08/04
27847 Rev H
29
HST-38 SERIES INSTALLATION INSTRUCTIONS
Figure 27 - Flue Draft
Flue draft is measured as negative pressure in the
chimney. The amount of negative pressure determines
how strong the draft is. The draft is important because
it draws the combustion air into the fireplace and pulls
the smoke out of the chimney.
There are three basic criteria essential in establishing
and maintaining flue draft:
•
Availability of combustion air.
•
Heat generated from the fire.
•
Diameter and height of the flue system.
These three factors work together as a system to create
the flue draft. Increasing or decreasing any one of them
will affect the other two and thus change the amount
of draft in the entire system. See Figure 27.
If the fire is hard to start and smoke spills out of the
fireplace, or you find it difficult to establish and maintain
a moderately high burn rate, then the flue draft is too
low and corrective measures must be taken.
Be sure you have air available for combustion and
that your firewood is dry and well seasoned. Build
your fires properly and according to the instructions
given in operating instructions, “Starting a Fire”. Be
sure your flue system is installed correctly and that it
is the proper diameter and height. Check for the
following:
•
All chimney sections are properly installed and the
joints are tightly sealed.
•
The chimney is clean and free of creosote buildup.
•
Make sure overhanging trees and branches are
cut back within ten feet of the top of the chimney.
•
Ensure the chimney cap is clean and free of any
buildup of soot or creosote.
If you still suspect you have a low draft problem it may
be necessary to increase the volume of air in your flue
system. Since the diameter of your flue system is
matched with the size of the flue collar and should not
be changed, then the height of the system must be
increased. Add chimney sections a little at a time until
the draft improves.
In some cases, regardless of what you do, it can still
be difficult to establish the proper flue draft. This is
especially evident when using an exterior factory built
chimney or exterior masonry chimney. Try placing
several sheets of crumpled paper on top of your kindling
as close to the flue outlet of the fireplace as possible.
Light this paper first then the paper under the kindling.
The heat generated from the rapidly burning paper on
top of the kindling should help get the draft
established.
Still other factors can affect how well your flue system
performs. Neighboring structures, high winds, tall
trees, even hillsides can affect air currents around
the chimney. Well designed chimney caps are
available that can help. Your fireplace dealer is the
local expert in your area. He or she can usually make
suggestions or discover problems that can be easily
corrected allowing your fireplace to operate correctly
as it has been designed, providing safe and
economical heat for your home.