The Masonry Stove

How is a masonry stove different from your average wood burning fireplace?  The Masonry Heater Association of North America says:  "The main thing that distinguishes a masonry heater is the ability to store a large amount of heat. This means that you can rapidly burn a large charge of wood without overheating your house. The heat is stored in the masonry thermal mass, and then slowly radiates into your house for the next 18 to 24 hours."   They are also know as tile stoves, kachelofens, heat-storing fireplaces, ceramic stoves, grundofens, Russian or Finnish masonry stoves or Finnish contra-flow fireplaces, and fireplace furnaces.

We started looking into masonry stoves after reading about them in The Solar House:  Passive Heating and Cooling by Dan Chiras.  The slow heat release and large thermal mass seemed like a good fit to complement the passive solar design of the house.  I think it has worked out better than a standard wood burner, which is hot when it's burning and cool when it's not.  Our unit provides a slow even heat.  The doors are hot during the actual firing, but the rest of the unit never gets too warm to touch.  This makes it a little safer to have in our family room where the boys spend a lot of time.

The combustion efficiency is better than most stoves and the Environmental Protection Agency has identified as a clean burning heat source.  This is because dry wood is burned quickly in a very hot fire in the primary combustion chamber, while the secondary combustion chamber burns any escaping combustion gases.  This means minimal soot/creosote buildup (if any) and lower emissions. 

Our unit is from Tempcast in Canada.  We also got a quote from Gimme Shelter and Tulikivi, both of which were well out of our budget range.  Our unit was not inexpensive ($4300 plus install), but those units were more than double the price.  It is extremely massive (we're talking over a ton when finished), so you need to plan for the weight during home construction.  It's not an easy retrofit.  You also need to provide a fresh air feed (with dampener for when not in use) from outside the house to the firebox.  It acts like a bellows to a blast furnace and contributes to the high combustion efficiency.  It's a good idea to have wood storage nearby, too, like any woodstove.  You want to have your wood as dry as possible (moisture content no higher than 20%), so you either need more inside storage or dry outside storage.

We build fires once or twice a day, depending on the outside temp and available sunlight.  The stove radiates heat for over 24 hours, but the amount of heat drops off over time.  Typically if it's a "one fire" day, we burn in the evening and it radiates the most overnight when we don't have any solar gain.  The goal is a quick burn, after which the fresh air feed and chimney are both closed, maximizing the heat trapped within the stove and chimney (masonry chimneys are recommended).  Although the combustion temperatures range from 1500-2000 degrees, by the time the exhaust gases exit the serpentine flue system they have dropped down to around 350 degrees.  This means that more of the heat from the wood stays in the house instead of being lost up the chimney.

The wood has to be cut up small, we cut ours to 18 inches in length. It should not exceed four to five inches in diameter, otherwise the burn is slowed down and you lose more heat up the chimney.  This makes it a good candidate for burning scrap wood, which is what we actually do.  My brother-in-law owns a saw mill where they make wood stakes, and we get a flatbed of trimmings to take us through the winter.

If your house is small it could probably provide all your heating needs.  Our house does require supplemental heating at times (in addition to the wood and passive solar).  I'll be honest, the house isn't as small as it could be, but we built to accommodate aging family members and/or other extended family as time goes on.  We use in floor radiant heat from a 55 gallon Combi-Cor water heater.  In the winter of 2005-2006 the backup heater provided around 2.5 btus/sqft/HDD.

Tempcast has a bake oven option, which is basically a door to access the secondary combustion chamber (door with orange flame in photo).  You bake in it after the fire has burned out.  So far I've made pizza and baked beans, both of which were tasty.  It takes a bit of getting used to, as you have to time your fires to aim for the right oven temp at the right time.  I usually do a small morning fire followed by a larger fire around 3pm to have it around 350 to 400 degrees F by 6pm for pizza.  Beans just go in after a morning fire and slow cook all day in a covered stoneware crock.

A stainless steel heat exchanger can also be integrated into the design to provide hot water, but this is more for folks who are "off grid".  I think I'd go for a solar water heating system first, but that would come in handy for when the sun doesn't shine.

I like the stove, but it is messy (you are still hauling the wood around) and was fairly expensive.  If you're committed to wood burning for heat, I'd definitely recommend it.