Unusuallytightconstruction isdefinedas construction where:
a)wallsandceilingsexposed totheoutside atmospherehaveacontinuouswatervaporretarderwitharatingofone
perm(6x10-11kgperpa-sec-m2)orless withopeningsgasketed orsealed and
b)weatherstrippinghasbeenadded onwindowsthat can be opened anddoors and
c)caulkingorsealantsareapplied toareassuchasjointsaroundwindowanddoorframes, betweensoleplates
andfloors, betweenwall-ceilingjoints, betweenwall panels, at penetrationsforplumbing, electrical, andgas
lines, andat otheropenings.
If yourhomemeetsall ofthethree criteriaabove, youmust provide additionalfreshair. See VentilationAirFrom
Outdoors(page 8). If yourhomedoesnot meet all ofthethree criteriaabove, proceed to"DeterminingFresh-AirFlow
ForHeaterLocation"(below).
DETERMININGFRESH-AIRFLOWFORHEATERLOCATION
DeterminingifYouHaveaConfined orUnconfined Space
Usethisworksheet todetermineifyouhaveaconfined orunconfined space.
Space: Includestheroominwhichyouwill install heaterplusanyadjoiningroomswithdoorless passagewaysor
ventilationgrillsbetweentherooms.
1. Determinethevolumeofthespace
Length×Width×Height=cu.ft. (volumeofspace)
Example: Spacesize20ft. (length) ×16ft. (width) ×8ft. (ceilingheight)=2560cu. ft. (volumeofspace)If additional
ventilationtoadjoiningroomissupplied withgrillsoropenings, add thevolumeoftheseroomstothetotalvolumeof
thespace.
2. Divide thespacevolumeby50 cubicfeet todeterminethemaximumBtu/Hrthespacecan support.
(volumeofspace) ÷50 cu. ft. =(MaximumBtu/Hrthespacecan support)
Example: 2560 cu. ft. (volumeofspace)?0 cu.ft.=51.2or51,200(maximumBtu/Hrthespacecan support)
3. Add theBtu/Hrofall fuelburningappliancesinthespace.
Vent-free heater
Gaswaterheater*
Gasfurnace
Vented gasheater
Gasheaterlogs
Othergasappliances*+
Total=
Btu/Hr
Btu/Hr
Btu/HrExample:
Btu/HrGaswaterheater30,000 Btu/Hr
Btu/HrVent-free heater+26,000 Btu/Hr
Btu/HrTotal=56,000 Btu/Hr
*Donotinclude direct-ventgasappliances. Direct-ventdrawscombustion airfromthe outdoorsand ventstothe outdoors.
4.ComparethemaximumBtu/Hrthespacecan support withtheactualamount ofBtu/Hrused.
Btu/Hr(maximumthespacecan support)
Btu/Hr(actualamount ofBtu/Hrused)
Example: 51,200 Btu/Hr(maximumthespacecan support)
56,000 Btu/Hr(actualamount ofBtu/Hrused)
Thespaceintheaboveexampleisaconfined spacebecausetheactualBtu/Hrused ismorethan the
maximumBtu/Hrthespacecan support. Youmust provide additionalfreshair. Youroptionsareasfollows:
a)Reworkworksheet, addingthespaceofan adjoiningroom. If theextraspaceprovidesan unconfined
space, removedoortoadjoiningroomoradd ventilationgrillsbetweenrooms. See “VentilationAirfrom
Outdoors,”page 8.
b)Vent roomdirectlytotheoutdoors. See “VentilationAirfromOutdoors,”page 8.
c)Install alowerBtu/Hrheater, iflowerBtu/Hrsizemakesroomunconfined. If theactualBtu/Hrused is
less than themaximumBtu/Hrthespacecan support, thespaceisan unconfined space. Youwill need no
additionalfreshairventilation.
WARNING: If theareainwhichtheheatermaybe operated issmallerthan that defined asan unconfined
Spaceorifthebuildingisofunusuallytight construction, provide adequatecombustionandventilationairby
oneofthemethodsdescribed intheNationalFuelGasCode, ANSIZ223.1, Section5.3orapplicablelocal
codes.
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