48) Masonry Mass Heaters – Guest post

by Gail on March 28, 2010

(D’s contribution. The following article is pretty detailed, for anyone considering using a masonry mass heater. Guest appearance by Liam if you read all the way to the end.)

Thermal mass fireplace kit installed

Thermal mass fireplace kit installed

With masonry heaters the mass isn’t just with the masonry: really, they’re a mass of contradictions, too.  In a world that’s moving to high-tech heating technology masonry heaters work on decidedly old technology.  Where convenience is becoming more and more the hallmark of modern life, masonry heaters involve a little good old fashioned work and elbow grease.  It seems anomalous, but true, that burning wood can offer an environmentally better way of heating your home, and where the energy for most people’s domestic heating travels a long distance, the energy for mass heaters is extremely local.

The concepts behind masonry heaters are not new: apparently in Roman times it was not all that unusual for smoke and combustion gases to be vented beneath floors and beds, thereby warming the rooms.  In Europe they are known as Russian or Finnish fireplaces, although they are typically designed as room-heaters, not whole-house heaters.

As Gail has described, a masonry heater, basically, is a high-tech fireplace.  What distinguishes it from the familiar fireplace, however, is

-            the air and smoke channels are carefully planned and more complicated;

-            the design requires at least 1800 lbs. (800 kg.) of bricks/rocks/masonry around the burn chamber;

-            the unit requires an air tight, closed door, much hotter burn;

-            the heater burns only intermittently, not continuously, even when intended to heat continuously;

-            location/siting.

Taking some of these features in order: probably the most important aspect is the channelling of the gases involved in the combustion.  The design makes this substantially more complicated or convoluted.  That’s why you purchase one as a kit and don’t build it from scratch; the kit includes all the pre-designed smoke channels. The incoming air is carefully controlled so that only enough for combustion is allowed (excess air will simply cool the burn and create smoke and soot) and for some the burn is top-down.  You prepare the fire, in other words, by loading the kindling on top.

The real difference in venting concerns the exhaust gases, which are, first, directed around inside the firebox so that there is a kind of double-burn.  Instead of having the initial gases from combustion going up a chimney, they are circulated around the burn chamber for a secondary burn.  After that the (heated, of course) exhaust gases are directed into channels throughout the masonry mass before heading up the chimney.

The mass is also part of the equation; you need at least 800 kg. to do the job which, in simple terms, is to absorb a lot of energy very quickly from a very hot fire and then release it very slowly into the home.  The rate of absorption/release is, of course, affected by the quantity of mass; the less the mass, the quicker the total masonry absorbs the energy and begins to release it.  Manufacturers suggest, for example, that cottages might be better suited to mass heaters with facings that are thinner , so that the heat release can begin more quickly.  For a conventional home you might want a mass that is thicker, typically 4”-5”.  The mass can take any form; it obviously includes the actual burn chamber, but also the surrounding masonry facing.  Interestingly, the burn chamber is separate and distinct from the facing, so much so that they cannot be connected by, for example, brick ties.  There must be space/joint between them that allows for expansion and contraction of the burn chamber.

burning chamber covered with 1/4" cardboard for expansion

Heater kit covered with 1/4" cardboard for expansion, before adding 4" facing rocks. Note chimney on the side.

With a burn chamber smaller than most fireplaces, the burn is much hotter than a typical fireplace.  The burn chamber is enclosed, generally using a close-fitted door and venting air (via a damper) from outside the home.  The damper can be used to create a slower, longer lasting fire for the occasional traditional fireplace feel.  The exceptionally hot burn means that the fuel is used more efficiently than with most wood-burning heaters, as this report confirms.  One of the collateral advantages to masonry heaters is that you can incorporate ovens into them, as these gallery photos show.  We’re incorporating one into our unit, and we’re really looking to using it for, obviously, baking, but also slow cooking in general.

The heater is not burned continuously.  This is almost counter-intuitive, but if you think about it it makes sense.  The idea is to create, say, two intense ‘burns’ per day; one in the morning and one in the evening.  With each burn the surrounding masonry is quickly re-heated, then, for 8-10 hours, slowly releases the heat into the home.  Continuous burning can actually damage the refractory lining, as this study and report indicates.

Where you locate the heater in the home is key.  The idea is to encourage as much free air circulation around the heater as possible.  For that reason the heater should be in a central location, and never part of an outside wall.  You can see this, and how various heaters look, in the gallery photos, above, from the Stovemaster site.  Clearance of at least 5” from adjacent walls or structures is best.

Masonry heaters, properly used, are more efficient than good woodstoves.  This is the conclusion of Omni Test Labs, a Portland, OR testing facility.  Their reports are online and, while technical, are certainly readable.  They show that emissions from masonry heaters are remarkably low.

To check out various manufacturers and vendors, go to The Masonry Heater Virtual Mall. For Canadian manufacturers or distributors, you could try Lakeshore Design or StoveMaster Expect to pay in the neighbourhood of $3000 plus shipping (it cost us $417 to have the kit shipped from Ontario to Vancouver).  We ordered a second door ($900) for a see-through effect.

So: all of this is talking through our hats, as we haven’t tried the heater and oven yet, but we’re really looking forward to it.  Gail has found a source of mill scraps very close to our home, so we can probably get a continuous supply of fuel there.  It all means being a little more involved in heating your home, whether it’s collecting mill scraps or taking the chainsaw down to the beach.  Either way, it will almost certainly give us the satisfaction that always comes with doing something yourself.  As my father used to say when he press-ganged me and my siblings into loading firewood: “You get warmed twice – once when you cut, load, and split the wood, and once when you burn it”.

Liam cuts firewood

Liam cuts firewood


{ 4 comments… read them below or add one }

Tess March 29, 2010 at 7:07 am

How controllable is the temperature of the oven?
-t

Reply

D March 30, 2010 at 9:26 am

Don’t know, but I suspect the only control would be to leave the door open. To try and control the oven temperature you’d have to control the whole fireplace and draft as there is no separate oven damper control (as there is, for example, with the old-fashioned wood stove flue-ovens). I understand that it just sits at about moderate (350 + or -) temperatures, which should work for most things. We’ll see.

Reply

Scott Lemieux November 28, 2010 at 1:18 am

Harpswell,Maine – We are up and running for past 6 weeks w/ a new cobblestone faced heater. On oven temps, consider monitoring amount of fuel, time of burn, oven temp rate of decline. The most interesting, but hard to digest the fact is that the firebrick and stone release heat very slowly and with a consistent lag time. (specific to the heater & space). The heat output on a small or large fire is noticed 8 hours after the fire. It pays to watch the predicted outside temparature to plan for heating. For baking, we plan to simply change up the time of firing to be ready for ideal baking conditions. Our heater holds oven temps at 350 for several hours at 3-5 hours after burn and 200-225 for about 6-8 hours on a 2/3 load. Opening the oven door is a poor way to control temp as the tiny loss to the open door is far outweighed by the mass of radient heat coming off the brick. Would love to discuss.

Reply

Norma August 31, 2011 at 8:16 am

How did you first hear or see of this heater kit?

Reply

Leave a Comment

{ 3 trackbacks }

Previous post:

Next post: