February 15, 2010
Heater Chimney Discussion with Doug Hren - from the Yahoo masonry heater list
Here are some comments on the recent discussion about flues. Clarifications by
knowledgeable people in the industry are welcomed.
If we are talking about properly built and operated masonry heaters and flue
systems, the UL-103HT (old terminology is Class A) or masonry flues should cause
no problems. If we digress into wood stoves, open hearth fireplaces and
improperly built and operated masonry heaters than you will run into the
problems discussed in earlier posts.
I have inspected masonry heater flue systems (metal and masonry) that have been
properly operated over a decade or more and there was really no reason for me to
be on the roof or have chimney brushes with me.
(1) Terminology-The class A terminology has been replaced in the US with
UL-103HT (Canada ULC-S629). It is important to use this rating when specifying
your chimney or you might not get what you expected.
(2) Interior, properly constructed, approved, clay lined masonry flues are
preferred for base vented masonry heaters if project allows for many reasons. a)
additional mass, (b) easier to incorporate damper (c) longevity (d) other....
(3) Try to avoid exterior flues, but if you must, use IsoKern (or some other
insulated masonry flue) or a UL-103HT system in an insulated chase
(4) I believe the airspace between a clay flue liner and the exterior masonry is
to create a mechanical break to prevent conduction of heat to the exterior
masonry. This serves 2 purposes: clay flue warms up faster, exterior masonry
exposed to cold temperatures does not come into direct contact with clay flues.
(5) Integrating dampers into UL-103HT systems can be a nightmare (as Tom
mentioned). When using UL-103HT systems, in all cases I try to avoid using T's
and opt for either an anchor plate with an IsoKern breach block or use a support
box for top vented heaters. Damper is integrated between heater and anchor plate
or support box.
(6) This is not a complete list and each project may differ but the typical
parts of a UL-103HT system from start to finish are:
- anchor plate, support box or T (depends on project, you only need one of the
- support box and T listed above may require additional UL-103HT listed parts
for flue connectors, wall passthroughs, end caps, support or other reasons
(always sketch your project and list each component using the manufacturers
-pipe sections and possibly offsets
-attic insulation shield and collar (very important and often overlooked)
- support brackets
- custom made top flashing or roof flashing from pipe manufacturer (depends on
- storm collar
- rain cap
-roof brace kit (if chimney extends 5' or more above roof line)
- ****in all cases follow manufacturers installation instructions and consult
with a competent installer of this type of flue system or someone with
experience, do not skimp on needed components or disregard clearances***
Masonry Heater Design House