Heater Project Diaries

Masonry Heater Firebox Design

By March 11, 2010 3 Comments

One of the great masonry heater designers and researchers of the mid twentieth century was a Finnish researcher named Asp. When I (Albie) began my journeys to Finland in the late seventies, my Helsinki, Finland friend, Heikki Hyytiainen, made mention of Asp’s work and we were able to find one of his books in Finnish which I now have. Among other things, Asp did a lot of work with the introduction of what masonry heater and stove designers have often called secondary air, or air which does not come through the grate, but is instead introduced to the fire through the walls or the throat of the firebox over the fire. When Albie organized the first hands-on Finnish fireplace construction workshop in North America in the late seventies, Heikki traveled to Lincolnville, Maine to lead the workshop. As I recall, about sixteen interested masons and others from the U.S. and Canada came to the heater workshop, many of them on motorcycles. On site we discussed the issue of secondary air and introduced in that first heater a very simple system of over fire air with a couple small pipes that brought in air over the fire, but we had no clear or scientific knowledge about what we were doing.

The masonry heater we built in Lincolnville worked well for about twenty years. Our friend Chris Chatfield called one day and said that he was doubling the size of his little Cape and wanted a second heater built and while we were there could we look at his original heater. As we built the new heater with an Albiecore in the expanded section of the house, we also assigned one mason to gently open up and repair the firebox of the original heater. Both the rebuilt heater and the new heater are now being used in the Lincolnville home. The metal tubes bringing in the secondary air had plugged or eroded so we took them out during the rebuild.

masonry heater firebox

Since that very first heater, we have tried to continually improve our system, especially in the firebox and over the years developed the Albiecore concept based on the idea of making all the firebox and fire tube components easily repairable and replaceable. Recently we increased the number of floor components from four pieces to eight so that the floor could be easily replaced at any time. And most recently we have begun to do prototype firebox work for certain clients who wanted to be willing experimenters with us using over fire air.

About ten years ago I was in Finland visiting Heikki and at a mason’s supply and building center, we attended an open house where several masons had built up samples of their masonry heater work for new house customers to come in and see. Customers were meeting the masons and picking both craftsmen and designs that they liked. I noticed that a number of the fireboxes offered very simple over fire air supply ideas. All were quite small.

About fifteen years ago, Heikki started experimenting in his shop with a firebox design idea to create a cassette that would be an insert that would feature the cleanest possible burn. His design work matured over fifteen years and had over 1500 computer monitored test burns in his system as it evolved. The system now has a worldwide patent and is in the prototype pre-production stage. We expect to receive two of his prototype cassettes shortly and one of them will be tested for emissions at an American West Coast lab run by Ben Myren. A second unit will be shipped to the annual MHA event in April at Wildacres in Little Switzerland and Heikki and others will build up a heater using his cassette and do some test burns on it. Heikki will also lecture to the entire group on his research work. I will then take the cassette home and install it sometime soon in a test unit here or in a permanent wood burning heater, perhaps a soapstone heater that we are working with.

fetze masonry heater

A few years ago, I went to Holland on my way to visit our wood fired oven partners in France, accompanied by my daughter Anna. We wanted to see the work and shop of a remarkable masonry heater designer in Holland called Fetze Techlar. He had just expanded his little shop into a rented disused creamery and had a production line set up being run by his son. Fetze was making a number of stacking modular heater lifts in a variety of ceramic colors, which you can see on the left. All the colors were standard mortar colors ranging from cream to red to blue and he was using a castable refractory system and molds that allowed him to cast beautifully colored refractory elements that looked like fired ceramics. Aside from having designed a system that used no mortar, Fetze was also experimenting with a second or third generation of over fire air ports which was produced in a heater at his son’s home that we visited, one of the cleanest looking and beautiful burns that I have ever seen.

construction of scott's heater firebox

At Maine Wood Heat Co., Scott Barden needed to build a small heater for his own home, so inspired by photos of some of Fetze’s work, he went about designing and building a soapstone veneered masonry heater with firebrick and castable refractory guts which included secondary air over fire air ports. He has since built a few more of his heaters for clients of ours and each have included these over fire air ports. Once the fire is underway, the door drafts can be closed and then all the air comes to the fire at the level above the logs where the gases are hot and rich and looking for oxygen. The burn is quite clean. The burn time is extended and the heat absorption is extended as well.

When I go to babysit my grandchildren at Scott and Cate’s home in the winter months, we always light the fire together and are mesmerized by the beauty and warmth of the heater and its over fire burn. Sometimes it is hard to get a clear view of the fire, because June Bug, their dog, sits directly in front of the door for a half hour at a time looking at the fire and soaking up the warmth.

scotts soapstone veneer masonry heater

We will be building a Scott design custom soapstone veneered heater in April for our friend Marty Cain before Wildacres. We are building the unit as a permanent unit in Marty’s Black Mountain cottage home. We have cut both the core and the veneer here in Maine, and prepped it all as a kit for a three-day hands-on workshop which we estimate will cost about $300.00 to attend. For this unit, based on Scott’s heater, we have again worked with over fire air and we have quadrupled the amount of air we are bringing in over the fire comparing this unit to Scott’s heater in his home. Chris Pelletier at our shop designed a very nice pair of adapted small soot doors as the secondary air draft controls. This workshop will be documented and presented in a future blog.

Last year (2009) we built two prototype full-sized fireboxes in heaters for clients who wanted masonry heaters and who were willing to experiment with us to try to achieve a cleaner burn. Both heaters used a similar system of bringing in air through the ash box door, closing off the grate, except for ash dumping and removal, and bringing combustion air in under the firebox floor and between the inner and outer walls of the firebox to be directed back into the firebox higher up in a series of small ports spread around the firebox. Both clients have kept in close touch with us and report good satisfaction and pleasure from their over draft fireboxes. The doors stay cleaner. The fire burns longer and the upper chamber oven temperatures are higher. The couple in Strong, Maine are routinely baking wood fired pizzas in their upper chamber oven from the accumulated heat of the fire.

brick masonry heater

In New Zealand, with an all-handmade masonry heater construction workshop, Albie decided to build with his students another version of a firebox with lots of over fire air as an option (the same firebox can be converted at any time to a through-the-grate air delivery firebox). We measured the grate that Sampsa had bought and determined that we had twenty-four square inches or more of opening in the grate through which air could pass so we decided to design a secondary air over fire air delivery system of about twenty-four inches of air. This would be significantly larger volumes of air than our first experiment in Lincolnville, Maine.

firebox grate

The firebrick that Sampsa had purchased was the same length and width dimension as standard American firebrick (4.5″ by 9″) but the thickness of the large firebrick was 75mm (2 l5/l6″) and the thickness of the thinner firebrick was 1 3/8 inch or 35mm. We had lots of height in the cathedral ceiling room so we decided to build a three brick thick firebox floor. The first layer of thick firebrick bridged over the rear channel and along the sides were cut at a skewed angle to allow the expanded room we normally leave at the base of the heat exchange channels to keep “Mr. Smoke” from getting his shoulders stuck as he makes the difficult turn upside down to enter the base channel and find his way in the dark to the chimney.

The next layer of firebrick was the 35mm brick and with this we created a sunburst pattern so that arranged around the two sides and the rear of the firegrate opening we were able to lay out eight three inch wide channels or “rays” leading to the side walls and rear wall. We made the firebox walls out of two layers of firebrick turned on edge and cut slots into the inner layer of thick firebrick so that the three-inch channels extended vertically much of the way up the firebox wall. Opposite these eight three inch channels we made three stacking one inch square openings for a total of twenty-four square inches of openings around the two sides and rear of the firebox. As you can imagine, cutting this system was very labor intensive and required a lot of patience but we had very gifted craftsmen on the site able to handle any challenge.

sunburst

The third layer of firebrick we again made out of the 785mm thick stock and completely covered the floor. We worried a little bit about ash getting down into the channels or rays and I ultimately decided to have a local welding shop (locally called an engineering shop) make up a plate that would just cover the second course of floor firebrick with a hole aligned over the grate opening. We made some rather hasty measures and the plate came back in a timely manner but too wide. We made more hasty measures and the plate came back the correct width but still too long. Finally, on a third try with the friendly help of the remarkable mechanic neighbor, Lionel, we were able to get the extra length on the front cut off for a perfect fit. With the plate in place we were able to lay down a solid third course of firebrick on the plate guaranteeing that nothing could get into our channels or destabilize the floor. We assumed that the somewhat cooled firebox floor plate (incoming air) and the firebrick on top of it, would keep the metal from warping.

As we laid up the inner firebrick wall against the open channels we kept them clear at the top with little paper covers and reamed them out before capping them with a little sponge screwed to a stick. We did not fire the heater during the workshop, nor did stucco get put on the heater during the workshop, nor was a chimney for the brick to metal transition through the roof available or fabricated during our stay. This will all be documented and reported on in later stories. For the next New Zealand blog I will discuss the heat exchanger that we installed at the top of the heater and following that will describe the remarkable experience of acquiring and then milling on site several thousand pounds of soapstone that arrived in raw form and ended up on the heater sized, polished and fitted for all the trim that Sampsa has desired.

sampsa masonry heater firebox

The photos accompanying this blog show the firebox design and construction process. When the heater is fully installed, the unit will be monitored and tested for efficiency and emissions and we will report those results to you as well. It feels to us that 2010 will be a year in which a lot of information on over fire air delivery systems and principles will be discussed and worked with. As with any product, there is always room for improvement and we are happy to keep doing our small part to push this design envelope.

Join the discussion 3 Comments

  • Jess Tremaine says:

    Follow many of your reports, it is February 2011, are there any continuations?

  • Denny Duello says:

    Am designing a metal firebox for a stone fireplace with a metal enclosed outer chamber through which air is to be circulated via convection or electric fan. Am very interested in placing the primary and secondary combustion air inlets (orifices) optimally. I’ll be happy to pay for patented info to design this best-shot firebox in this last house I’ll ever build. Any help would be appreciated. Thanks from Iowa.

    • albie barden says:

      The heater was finished with stucco and is working well. Sampsa married a woman from Finland and she joined him in New Zealand. They have two children. Our photographer/videographer followed up the heater experience with a trip o the US. He came with his bicycle as luggage and unpacked it at the airport in Portland Maine and rode his bicycle from Portland to Norridgewock seventy miles and stayed with us and went to the Kneading Conference as a participant and also visited some bakers and bakeries in Maine and Quebec before flying home. What a treat.

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