Unlike the fireplace, the masonry stove is
airtight. Unlike the woodstove, the
masonry stove has a combustion pathway
built entirely from refractory materials that
can take the heat. In fact, the flue path
is long enough to capture as much
combustion heat as is desirable; leaving
some heat in the exhaust gases is
important, or the heater won’t draw, that
is, direct combustion gases up the
chimney rather than into your home.
airtight. Unlike the woodstove, the
masonry stove has a combustion pathway
built entirely from refractory materials that
can take the heat. In fact, the flue path
is long enough to capture as much
combustion heat as is desirable; leaving
some heat in the exhaust gases is
important, or the heater won’t draw, that
is, direct combustion gases up the
chimney rather than into your home.
Finally, the stove contains enough thermal mass to capture all the heat of an
intense, clean burn and radiate it back gently and slowly, over the designed time
period, usually 18-24 hours. From one firing to the next, the stove ranges from
warm to very warm, but never hot to the touch. No need to add more wood; in
fact, it’s not a good idea to do so. (Burning too much wood in one firing is called
“over-firing” and can damage the heater.) Combustion is so complete in a well-
managed masonry stove that with the exception of a bit of smoke released when
the fire is first started, most of what comes out of the chimney is water vapor and
Co2. You build the fire, light it, and let it burn. That’s it!
Best of Breed
intense, clean burn and radiate it back gently and slowly, over the designed time
period, usually 18-24 hours. From one firing to the next, the stove ranges from
warm to very warm, but never hot to the touch. No need to add more wood; in
fact, it’s not a good idea to do so. (Burning too much wood in one firing is called
“over-firing” and can damage the heater.) Combustion is so complete in a well-
managed masonry stove that with the exception of a bit of smoke released when
the fire is first started, most of what comes out of the chimney is water vapor and
Co2. You build the fire, light it, and let it burn. That’s it!
Best of Breed
According to the U.S. Department of Energy, heating and cooling uses 46% of all
energy used in our homes. (Water heating – quite unnecessarily -- uses another
14%.) In coastal California, where extreme heat is rare and winters are mild, a
properly sited, well designed passive solar home can do without heat or air
conditioning. Given that few homes are so well sited or built, and that we have
more heating days than cooling days, most of us find they can make do with just
heat.
Heating our Homes – Radiant Floors, and Fireplaces Revisited
Heat can be transferred to us through
three different mechanisms: radiation,
conduction, and convection. Radiation
is what you feel when you stand in
front of a fireplace; conduction is how
a coffee cup warms your hand.
Convection heats by moving warm air
across cooler surfaces (i.e., you.)
Of the three, convection, i.e., forced
air, is the cheapest to install. Small
wonder it’s the most common system in
California. Forced air heating systems
have two big advantages: they are
three different mechanisms: radiation,
conduction, and convection. Radiation
is what you feel when you stand in
front of a fireplace; conduction is how
a coffee cup warms your hand.
Convection heats by moving warm air
across cooler surfaces (i.e., you.)
Of the three, convection, i.e., forced
air, is the cheapest to install. Small
wonder it’s the most common system in
California. Forced air heating systems
have two big advantages: they are
For the health and well-being of its occupants, a home must exhaust stale air and
refresh it with new air drawn from outdoors. Forced air systems heat air and blow
it, through ducts, throughout your house. Since new air is continually entering and
leaving, you are heating the outdoors as much as anything else. (Heat Recovery
Ventilators – HRV – are systems that transfer heat from warm outgoing air to cold
incoming air. They’re rarely considered cost-effective, or used, in our climate.)
Even with HRV installed, air is lost via opened and closed doors, combustion
devices, bath and range exhaust fans.
I like to think of the inherent inefficiency of heating or cooling air in a tightly
insulated space as the Empty Refrigerator Effect. Refrigerators achieve their
rated efficiencies only when full.
Perhaps just as bad, the ducting that moves the air you’ve heated is a network of
hidden spaces that can collect and redistribute debris and moisture. The moisture
forms when the heat cycles off, as warm air cools and condenses inside the
ducts. The moist, protected environment is an ideal space for mold and mildew to
breed.
Ducts also leak. PG&E estimates that the ducts in a typical home leak as much as
30% of the air they move. This warm, moist air can escape into an unconditioned
space (for example, an unheated attic or crawlspace), cool, condense, and
deposit water where you don’t want it, and can’t see it, like your roof or floor
framing. Subjected to enough regular wetting, this wood rots.
Forced air is inherently uncomfortable. Since the blown air needed to heat the
rest of the air in a room must be substantially warmer, many people find that it
dries their eyes, skin and sinuses. It also distributes any dust, bacteria, mold or
mildew that might be growing in ducts throughout the home.
refresh it with new air drawn from outdoors. Forced air systems heat air and blow
it, through ducts, throughout your house. Since new air is continually entering and
leaving, you are heating the outdoors as much as anything else. (Heat Recovery
Ventilators – HRV – are systems that transfer heat from warm outgoing air to cold
incoming air. They’re rarely considered cost-effective, or used, in our climate.)
Even with HRV installed, air is lost via opened and closed doors, combustion
devices, bath and range exhaust fans.
I like to think of the inherent inefficiency of heating or cooling air in a tightly
insulated space as the Empty Refrigerator Effect. Refrigerators achieve their
rated efficiencies only when full.
Perhaps just as bad, the ducting that moves the air you’ve heated is a network of
hidden spaces that can collect and redistribute debris and moisture. The moisture
forms when the heat cycles off, as warm air cools and condenses inside the
ducts. The moist, protected environment is an ideal space for mold and mildew to
breed.
Ducts also leak. PG&E estimates that the ducts in a typical home leak as much as
30% of the air they move. This warm, moist air can escape into an unconditioned
space (for example, an unheated attic or crawlspace), cool, condense, and
deposit water where you don’t want it, and can’t see it, like your roof or floor
framing. Subjected to enough regular wetting, this wood rots.
Forced air is inherently uncomfortable. Since the blown air needed to heat the
rest of the air in a room must be substantially warmer, many people find that it
dries their eyes, skin and sinuses. It also distributes any dust, bacteria, mold or
mildew that might be growing in ducts throughout the home.
simple and cheap to install, and they provide heat at a moment’s notice. On the
other hand, they are often the least energy efficient and least comfortable way of
heating a typical home.
other hand, they are often the least energy efficient and least comfortable way of
heating a typical home.
Slab floors are a pragmatic and very effective way of adding thermal mass to a
house with contemporary stick framing. Typically, air plenums or tubing run
through them. The slab can be heated (or cooled) by circulating warm air through
the plenums, or pumping water through the tubing. This setup with water
circulating through tubes is called a hydronic floor heating system, but is often
referred to as a radiant floor, or simply as radiant heat. In most cases, for weight
reasons, the heated slab for radiant heat has to be included in the original
design. Thin slabs, commonly gypcrete, with correspondingly less mass, are more
typical in retrofits because many suspended floor systems can support them with
little or no additional support.
Overall, radiant floor heat is amazingly efficient and effective. Cats, humans and
lizards all find they are comfortable at a lower ambient air temperature when
standing (or laying) on a warm, radiant surface. This means you can maintain the
same level of comfort with a lower thermostat setting.
Mass Heaters
Another approach is to install a heater that becomes part of your house. That is,
the heater contains the thermal mass your house lacks. When it burns wood,
such a heater is called a masonry stove, a Russian stove, or a mass heater.
Though mass heaters often look like traditional open fireplaces, don’t be
confused. An open fireplace, when well-managed, burns wood fairly efficiently
(cleanly), but by drawing more air than is needed for combustion, it pumps warm
air from the room up and out the chimney. The optimal amount of air for
combustion is called the stoichiometric air-fuel ratio. It contains just enough
oxygen to oxidize – burn – the evaporated wood products available for
combustion. Under some circumstances, fireplaces can suck more heat (in the
form of warm air) out of a house than they radiate back into it.
The usable heat produced from the open fireplace is primarily radiation -- the
same heat you feel on your face when you look at the flames. Since there is
usually little thermal mass available to capture that heat, most people elect to burn
the wood more slowly, so that the fireplace makes heat available for a longer
time. This means that the wood is burning at a lower temperature. At a lower
temperature, less of the wood combusts completely, and more leaves the chimney
as smoke.
Thermal Mass
Another approach to heating your house is to, literally, heat your house – not the
air cycling through it. When you heat the building itself, you can open all the
doors and windows, let all the warm air escape, close everything back up, and,
instantly, be warm again – without adding any more heat! Warm masses, like
sunshine, heat you by radiation. This is the full refrigerator working for you.
Another approach to heating your house is to, literally, heat your house – not the
air cycling through it. When you heat the building itself, you can open all the
doors and windows, let all the warm air escape, close everything back up, and,
instantly, be warm again – without adding any more heat! Warm masses, like
sunshine, heat you by radiation. This is the full refrigerator working for you.
To heat your house, your house must be
capable of storing heat. That is, it must
have substantial thermal mass. More
traditional building materials – stone,
brick, and earth – all have high thermal
mass. Concrete (and water!) also have
high mass. Note that, to do you any
good, the mass has to be on the inside of
your home – ideally, with insulation
behind it. Unfortunately, few walls are
built with sufficient mass, and fewer still
with insulation behind the mass and the
outdoors.
capable of storing heat. That is, it must
have substantial thermal mass. More
traditional building materials – stone,
brick, and earth – all have high thermal
mass. Concrete (and water!) also have
high mass. Note that, to do you any
good, the mass has to be on the inside of
your home – ideally, with insulation
behind it. Unfortunately, few walls are
built with sufficient mass, and fewer still
with insulation behind the mass and the
outdoors.
Metal woodstoves are a
significant improvement from
the standpoint of producing
more useable heat, but not
of conserving wood or
preventing air pollution. One
improvement is that they limit
incoming air, thus avoiding
heating air not needed for
combustion. (When a stove
can limit combustion air, it’s
called airtight.) Another
improvement is that they use a
lengthened heat exchange
pathway to improve heat
transfer from the heated
combustion gases (outside
significant improvement from
the standpoint of producing
more useable heat, but not
of conserving wood or
preventing air pollution. One
improvement is that they limit
incoming air, thus avoiding
heating air not needed for
combustion. (When a stove
can limit combustion air, it’s
called airtight.) Another
improvement is that they use a
lengthened heat exchange
pathway to improve heat
transfer from the heated
combustion gases (outside
combustion-heated air plus combustion bi-products like Co2 ) before they exit the
chimney. Metal (cast iron) woodstoves, by necessity, operate at low, inefficient
(and polluting) combustion temperatures. Most people don’t have any way of
storing the heat that radiates from their woodstove. Like the forced-air folks, they
are heating only the air in their home, not the home itself. To compensate, they --
like fireplace owners -- elect to slow the fire (by burning larger pieces, for
example) for a longer burn. The problem is that efficient solid fuel combustion
requires fairly high temperatures, ideally between 1200 and 2000 degrees
Fahrenheit. Since cast iron begins to distort and fail at around 1200 degrees,
burning your woodstove hot enough for clean combustion (red-hot!) will destroy it
in short order.
Modern catalytic stoves use tricks to work around these problems, at least
partially. Brick-lined fireboxes add longevity, and catalytic converters assist in
reburning smoke at lower temperatures. In my opinion, these are Band-Aids.
Lacking the needed mass and industrial-strength combustion chambers, cast-iron
woodstoves are still inferior to masonry stoves for convenient, whole-house
heating.
Heat Storage Ability: Large Mass + Good Design
= Maximum Heat Storage
= Maximum Heat Storage
After experimenting with building my
own masonry stove, I found a design
better than I could hope to
duplicate on my own. The
engineered “core” of the system is
marketed as a kit. You build it by
stacking and mortaring the pieces,
then cutting in and installing doors
and hardware. Next, you wrap this
core in a decorative masonry façade
to add to its mass and integrate it with
your house. The company is called
own masonry stove, I found a design
better than I could hope to
duplicate on my own. The
engineered “core” of the system is
marketed as a kit. You build it by
stacking and mortaring the pieces,
then cutting in and installing doors
and hardware. Next, you wrap this
core in a decorative masonry façade
to add to its mass and integrate it with
your house. The company is called
Second, the long, vertical, and relatively straight heat exchange path channels fly
ash, an inevitable by-product of solid fuel combustion, into a few easily accessible
areas. Since there are very few horizontal surfaces, there’s nowhere else for ash
to collect. This is a major problem with a few of the brick-based designs I’ve seen,
which use numerous horizontal channels.
Third, because what you purchase is a core designed to be customized for
different chimney configurations, the layout and finish of the heater is infinitely
customizable.
Mass heaters are a new approach to burning wood. If you don’t take the trouble
to learn how to do it right, you are sure to be sorely disappointed, and will
regularly fill your house with smoke. One key to lighting these heaters is to
understand and move the cold air “plug” that can form inside the chimney of a
cold stove. You purge the plug by lighting a small fire (e.g., single piece of
newspaper) and by placing it directly underneath the chimney via one of the ash
clean-out doors. When you hear a dull “roar” from the newspaper, it means that
hot air is moving up the chimney and it’s safe to light the main fire (which you
should do immediately.)
The next trick is to gauge when to shut the stove down. If you don’t close it up
when the fire burns out, the heater will continue to draw air into its vast mass, heat
it, and send it up the flue. Eventually, the stove will send all its heat up the flue
and stop heating your house. So, when the fire burns down to small embers, you
must manually close the flue damper and the air intake door. Using the stove
correctly requires a little planning: You want to be around when the fire burns out.
When you understand how the stove works and how to light it, it draws and burns
like nothing else. Equipped with an optional bake oven, you can watch the flames
through the glass of the main fire doors as they merge into a single jet of plasma.
If net-zero passive solar isn’t a possibility for you, the next most comfortable,
healthy and sustainable option may be a combination of radiant floor and a
masonry stove. You might consider letting one serve as a backup for the other.
ash, an inevitable by-product of solid fuel combustion, into a few easily accessible
areas. Since there are very few horizontal surfaces, there’s nowhere else for ash
to collect. This is a major problem with a few of the brick-based designs I’ve seen,
which use numerous horizontal channels.
Third, because what you purchase is a core designed to be customized for
different chimney configurations, the layout and finish of the heater is infinitely
customizable.
Mass heaters are a new approach to burning wood. If you don’t take the trouble
to learn how to do it right, you are sure to be sorely disappointed, and will
regularly fill your house with smoke. One key to lighting these heaters is to
understand and move the cold air “plug” that can form inside the chimney of a
cold stove. You purge the plug by lighting a small fire (e.g., single piece of
newspaper) and by placing it directly underneath the chimney via one of the ash
clean-out doors. When you hear a dull “roar” from the newspaper, it means that
hot air is moving up the chimney and it’s safe to light the main fire (which you
should do immediately.)
The next trick is to gauge when to shut the stove down. If you don’t close it up
when the fire burns out, the heater will continue to draw air into its vast mass, heat
it, and send it up the flue. Eventually, the stove will send all its heat up the flue
and stop heating your house. So, when the fire burns down to small embers, you
must manually close the flue damper and the air intake door. Using the stove
correctly requires a little planning: You want to be around when the fire burns out.
When you understand how the stove works and how to light it, it draws and burns
like nothing else. Equipped with an optional bake oven, you can watch the flames
through the glass of the main fire doors as they merge into a single jet of plasma.
If net-zero passive solar isn’t a possibility for you, the next most comfortable,
healthy and sustainable option may be a combination of radiant floor and a
masonry stove. You might consider letting one serve as a backup for the other.
Tempcast (www.tempcast.com). As I
see it, here’s what sets their product apart from the other designs I found.
First, when you subject any material to a range of temperatures of almost 2000
degrees f, it will move. The Tempcast core is a set of blocks designed to fit
together, but move independently. The assembled core itself then moves in
isolation from the masonry façade. Ceramic fiber gasket material isolates and
seals critical joints like those between the metal doors and the core. The unit is
carefully designed so that it won’t rack itself apart, even after many years of use.
see it, here’s what sets their product apart from the other designs I found.
First, when you subject any material to a range of temperatures of almost 2000
degrees f, it will move. The Tempcast core is a set of blocks designed to fit
together, but move independently. The assembled core itself then moves in
isolation from the masonry façade. Ceramic fiber gasket material isolates and
seals critical joints like those between the metal doors and the core. The unit is
carefully designed so that it won’t rack itself apart, even after many years of use.
Copyright ©2008, Rustic Precision | Contractor License #911827
Combustion Efficiency Comparison Estimated
Average By Group
Average By Group
