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 . . .
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| Photo credit: Batchrocket.eu |
The entire unit is referred to as the core. Here's Dan's dried-in practice build on the front porch.
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| The sides are firebrick and the tops are ceramic fiber board. |
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.
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| We ordered ours from Dragon Technology. |
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.
Continued here ⇢ Progress on the Thermal Mass
Masonry Heater Project © August 2024
15 comments:
Very interesting. The fresh air pipe has to be able to handle the heat of a wide-open burn chamber.
That's what most folks don't seem to understand about rocket stoves. It's the closing of the damper of a wood stove that generates most of the creosote and waste fuel as the flammable gasses slowly go up the chimney and like the moisture in the air condensing on that ice cold glass of water it sticks to the pipes.
You're not closing a damper; you're adding more fresh air to burn up those flammable gasses and thus heating up the thermal mass of the stove.
Thermal mass is the issue. I hope your gentleman has really strengthened the floor structure under that stove.
Michael, excellent explanation. It's puzzling to people because it's a completely different technology than fireplaces and conventional woodstoves.
The original occupant of this space was a fireplace. It was made when the house was built, with a base of brick and concrete built directly on the ground. When Dan built the alcove, he tore out the fireplace and chimney, and then used the same base, building upon it with more bricks and concrete. So that's what the masonry stove is going, with no wooden floor structure underneath to worry about.
I suspect there are geography affects at play here because I'm struck by how small your original soapstone firebox is compared to what would be needed to heat a house up here, much further north. Even with house air being added to our wood burning insert, it can be extremely difficult at times to get the draft going.
Part of our struggles of starting a fire and drafting has to do with a large column of heavy cold air residing in the flue (25 feet tall) that must be lifted up and out by hot air before a draft can form. This can be really challenging for us in minus 10 F temperatures. I typically open up the flue to let all that cold air rush into the house first for a few minutes and mix with some escaping warmer house air before then attempting to start a fire and get the draft going. I think this lightens the weight of the air since it is now less dense. In the warmer months of autumn and spring, maybe similar to your winters, it isn't hard to do this. In the depts of winter, it requires a heavy jacket until the draft gets established and the basement heats back up to a reasonable temperature.
Ed, you have an intriguing problem! Sounds like your chimney is plenty tall enough to get a good draft; how high above the roof peak does it extend? We had backdraft problems with ours until we got it 3 feet above the roof line. I know some people preheat the flue with a candle, small fire, or heating device to help get the draft going. But I'm not an expert. Surely there must be an easier answer!
As far as our soapstone stove, we chose the model sized according to our square footage. It has worked pretty well, except for heating the kitchen, which is at the other end of the house. But we have the wood cookstove for that. Sizing is similar with the masonry stove, except we already have the six-inch chimney, so the stove had to be sized to match that. Larger stoves are built for larger spaces. Or smaller for smaller spaces.
It is three feet, probably a bit more above the peak. It drafts fine in use, just not starting a fire on really cold days. I have tried candles and such but it is an exercise in patience. Leaving the firebox door open and just letting the cold air down and warm house air up is a lot faster though probably not as efficient.
Ed, it sounds like you're doing all the recommended things. Do you have to do many cold starts? I wonder if using a fan to push the cold air out of the chimney would help. At least you've worked out a solution, even if it doesn't seem like the best option.
I suggest a blower and some steel duct to get the chimney air moving. Or a small stick fire and a small fan to help the smoke moving up the chimney.
Worked for my Crimean oven.
Anonymous, you mean for Ed? I agree, those are good suggestions.
Sorry for some reason this AM same computer I was anon and now again Michael.
Yes for Ed. The pipe is to get the blower air past the starting fire to get the chimney air moving properly.
Michael, great idea!
Fascinating Leigh, thank you! I assume the bricks are especially designed for this sort of thing?
Thanks TB. The bricks aren't special for this, except, of course, firebrick for the firebox. Ours are actually all bricks we've already had on hand. The mortar for the firebox needs to be refractory cement, which is heat rated for fire.
What a beautiful door!
RT,I think it will make the stove. Can't wait!
Since we only burn wood for ambiance, every fire is essentially a cold start. But the difficulty only occurs on the coldest of days, generally when temps are below 0 F.
We have a wood burning insert due to the fireplace being butchered by a previous owner to fit in a gas fireplace. Due to it being an insert, there is no direct shot up the flue to point a fan. Heat goes through a baffle of sorts that leads to the flue. This is part of our draft problem. At a previous house we had a regular inefficient fireplace and I could just hold a piece of burning newspaper to the bottom of the flue and get it drafting. But that flue was probably ten feet shorter than our current one too which also helped.
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