Masonry Heater Clearance to Combustibles

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I was invited to examine a masonry heater that was built by another mason in a town one hour north of us.  One of the wooden studs immediately behind the heater, in the wall adjoining their bedroom, had been heated to the point that it had at least reached a smoldering point inside the wall.  They smelled smoke and called the fire department who came promptly and tore the drywall off from the bedroom side and were able to extinguish the smoldering wall framing with relative ease compared to the possible consequences had such a fire gotten out of control. 

I am a professional heater mason with 10 years of experience in the trade and have been a full voting member of the Masonry Heater Association of North America for as long.  I underwent mentorship in my training and continue to maintain ties with my colleagues to discuss best practices. I have been a part of over 25 masonry heaters installations.

While the stone work on the masonry heater in question is impressive and clearly the work of a craftsperson, the details of how the functional parts of the masonry heater was built raise several points of concern. 

The most important oversight was a complete lack of respect to necessary clearances to combustibles.  The proper clearance to combustibles behind a masonry heater placed in front of a wall made of combustible material is 4” (refer to attached excerpt from ASTM standard for site-built heaters).  The 4” cavity should be open to air movement to prevent static heat from building up by providing vents low and high to provide for natural convection. In order to remedy this particular situation, all the walls adjoining the heater would have to be re-built such that they respect this clearance.  Also see ASTM standard for a full list of clearances, including vertical clearances, 2” clearance to combustibles for framing around heater foundation, as well as for “wing walls” in the case of masonry which abuts combustibles.

I also observed cracks in the masonry facing near the smoldered framing.  Cracks appear in masonry subject to relatively high temperatures because the materials expand when heated.  Apparently this heater had previously experienced enough cracking in the mortar joints between the stone on the front that the mason had to tear off a good portion of the stone work retroactively and installed a ceramic wool/paper gasket on a portion of the heater before rebuilding the stone work.

In order to prevent such issues, a well-built masonry heater has a refractory “core” and a masonry “facing” which are separated by an expansion joint.  This allows the inside of the heater to expand and contract where it is hottest and maintain the more decorative facing.  Even with these precautions, heaters may experience hairline cracks in the facing which do not impair the operation of the heater. 

While it is likely that the existing cracks in this heater are not sufficient to affect the function of the heater, they should be addressed before the wall cavity is closed back up.  In order to remedy this situation, all mortar slush should be chiseled to a flat plane, a full sheet covering of ¼” ceramic paper should be placed against the back of the heater and then secured with sheets of galvanized sheet metal attached with masonry anchors.  This remedy does not reduce required clearance to combustibles and should only be considered as a retroactive patch.  In new construction, the masonry facing should not be mortared directly to the refractory core.

An additional area of concern includes insufficient inpection to have observed such oversights before they caused a problem.  A safe installation requires competence at the point of design, construction, and inspection.  Plans for a heater installation should address building safety including required clearances to combustibles.  A mason who enters into a contract to build a masonry heater should have sufficient education and training to address all safety concerns.  An inspector called to sign off on a heater installation should be aware of the code for site-built heaters.  The current code for site-built heaters is initially prescribed by Chapter 10 of the IRC and further specified by the ASTM standard E1602.

Masonry heaters are efficient, clean-burning solid fuel appliances which contribute a high level of warmth and comfort.  They are a solution to meeting sustainable residential heating (and cooking) needs.  It is important to distinguish this instance of a heater that does not meet best practices with those which are well-built in order to preserve perception of their safety in normal conditions.

-Max Edleson

Design Development for a Tulikivi Masonry Heater

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This is a chronicle of design development for a custom Tulikivi masonry heater that shows the process of communication between us, our clients, Tulikivi, and Tulikivi’s design team.

After having some initial conversations with the clients by phone and obtaining a .DFX file of the floor plan from the architect, I made the following rough sketch proposals…


The clients were leaning in the direction of the TTU 2700.  They were interested in adding some benches so I made the following more specific drawing that included benches and a proposal for how they could store their wood…  

Upon making a site visit and meeting with the clients and the home builder at the rough framing stage, an issue with the way the chimney placement was dictated by a minimum hallway clearance of 36″ on the second floor was identified.  This caused me to change the proposal from a top-venting unit to a bottom-venting unit with a side chimney extension so that we could shift the chimney location and meet the necessary clearance on the second floor while maintaining the relationship between the heater and the living room on the first floor. Hence…This pivot was facilitated by a design precedent by Warmstone Fireplaces & Design, which was available to us via the Tulikivi dealer intranet….

I flipped the image horizontally in a graphics program to show the clients what I had in mind…

…we honed some details together and I sent our request to the Tulikivi design team in Finland via our North American representative Boris Kukolj…
The design team led by Jari Murto responded with the following…

We liked it all except for one small detail:They made the edit we were looking for:

….and now we are looking forward to receiving the unit for this project which is slated to built in March 2019.

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Wintergreen Heater Core Design

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These are the kinds of drawings that I wish I had had more exposure/access to when I was starting out in masonry heater building.

It still takes a somewhat trained eye to understand the overall flow of gases through these sections.  Further refinement and artistry would render them more understandable still.  Basically, wood is burned very efficiently in the firebox, goes up and around a “white” oven and then down and around into a heated bench before coming back up and exiting through the chimney.  A bi-pass damper will be installed in order to be able to prime the flue.

It has long time been a desire of mine to contribute to sustainability and ultimately to peace and happiness.  I am hoping to put more energy into communicating through writing and drawings, and also into sharing both the artistic and emotional parts of this process.  Subscribing to Firespeaking’s mailing list and/or following us on Facebook are two good ways to stay in touch and show your support!

Decorative Brick & Tile Masonry Heater

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This is an initial photo report of a recently completed masonry heater with inline wood-fired oven built in Grants Pass, OR.  The heater uses Balmoral brick and tile details that the client provided, along with polished concrete details made in our shop.

The core:

The craftsman: Max Edleson of Firespeaking and Jeremiah Church of Boreal Heat!    

Detail of Polished Concrete:

The Rocket-Fired Barrel Oven

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Here is an exciting innovation combining the advantages of a rocket-fired combustion box with the practicality and efficiency of a wood-fired barrel oven.  This idea was developed by Flemming Abrahamsson of Fornyet Energi in Denmark.  See also his brilliant Rocket-Fired Griddle Oven Design, which further combines a cooktop griddle to the configuration.

This is an important evolution in the barrel oven concept because it both improves combustion and does away with the need for the additional hardware beyond the barrel of a firebox door, grate and ash drawer.  Notice the very simple yet clever sections sketched at 1 and 2 which demonstrate how the flue path is baffled to improve heat exchange.

Flemming has agreed to provide these drawings and the photos of a build sequence for free in order to maximize their potential reach.  You can contact Flemming for project specific design/consultation and also for barrel oven hardware in Europe.