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.