Wednesday, December 28, 2011

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.


  1. Jared,
    I'm still clearing my land of hardwood and staying inspired by your progress. Also plan on a masonry heater. Look forward to hearing how you like it once installed. Thanks for the inspiration.

  2. All that extra hardwood laying around just begs for a masonry heater. You will have to keep me posted.

  3. will do. Not sure how to contact except on here. Currently have a brush pile about 12 feet x 20 begging for a match. SO much to do before grading, footings, etc. Plans include a slipformed 1st story with timber frame 2nd. We have alot of poplar to mill to dimensional lumber for interior use.

    I have a question about insulating your walls. I see your walls are insulated on the inside. Living in a much more humid climate in the summer I wonder if that would change where the insulation should go? It can't go on the outside obviously. A big concern is condensation. I haven't asked Elpel because he's in arid Montana. No one experienced down here in slipform that I can find. Any books cover that?

    FYI, planning on going to the MHA annual meeting just south of us to learn masonry heater construction.

  4. I'm jealous that you are going to the MHA meeting.

    I don't have any books to recommend, but you might try to check with a builder in your area and ask them about ICF or even just concrete walls. The principles would be the same. Also if humidity is a huge concern - you could always try running a dehumidifier. What side of the wall do builders put the vapor barrier in your area? That would be a good place to start your research.

    I don't like to put my email address on the site, but if you wanted to post yours, I would send you my address. Once we have each others email addresses I can delete the comment off the blog.

  5. we are all waiting for thew next sequence!!!...what you do is beautiful..I'm doing something like also! may want to take look of it!...enjoy from Tuscany!..(P.S. don't forget to leave 1/2 inch of air betw the combustion chamber and the outside layer!!..otherwise some cracks may happen!..)


    1. Thanks! I can't wait for the next sequence either. Unfortunately we had to switch gears for a bit in order to get the wiring put into the walls. When that is done, I plan to put the facing on the masonry heater. Stay tuned!

  6. Great heater yet on the net! I'm preparing to build the same 22" heater from the MHA portfolio. Where did you get your doors and clean-outs? from Montana

  7. I got all my hardware from:

    Excellent customer service and the hardware is imported from Finland. I should be putting the facing on my heater in about a week from now. Just before the cold sets in! Yikes!

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