Masonry Fireplace Construction Details - Part I
What's under all that pretty granite stone on my fireplace? You've seen smidgens of it being constructed, now let's get an up close and personal look.
What exactly a Rumford designed fireplace is, has been discussed here before, so we'll skip all of that. Go back through this thread or Google it for more background information:
https://www.rumford.com/articleRumford.html
What we're going to look at is how this masonry fireplace was actually constructed and the little nuances in that construction that will provide for an efficient, safe, trouble free and long life, masonry fireplace. Some of this is material is repetitive information but most of the pictures are new.
I'll start with the basic firebox. An overlooked design/construction factor is sizing the firebox mortar joints themselves. On the firebox, those joints are
much thinner than normal mortar joints. The firebrick itself is high temperature resistant and high temperature refractory cement was used in the construction. But to aid in making the firebox last longer before the joints breakdown, they are made very thin so the surface of the firebox is almost exclusively firebrick with very little of the joints showing.
The damper I used is an infinitely adjustable damper. That will allow for fine tuning of the damper opening when in use. The hotter the firebox, coals and flue are, the more closed the damper can be resulting in more heat output into the room. It will sit on top of the firebox as seen here. The damper is made of cast iron which can rust over time. It is literally buried within the fireplace during construction and should it ever rust out, it would be an
enormous and expensive undertaking to replace it, so I had my entire damper powder coated. That will give it an almost indefinite, useful life.
This is the backing block competed which surrounds the sides and back of the firebox.
Looking at the rear of the firebox you can see a 2" (5 cm) insulating air gap is used. The hottest area of the firebox is at the rear of it. Heat migrates out the backside and if there were combustible material behind it, over time it could ignite. To prevent that, behind my firebox we used only masonry material, the air gap, a shinny metal sheet to reflect heat back into the firebox, metal wall studs, fireproof insulation in the wall cavity and finally fireproof plywood on the outside of the wall which was then faced with exterior brick.
The space between the firebrick and the concrete block was
loosely filled with masonry rubble if you will. Pieces of brick, stone and hardened mortar were placed there. The reason this material was left loose was to allow for expansion and contraction of the firebrick. If that space behind the firebrick were filled and hard packed with mortar or cement, the firebrick would eventually crack from being restricted in movement.
Lastly, as seen, the backing block cores themselves were mortar filled. This added to the masonry mass of the fireplace, giving a place for heat to be absorbed and held until the fire is out and then, releasing heat slowly back into the room. This is why the entire fireplace is located within the heated envelope of the home. I don't lose any of the heat out the back or sides of the fireplace to the outdoors. It greatly enhances it's efficiency.
Here's a look at the blocking area behind the firebox once it was completed. On top of the loose fill material next to the firebrick, about a 1" top layer of mortar was placed to seal it off. Note where the damper will be placed, right over the top opening of the fire box.
Next is the start of the construction of the smoke shelf and smoke chamber. The damper in set on top of the firebox as previously seen. Then, 2 courses of ordinary brick are laid up around the opening at the top of the firebox. On top of the second course of brick, a secondary, steel angle lintel is laid. It's the grey angle iron Steve is seen installing above. The magenta colored line, running top to bottom behind Steve, is the plumb bob line that was used as a reference point when laying the stone corners.
Here's another view. The black object in the center is the damper and above it is the grey, secondary lintel. I had it powder coated as well. I'll explain what the fiberglass insulation was used for in a moment.
From the other side here's a good view and description of all the various parts. One thing to note is to leave clearance between the 2nd course of brick and the side of the damper blade. The damper blade needs to move freely and it needs to be removed when properly cleaning the flue.
Here a 3rd course of brick has been laid, starting the forming of the smoke chamber behind the damper. Now the reason for the small, little rolls of fiberglass insulation, placed on the ends of the damper and secondary lintel. They prevent mortar from getting into the ends of the metal damper and secondary lintel. Those metal pieces need room to expand from heat longitudinally so the fiberglass gives it a little cushion room for that expansion. If mortar gets in there, solidifies and tries to prevent the metal from expanding, the metal will expand from heat and crack the mortar joints that will eventually gravitate to the outer stone and crack those mortar joints.
This is an overview of the smoke chamber being completed using taper bricks on the 5th course of brick.
From the side...
...and as seen from the front. The course that is being pointed out; those bricks have been cut on a taper or wedge shape, to start the arch of the top of the smoke chamber. You can now easily see the need for the grey, secondary lintel. It is necessary to support the bricks that are above the front of the damper. The front of the damper slopes and cannot support any bricks above it, hence a secondary lintel is needed.
The first or primary lintel...
...is the one across the top of the face of the firebox opening.
Note that fiberglass was placed on the ends of it as well to provide room for longitudinal expansion of the metal.
That steel lintel supports the lintel stone as seen here before its installation. Using steel angle iron will help the lintel stone carry the weight of all the stone above it. Due to expansion and contraction from the heat of the firebox and the wide opening it spans, the lintel stone, without any additional support, would always be at risk of cracking.
So this concludes Part I of this exciting chapter of,
" What in the world does masonry fireplace construction have to do with Garage Journal?!!"
Thanks everyone. More information you can't live without is coming your way!
Thomas