
Vermont Castings • Deant Flexburn 1975 Installation Manual_R1 • 2021 - ___ • 08/21 138390-035i
44 Chimneys & Venting
A. Venting & Draft Management
Astoveispartofasystem,whichincludesthechimney,the
operator,thefuel,andthehome.Theotherpartsofthesystem
willaecthowwellthestoveworks.Whenthereisagood
matchbetweenalltheparts,thesystemworkswell.
Woodstoveorinsertoperationdependsonnatural(unforced)
draft.Natural draftoccurswhen theexhaustis hotter(and
thereforelighter)thantheoutdoorairatthetopofthechimney.
Thebiggerthetemperaturedierence,thestrongerthedraft.
Asthehotgasesrisethroughthechimneytheyprovidesuction
or‘draw’thatpullsairintothestoveforcombustion.Aslow,lazy
rewiththestove’sairinletsfullyopenindicatesaweakdraft.
Abriskre,supportedonlybyairenteringthestovethrough
thenormalinlets,indicatesagooddraft.Thestove’sairinlets
arepassive;theyregulatehowmuchaircanenterthestove,
buttheydon’tmoveairintoit.
Depending on the features of your installation - steel or
masonrychimney,insideoroutsidethehouse,matchedtothe
stove’soutletoroversized-yoursystemmaywarmupquickly,
oritmaytakeawhiletowarmupandoperatewell.Withan
‘airtight’stove,onewhichrestrictstheamountofairgetting
intotherebox,thechimneymustkeepthestove’sexhaust
warmallthewaytotheoutdoorsinorderforthestovetowork
well.Somechimneysdothisbetterthanothers.Here’salist
offeaturesandtheireects.
Masonry Chimney
Masonry is a traditional material for chimneys, but it can
performpoorlywhenitservesan‘airtight’stove.Masonryisa
veryeective‘heatsink’-itabsorbsalotofheat.Itcancool
thechimneygasesenoughtodiminishdraft.Thebiggerthe
chimney,thelongerittakestowarmup.It’softenverydicultto
warmupanoutdoormasonrychimney,especiallyanoversized
one,andkeepitwarmenoughtomaintainanadequatedraft.
Steel Chimney
Mostfactory-madesteelchimneyshavealayerofinsulation
aroundtheinnerue.Thisinsulationkeepsthechimneywarm.
Theinsulationislessdensethanmasonry,soasteelchimney
warmsupmorequicklythanamasonrychimney.Steeldoesn’t
havethegoodlooksofmasonry,butitperformsmuchbetter.
Indoor/Outdoor Location
Becausethechimneymustkeepthesmokewarm,it’sbestto
locateitinsidethehouse.Thisusesthehouseasinsulation
fortheueandallowssomeheatreleaseintothehome.An
indoorchimneywon’tloseitsheattotheoutdoors,soittakes
lessheatfromthestovetoheatitupandkeepitwarm.
Single Venting
Each‘airtight’stoverequiresitsownue.Ifanairtightstoveis
ventedtoauethatalsoservesanopenreplace,oraleakier
stove,it’seasierforthechimneydrafttopullairinthrough
thosechannelsandperformanceofthestovesuers.Imagine
avacuumcleanerwithaholeinthehosetounderstandthe
eecthere.Insomecasestheotherappliancecanevencause
anegativedraftthroughthestove,andresultinadangerous
draftreversal.
Chimney Height
Thecommonwisdomtellsusthatatalleruedrawsbetter
thanashortone.Aruleofthumbforminimumheightstates
thatthetotalsystemheight(fromtheoortheapplianceis
mountedontothetopofthechimney)shouldneverbeless
than 4.8 m (16 ft.). Most normal installations exceed this
height,butinstallationsincottageswithshallow-pitchroofs
maynot.Don’tmakeachimneytallerunlessyoumustin
ordertomeetthesafetyrules,orunlessthere’ssomenearby
featurecausingadowndraft.Eventhen,therearedowndraft-
preventingchimneycapsavailable,whichareprobablythe
smarterchoice.
Flue Sizing
Theinsidesizeofachimneyforan‘airtight’stoveshould
matchthesizeofthestove’sueoutlet.Whenachimney
servesanairtightstove,moreisnotbetter;infact,itcanbea
disadvantage.Hotgasesloseheatfasterastheytravelslower
throughachimney;ifweventastovewithasix-inchuecollar
(28squareincharea)intoa10x10"ue,thegasesslowto
onethirdtheiroriginalspeed.Thisallowsthegasestocool
morerapids,whichweakensdraftstrength.Ifanoversizedue
isalsooutsidethehouse,theheatitabsorbsgetstransferred
totheoutdoorairandtheueusuallystayscool.
Itiscommonforamasonryue,especiallyoneservinga
replace,tobeoversizedforthestove.Itcantakequiteawhile
towarmupsuchaue,andtheresultscanbedisappointing.
Thebestsolutiontoanoversizedueisaninsulatedsteel
chimneyliner,thesamediameterasthestoveorinsertsue
outlet;thelinerkeepstheexhaustwarm,andtheresultisa
strongerdraft.Annon-insulatedlinerisasecondchoice-the
linerkeepstheexhaustrestrictedtoitsoriginalsize,butthe
hotgasesstillmustwarmuptheairaroundtheliner.This
makesthewarm-upprocesstakelonger.
Pipe & Chimney Layout
Everyturntheexhaustmusttakeasittravelstothechimney
topwillslowitdown.Theidealpipeandchimneylayoutisto
ventverticallyintoacompletelystraightandverticalchimney.
Ifyouarestartingfromscratch,usethislayoutifpossible.
Ifthestovepipemustelbowtoenterachimney,locatethe
thimbleaboutmidwaybetweenthestovetopandtheceiling.
Thisachievesseveralgoals:itallowsthegasestospeedup
beforetheymustturn,itleavessomepipeintheroomfor
heattransfer,anditgivesyoulong-termexibilityforinstalling
adierentstovewithoutrelocatingthethimble.
Thereshouldbenomorethaneightfeetofsingle-wallstove
pipebetweenthestoveandachimney;longerrunscancool
theexhaustenoughtocausedraftandcreosoteproblems.
Withprefabricatedchimney,bringitdowntosixtoeightfeet
fromthestove.Withamasonrychimney,arrangethepipe
sothatitturnsintothechimneywithineightfeetofthestove.