Last time, I explained a bit about masonry stoves, how they work, and how they've evolved over the centuries (
Masonry Heaters: A Bit of a History). Our stove design is based on the wisdom and experience of the latest technology.
The two main components of the stove are the thermal mass and the firebox. Typically, the thermal mass is built as a bench (often used for seating) or as a "bell" (big hollow box). We have no place in our living room for a bench, so a bell will reside in
the alcove Dan built for our soapstone woodstove years ago.
Above the brick the walls are cement board, with a specific air gap between the alcove walls and the walls behind. The vents at the top of the alcove serve to vent warm air. This is the ready-made space we have for the new stove.
For the firebox, we chose what's called a double shoebox designed by Peter van den Berg. It's useful for smaller spaces, like we've got. Here's a cutaway . . .
The entire unit is referred to as the core. Here's Dan's dried-in practice build on the front porch.
So what makes this firebox more efficient? And how does it work without filling the house with smoke because of the retained exhaust pathway? The answers are in the way it's designed.
The narrow opening at the back of the firebox is called the port. By slowing the escape of the heat and smoke from the firebox, it forces the exhaust gases to be burned. It's these gases that deposit soot in the chimney as they cool on their way out. In this design, the gases and ash are converted to heat energy before they exit the firebox. So the fire burns more completely and more cleanly. It's the same principle as
the catalytic combustor on our soapstone stove. You can read a more detailed explanation at Peter van den Berg's
website, here.
Besides cleaner, safer smoke, people report a decrease in wood fuel consumption by up to 75-80%. What's not to like about that?
It must be noted that the dimensions of the design are important: firebox, port, thermal mass, and chimney must all be sized properly. A sizing chart can be found at Peter's website.
The other question is, given the convoluted chimney path, how do we get enough draft to direct the smoke out the chimney instead of into the room? The answer is with two air intakes.
The first is in the firebox door.
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Photo credit: Firespeaking.com. This is the door we
purchased and which you will see in future blog posts. |
The second is the secondary air tube.
It fits in the channel in the firebox,
. . . and will be under the door. It delivers continuous fresh air to the port in the back of the firebox, supplying steady oxygen to keep the fire hot enough to burn the exhaust gases. It will be cut to fit after the core is built in the bell and the door installed.
Progress so far has been building the bell. Following are a few photos I took over the past several weeks.
If you look carefully, you can see where the chimney will exit the bell. It's placed low in the bell where the coolest air is.
We're going to re-use the chimney pipe from the soapstone stove.
The sides are being constructed first because the space is tight and Dan needs working room for the top of the heater. The core will be added after that.
The other question was about cleaning out the bell. For that, a clean-out door will be added later (something like
this), roughly where I made the red square below, next to the chimney flue.
We'll access it from the
adjacent bedroom (where my big floor loom now resides. Photos at that link).
Dan cleans out our chimneys every autumn with a chimney brush, and then uses his shop vac to vacuum out any residue that falls to the bottom. Even with our soapstone stove there has been very little soot build-up. The clean-out door is large enough to accommodate the nozzle and hose of the shop vac. The fire box, of course, is swept out with a whisk broom.
