnace only. If other fuel burning appliances
are present, their combustion air and ventilation
air requirements must be added to those of the
furnace.
IMPORTANT:
IF AN ExhAuST FAN, FIREplACE, CloThES DRyER oR ANy SImIlAR DEVICE IS pRESENT IN ThE
INDooR AREA FRom WhICh ThE CombuSTIoN AND VENTIlATIoN AIR WIll bE DRAWN, NEgATIVE
pRESSuRE CoulD bE A pRoblEm IF NATuRAl INFIlTRATIoN FRom ThE ouTDooRS DoES NoT
mATCh ThE RATE AT WhICh AIR IS ExhAuSTED.
TAblE 2 - VERTICAl AIR Supply (CASE 3)
Input
Free Area
Round pipe Size
k/btu/hr
Ea. opening
45
15 in. ²
5 in.
60
15 in. ²
5 in.
80
20 in. ²
6 in.
100
25 in. ²
6 in.
120
30 in. ²
7 in.
8.1.3 Case 3: fURnaCe loCaTeD In a ConfIneD spaCe, oUTDooR aIR
fRoM aTTIC oR CRaWl spaCe
FIGURE 5 - OUTSIDE AIR FOR COMBUSTION, ATTIC OR
CRAWL SPACE (CASE 3)
IMPORTANT:
IF ThE ATTIC hAS AN ExhAuST FAN (poWER VENT), IT mAy CREATE A NEgATIVE
pRESSuRE SuFFICIENTly lARgE ENough To pREVENT ThE ATTIC FRom bEINg
AN EFFECTIVE SouRCE oF CombuSTIoN AND VENTIlATIoN AIR. poWERED ATTIC
FANS Do NoT CuSTomARIly RuN DuRINg ThE hEATINg SEASoN; hoWEVER,
SomE ARE CoNTRollED by A humIDISTAT AS WEll AS A ThERmoSTAT, WhICh mAy
AlloW SomE opERATIoN DuRINg ThE hEATINg SEASoN. ThE ChoICES ARE (A)
uSE ThE DIRECT VENT opTIoN; (b) obTAIN ouTDooR AIR FRom ElSEWhERE; oR
(C) INTERloCk ThE ATTIC ExhAuST FAN WITh ThE FuRNACE SuCh ThAT ThE TWo
CANNoT opERATE SImulTANEouSly.
