Installing a Soapstone Masonry Heater

If you’re looking for a beautiful, fuel efficient heat source that works even when the power is out, then a masonry heater is an excellent choice.

Here in Cascadia, we’re increasingly aware that a major earthquake will likely disrupt the power grid for an extended amount of time.  Therefore, when we recently remodeled our home we decided to compliment our existing electrical heating system with a wood fired masonry heater.

Why wood?  It’s low-tech, easy to store, renewable and locally available. We use the discarded slab wood from a local sawmill.  It’s easy to split and cut to length and makes for an efficient use of a waste product.

Why a masonry heater?  Unlike most wood burning appliances, masonry heaters are exceptionally clean burning and fuel efficient.  They use intelligent design rather than brute force to heat your home.  The principle is to burn a small hot fire quickly and then store the heat within stone or masonry.  The slow release of energy from the mass provides steady even heat for the duration of the day.  In practical terms, this means that one fire in the morning and evening will comfortably heat your home. The flywheel effect of heating your house intermittently conserves a lot of wood. Overall, we use less than 2 cords each year to heat our home … and we have a retrofitted 1940’s cottage with 2×4 walls. A modern home could be significantly more efficient.

The drawback to most masonry heaters is their size.  Some of them are positively huge!  Our house is relatively small and needed a solution that was scaled appropriately.  After a fair bit of research, we settled on the Tulikivi.  It’s a Finish design that utilizes soapstone as its thermal mass.  Soapstone is incredibly dense and can, therefore, store a vast amount of heat within a small footprint.  We contacted the west coast distributor:  Alaska Masonry Heat and they helped us fine-tune our selection.  Ultimately, we decided on the TU-1000T

Planning

As an architect, I enjoyed the planning process, but also found it challenging.  The fit was complex.  The fireplace was designed to center on the entry door but was also centered on an angled wall relative to the Living Room.  The flue pipe needed to precisely align with the second floor wall above, but steer clear of a structural beam passing through the area.  Finally, a ship’s ladder staggered through the cavity behind the fireplace to access the loft with a whole list of its own constraints.  Once everything was accounted for, there was about ½” of tolerance to make everything work. That meant the planning needed to be spot on.

The guides and resources available were a little hit and miss. For instance, in Europe, they use “Foam Glass” for their details (which isn’t available here in the US). Gradually I pieced together a set of materials from both metric and imperial sources, photos from other installations, and conversations with the distributor that helped me understand how everything was intended to go together.   

Design

We decided to elevate the fireplace about 6″ off the floor. That provided a nice visual base for the Tulikivi and allowed an even number of chimney surround blocks to meet the ceiling. It also aligned the bench tops with the stone coursing at the ergonomically correct height of ±18″ above the floor. The benches add some overflow seating to our modest living room and a “hot seat” if you want to get close to the stone on a cold night. They also serve to contain the wood storage beneath them.

Here’s a layout of the plan showing the fireplace in relationship with the hearth, benches, walls and the beam above. The stair has been omitted for clarity.

Here’s a partial section showing the transition from the foundation through the floor system upwards to the second floor.

Installation

Masonry heaters are heavy, so they need to be supported by a dedicated foundation or a carefully engineered solution. We located ours on the first floor where we were able to cut a hole in the floor and install a new CMU support structure and concrete footing in the crawlspace. Our intention is to have the unit survive a decent earthquake, so we didn’t skimp on reinforcing.

The Tulikivi kit is precisely machined, so it was important to get the base perfectly level. The conduit sticking up out of the corner provides power access to the interior of the unit for heating cables. The cables are an optional extra and allow the mass to function as an radiant heater during long absences. We wanted to have this option available to us, but would advise against it – as we’ve yet to use it.

Here’s what the project looked like prior to the arrival of the fireplace. As you can see, the concrete slab is in and the conduit has been trimmed flush to the surface. The chimney flue is in and ready for connection at the top of the unit.

The fireplace comes as a palatalized kit with all of the pieces precut and ready for assembly. Isaac, our master mason made the trip up from Oregon to do the assembly. As I recall, he had already built over a hundred of these before tackling ours and after watching the assembly process over the next few days, I came to appreciate how much practice and skill was required. This isn’t a DIY level job. Hire a professional if you’re planning to go this route.

In order to build the unit tight to the wall, a heat shield was required. We built it out of lengths of hat channel, metal drywall bead and sheet metal panels. The end result is three stacked 1″ cavities that all allow air to flow convectively from the bottom to the top. This protects the wall from heat build up and circulates the heat up and out into the room.

Slots are milled into the sides of the blocks in the photo above. During assembly, these receive metal tabs that mechanically connect the blocks to one another. In addition, a very thin layer of high strength stone “glue” is applied to the joints. It’s composed of soapstone dust and a liquid bonding agent which imparts exceptional strength.

Here you can see the unit going together. Isaac used our fireplace to train Marinko Kordich (a local mason) on the particulars of the assembly.

Here’s a cut-away view of the base layer. The cavity in the center will ultimately house the ash pan. The round holes cut into the side blocks will receive removable plugs for cleaning. Some ash residue can settle into the base of the interior baffles (where the flue gasses are routed prior to exiting the top of the chimney) and need to be vacuumed out once a year.

Here’s the unit mostly assembled.

And here’s a view with the benches under construction. Steel angle was fastened to the walls and then mortised into the soapstone slabs for a nearly invisible installation. You can see the damper installed at the top left. It’s fully open when the fireplace is in use and closed when it is not (to prevent the escape of heat up the flue). Unlike most wood stoves, the airflow is not reduced to burn the wood slowly. That results in poor combustion and a lot of pollution. As the stone collects the heat for distribution later, the fire is encouraged to burn hot and fast. A side benefit of this strategy is that it also keeps the glass clean and dramatically reduces soot buildup in the chimney.

Originally, we had planned a simple steel plate hearth for the front of the fireplace but we quickly noticed how much gear was needed: newspaper, lighters, clean up tools, etc. So, we decided to evolve the hearth into a piece of functional art. We enlisted the aid of Jeff Holtby (an exceptional craftsman and blacksmith) who came up with the idea of retractable finger pulls modeled on the action of a flute. When depressed, these provide easy access to the floor box below. Otherwise, they form a flush surface and prevent embers from dropping into the box below. As an added safety measure, we treated the box with a fire proof solution. The last bit was tiling over the concrete base with soapstone to make everything match.

Here you can see the final effect with everything complete. The grain in the stone feeds the imagination. Sometimes I feel like I’m looking at the surface of Jupiter. That is when I’m not mesmerized by the simple beauty of the fire itself.