W415-1007 / A / 07.18.12
17
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 the attic. If using the attic air, ensure that the
opening 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.
