90 Degree Thru Wall Chimneys
Class A Chimney 90 Degree Through The Wall Exterior Installation
A 90 degree through the wall installation is one where the stove pipe elbows out through a wall at 90 degrees. A wall thimble is installed to minimize radiant heat transfer of the Class A chimney, which connects to a tee on the exterior of the structure. The tee is supported by a shelf which carries a certain amount of Class A chimney. In multi-story applications, additional support is required.
- A solution for situations where there is no other option but installing the chimney system on the exterior of the structure due to layout, design, construction restrictions, or obstructions.
- On commercial applications, or applications where a central heating system is being utilized which does not allow the chimney to cool down, this is a viable chimney system.
- Expensive. 90 degree installations require additional components like a wall thimble, tee, supports, chase enclosure, additional labor, and more materials.
- Exterior chimneys are exposed to the elements. When the chimney cools down, it will not draft until enough heat is applied to force the cold air coming down it, back up again.
- Exterior chimneys require more maintenance as they don’t get nearly as hot as an interior chimney. The temperature variations create condensation inside the chimney, especially in the wall thimble and tee sections. Condensation + wood smoke = creosote build up, which can burn like rocket fuel.
Note: Exterior chimneys generally need to be warmed up to create a draft before a match is struck. We recommend all exterior chimneys utilize a draw collar if possible to pre-heat the chimney so it will draft instead of fill the structure with smoke. A cold chimney will not draft.
With 90 Degree Through the Wall installations, you first need to consider where the exterior chimney will exit the structure. What is the exterior wall constructed of that the wall thimble will install into? If it is framed, will you clear the studs or will you have to cut a stud? Is it a load bearing wall, or will it requiring additional framing? Is there anything on the exterior such as windows, powerlines, or overhangs where the chimney will attach to the side of the structure? If you live in an area where you receive heavy snowfalls, where will the snow from the roof slide too? If your chimney is on the downhill side, you should consider another location or use a snow break device to keep the chimney from being knocked over.
On multi-story applications, make sure you have secure places to mount the support brackets to carry the weight of the chimney, which can be several hundred pounds. Your chimney must be a minimum of 2′ higher than anything within 10′ of it.
Wall Thimble Installation. The wall thimble is designed to prevent heat transfer into the combustible exterior wall. It also stabilizes the chimney, provides a transition between stove pipe and Class A chimney, and seals out the exterior weather. The wall thimble is designed to fit between wall studs which are 16″ OC. It is one of the most critical components in the assembly and must be installed properly. If you are passing through a non-combustible wall (like concrete), you will need a core driller to punch out a large hole. If the wall is cinder block or brick, you can carefully remove the material by drilling small holes in the shape of the diameter of the chimney, then remove the material with a hammer and chisel. In this type of installation, your wall thimble can be a steel tube which is mortared into the masonry wall.
Please note: In the event of a chimney fire, heat will build faster in the horizontal sections of a 90 degree installation than in the vertical section. Keep this in mind, and take measures to mitigate any heat transfer into combustible materials.
Chimney Tee. The tee is the intersection between horizontal and vertical aspects of a 90 degree through the wall installation. This is the place where the most turbulence inside the chimney system occurs: Between the 90 degree stove pipe elbow, and the 90 degree bend inside the tee. This is also the place where warm air meets cold air. The indoor stove pipe will conduct ambient heat, while the exterior chimney will be cold and allow cold air to travel down when the appliance is not burning, producing condensation. This is also an area that will develop the highest concentrations of creosote. Routine maintenance of this section is strongly advised. Proper maintenance requires removal of the stove pipe from the inside, removing the clean out cap on the tee, and sweeping the horizontal section inside the wall so that it does not become plugged, or worse yet, ignite.
Another note: On the bottom of the tee is a clean out cap. Remove it to gain access to the chimney from the bottom. Keep this in mind as you install your chimney to provide as much access as possible. It will be far easier to clean any chimney installation from the ground in the winter, rather than climbing a ladder.
Tee Support. Tee support is another very important component in a 90 degree installation, as it supports the weight of the chimney resting on it, which can be a few hundred pounds. The legs of the tee support must be over studs inside the wall, unless the exterior sheathing is greater than 1.5″. If the legs don’t line up over the studs and the sheathing is less than 1.5″ thick, you should open the wall cavity and add the necessary framing to carry the load.
Wall Brackets. Wall brackets are used to stabilize the chimney side to side, and some are also capable of carrying a vertical load. In multi-story applications, additional vertical support is required in addition to the tee support. These should be attached at the rim joist at each story, which is essentially floor level, as this area offers plenty of material to attach to.
Soffits and Overhangs. Soffets and overhangs extend the roof line past the side of the structure. If yours do not extend further than 14″, you can project past the overhangs without having to cut or modify the fascia, gutters, or soffits. Overhangs that extend 24″ or greater are fairly easy to penetrate, without dealing with fascias or gutters. Some people choose to use offsets here and extend past the overhang, but this creates cleaning and support issues because there is nothing to support the chimney above the offsets.
Extended Chimney Support Brackets. Your chimney system must terminate no lower than 2′ above anything within 10′. This can put the chimney pretty high above the fascia of the structure of a roof with a steep pitch. These brackets act like tripods and stabilize the chimney against high winds.