The Hearth in the House as a System

Summary table





installed inside building envelope

half in/half out

installed outside envelope

higher than top of envelope

same height as envelope

lower than top of envelope

sized to match appliance outlet

one size smaller or larger

too big/too small

high mass

moderate mass

low mass

well insulated

some insulation

not insulated/air cooled

straight up

slight offset

severe/multiple offset(s)


somewhat leaky


Flue pipe (if applicable)



sized to match appliance outlet

one size larger or smaller

too large/too small

straight up

one elbow

more than one elbow

tight joints

loose joints

leaky joints

no obstructions/major leaks

key damper or barometric draft control

flue pipe heat exchanger

total length less than 6'

between 6' and 10'

more than 10'


gas/propane appliances



sealed/direct vent

induced draft

draft hood

oil-fired appliances



sealed with induced draft

 induced draft

barometric draft control

woodburning appliances



gasketed doors

leaky doors

open — no doors

adequate internal flow area

somewhat restrictive

very restrictive internally

updraft — no internal by-pass

sidedraft/downdraft/catalytic — internal by-pass damper


high mass

moderate mass

low mass

round firebox outlet

rectangular firebox outlet

slot firebox outlet

EPA certified (stable combustion)

some combustion design (may smolder)

no combustion system (may smolder)

Building environment

The relative tightness of the building envelope is a factor in venting performance but a reasonably tight envelope does not necessarily contribute to spillage and a loose envelope does not guaranty reliable venting. The related features are the total mechanical exhaust capacity and how it is used.

balanced ventilation 

no ventilation system

exhaust-only ventilation

no large mechanical exhausts

moderate exhaust capacity

high volume mechanical exhausts

Note: This table is presented as a convenient summary of the lessons learned about reliable chimney venting. Please be aware that this condensed treatment of such complex issues cannot be accurately applied to all circumstances. Use the table as a reminder of the key issues involved. Where details or precision are required, refer to the main text of the manual.

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