08/04
31068 Rev E
3
MHST36D INSTALLATION INSTRUCTIONS
DESIGN AND INSTALLATION CONSIDERATIONS
When selecting a location for your woodburning fireplace, it is important to evaluate a number of considerations. Modern
construction techniques can create conditions that may not allow your chimney to draft properly. This may result in smoke
spillage from your fireplace, as well as cause other combustion appliances to operate incorrectly.
Tightly sealed construction is important for energy efficiency. Unfortunately, a great deal of effort has been directed to
tightening up sidewall construction, while considerably less attention has been paid to tightening upper portions of the
warm air envelope (insulated ceilings). This has increased the “Stack Effect”, a condition that increases the negative
pressure generated by the structure. This negative pressure will directly affect the drafting performance of a fireplace
chimney. To minimize the negative pressure generated by stack effect, make certain that all duct work installed in the
attic spaces is sealed airtight. Minimize the number of recessed light fixtures installed in the insulated ceiling, and use
sealed recessed light fixtures. Finally, make certain the whole house fans and attic access panels are tightly sealed.
These are important design considerations that must be observed during the design and construction stage of the
home.
If you desire to put a fireplace in your basement, we recommend that you consider a direct vent gas fireplace. Basements
always have a significant negative air pressure that causes the fireplace system to be more susceptible to smoke spillage
and cold flue back drafting. Since direct vent gas fireplaces are sealed, they are not affected by the negative pressure that
exists in basements.
Finally, woodburning fireplaces perform best when their chimney (roof termination) is located on the upper half of the roof,
especially when cathedral ceilings are present. Chimneys that are located on the lower half of the roof realize what is
known as “lazy flue” and will not draft as well as a chimney that is located in the upper portion of the roof. The reason for
this is that the stack effect generated by the overall height of the living spaces inside the house will exceed the draft
generated by the chimney system. If you desire to place a woodburning fireplace in a location where the termination cap
would be located on the lower half of a roof, such as on an outside wall at the base of a cathedral ceiling, we recommend
that you consider using a direct vent gas fireplace. This will assure the homeowner a fireplace that operates correctly.
These properties do not affect just your woodburning factory-built fireplace. They can cause any woodburning fireplace as
well as any conventionally vented (B-vent) gas appliance to operate improperly. Careful planning at this stage of your
project will ensure satisfaction with the operation of your fireplace once it is completed.
