Masonry Heater Part III: A Finished Finnish

My 22" finnish contraflow heater core is finished. It probably isn't precision perfect, but considering I had never laid a brick before in my life; I'm pretty pleased with the outcome. The pictures below are my course by course outline (save for a couple courses I forgot to photograph).

I decided after I had the corners of the transition header in place, I would mortar four of the bricks together before I put them up. It worked fairly well except I was getting to the bottom of my mortar and a few hard chucks of mortar threw a couple bricks off kilter. Lesson learned.

Header bricks in place.

I only did a course by course of one side channel since they are both the same. This is the left side channel finished. It is worth mentioning that I used mineral wool on one side and ceramic paper on the other side. There were pros and cons of each, but overall I would probably use the ceramic paper again if it is available.

Course by course of the right side channel (ceramic wool gasket - 2 thicknesses of 1/8" paper).

This is a picture of the bond pattern. Each course contained a 4 3/4", 3 1/8", 3 3/8" and 2 full bricks.

1/8" ceramic paper gasket on the top. The pre cast capping slabs go on next. We did get them on with only two people but a third person would have been nice.

Capping slabs in place. Voila! Remember when I said that a couple transition header bricks got a little kittywampus. Those rogue bricks made it so the slabs had some air gaps in some locations. I remedied the situation by stuffing in chunks of mineral wool.

You can see the unevenness of the header bricks in this picture.

The ratchet strap was put on so the side channels wouldn't move while we were putting on the capping slabs. The side channels sit dry up against the ceramic paper gasket.

Now I just need to figure out the chimney and put the facing on. Excitement ensues!

Masonry Heater Part II

I wanted to finish the center channel of the masonry heater today before I updated the blog, but I ran out of time and almost out of mortar. I decided to do the update anyway.

Inner firebox finished.

Lintel in place. The slots underneath will hold firebrick splits to form a heat shield.

My inner firebox ended up a little shorter than my outer firebox so I used some leftover castable refractory cement to bring it up to the same level.

The 2 inch gaps form a bypass channel that runs underneath the bake oven floor.

Bake oven floor is set in a bed of refractory mortar.

The back wall is gasketed with 1/8 inch ceramic paper and it sits dry.

Firebrick walls and back wall of the bake oven is gasketed with 1/8 inch ceramic paper.

Top of bake oven in place.

Channel behind the bake oven.

Flared rear channel tilted up so you can see the cuts.

Flare bricks in place.

In order to bring the front and back up to the same level we needed to rip 5/8 inch off the top course of the front bricks.

This is the start of the angled transition header that holds the capping slabs.

This is a view from the front.

Once the angled transition header is finished we will start to lay up the side channels.

Masonry Heater Part I

Our latest project is the masonry heater. I've been excited to start building this monument since day one. We are building a 22" finnish contraflow. If you are unfamiliar with masonry heaters, think of it as a fireplace with a lot of mass and a flue that winds about a bit before leaving out the roof of your house. The mass is for heat storage and the winding flue is for gathering up all that heat. It is one of the most efficient ways of burning wood.

Here is our account thus far. The first thing we did was to make some molds in order to pre-cast a few pieces of the heater. A masonry heater burns really hot. So to make the pre-cast pieces we needed to purchase what is called castable refractory cement. When all is said and done it is basically a concrete with a very high heat tolerance.

We ended up using two different types of castable refractory - Harbison Walker KS-4 and Alsey Hi-cast. I prefer the Alsey. It seemed much more workable.

The molds were made out of lumber, but we had to take extra care to make them water tight. Castable refractory (and common concrete) get their strength by curing; the slower the better. So in order to make our molds water tight we wrapped the interior surface with house wrap tape, stapled on plastic and mounted them to a plywood sheet. A side note, use heavier plastic than painters plastic. It tears too easy.

After they were mounted, a bead of silicone was applied to all the seams.

We had to pour the slabs in my garage since the castable refractory is temperature sensitive. This was a very messy experience.

After the refractory was poured into each mold, we vibrated it by putting a piece of scrap lumber up against the mold and beating on it with a hammer drill. This proved very effective for those of us with limited tools.

After each piece was poured, they were covered with plastic to cure.

Three bake oven slabs, two capping slabs and one hearth slab.

This is the base slab laid in place in a bed of common mortar. The base slab is not refractory. It is 3 parts vermiculite to 1 part portland cement. FYI vermiculite concrete cures extremely slow.

The following photos are a course by course synopsis of the progress so far. All joints are thin and laid with refractory cement - Meeco's Red Devil (because of availability)

The space in the front middle is the where the ash box is located. The hole in the back is where the flue connection is. The two side holes are clean outs for the side flue channels.

This picture shows a port I left open in order to try to have an under fire air source to speed up the hot coals phase of the fire. It is something I read about here, but wasn't included in the plans I purchased here.

Hearth floor with ash drop.

Firebox outer shell.

Expansion joint in the rear of firebox.

Firebox inner shell.

This is the progress so far. When I am completely done, I plan to compile the total photo sequence into one link to try to make things easier for anyone building the same heater.