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The Basics of Wood Burning

There are several factors which affect a wood-burning heater’s efficiency, which include the house itself, the heater, the heater pipe, the chimney, the fuel and the person feeding the heater.  How well matched these components are will determine the efficiency of the heater.  The heater does not stand or fall by itself.

The House

The house can be leaky, which affects how well the house can be warmed.  It may be worthwhile to install insulation and weather stripping, as well as consider replacing out-dated windows and doors to make your system more efficient.

Cathedral ceilings have a lovely ambience, but take into consideration that all the extra space needs to be warmed.

The location of the masonry heater affects efficiency.  Placing the heater in or near the center of the living space provides for maximum comfort as well as efficiency.  Positioning it between the living room and the kitchen is ideal as the living area will be warmer and the peripheral rooms, such as bedrooms, will be cooler.

Efficiency is also affected by the number and size of rooms. Larger central rooms which are adjacent to secondary rooms generally work best. A number of smaller rooms generally do not work as well, especially as distance from the heater increases. A good central floor plan is optimal.

The masonry heater is a beautiful addition to any home enhancing architectural detail and warmth to the home. Because of this reason, it is generally not best to place the masonry heater in the basement unless you actually live there. Try not to think of the masonry heater as a furnace, which it is not, but as a heater, which is accurate.

Notice the placement of this masonry heater.  It is not only positioned between the living room and kitchen, it is also open to the second floor.  The height of this heater has been expanded to the normal level of the ceiling, which increases its thermal mass and, thus, its heat storage capacity.


Matching Your Heater to Your House and Climate

The question of how large or small a heater should be has several considerations.

  • How large is the house and how central is the living space?  It is a good idea to plan the new home around the masonry heater.
  • How is the home insulated?  Typically in the United States insulation is between the wall studs, which actually lessen the mass that can be heated.  However, by moving the insulation to the outside of the house, the majority of the house becomes thermal mass in which to hold heat.  The greater the mass, the longer it takes to heat or cool off.
  • The location of the house has a major effect on its heating.  Heat loss occurs through windows, walls and air leaks.  Wind by itself can double heat loss through single-pane glass,  so locating in a sunny, sheltered spot is desirable where choice is possible.  If that is not possible, think of planting wind breaks or anything else you can think of.  If you are thinking of building on the valley floor, remember that cool air sinks making the valley floor the coldest place to be.  Accordingly, if the choice is available, look for a site that is off the hill top and out of the wind, but also above the valley floor.

Masonry Heater Size and Mass

A small masonry heater may weigh fifteen hundred to three thousand pounds.  A very large masonry heater can weigh twelve thousand pounds, and ought to be designed and built with the home.  However, a large masonry heater can be built in an existing structure by a crafts person who has the appropriate experience.
General principles that affect size are:

  • While greater mass gives a heater more heat storage capacity, greater wall thickness slows the heater’s response, or the amount of time it takes to deliver heat.
  • The climate, the house and the habits of the person(s) tending the fire determines the correct balance between heat storage capacity and the speed with which the heat is delivered.
  • Cold climates call for massive heaters for greater heat storage and to reduce stress on the masonry.
  • Moderate climates call for less massive heaters.  Speed of response becomes important where there are sudden changes in weather.
  • All heaters are by nature are area heaters, not central heaters.  Good natural air circulation in the house distributes heat in a wider area from the heater.  Mechanical means, such as pumps and blowers, are another option.

There are various methods of calculating heat requirements of a room.  If you have a method that you like, certainly go ahead and use it.  The following method is used in Austria, by taking the factor from the following table and multiplying it by the air volume of the room in cubic meters.

Rooms
Heat Required

Configuration

In Watts per Cubic Meters  Per Hour (W/m3/hr)

In Kcal per Cubic Meters  Per  Hour    (Kcal/m3/hr)

Room with one exterior wall

35-40

30-35

Room with one exterior wall, plus unheated adjoin rooms

45-50

40-45

Room with two exterior walls, facing south or west

50-55

45-50

Room with two exterior walls, facing east or north

65-70

55-60

Bathroom with north or east exposure

80-90

70-80



Firing the Heater
It is possible for firing techniques to vary, but most are minor. The principle remains the same, which is, fire quickly at high temperatures with sufficient air so that all gases burn. Never use the slow-burn, air-starved technique used to get long burns out of iron stoves.

For example, build a fire with paper and softwood kindling in a cold heater. Make sure the heater’s doors and damper are fully open. The hot kindling fire begins to warm the heater and drives the cold air plug out of the chimney. When the color of the fire changes from smoky orange to vivid and smokeless yellow-white, the cold air plug is gone. Then you can fill the firebox and close the heater doors. Adjust the damper to produce a fire hot enough to consume the gases and give good combustion. However, you don’t want to draw heat unnecessarily into the chimney. The wood should burn as long as possible, but completely. Too much air wastes heat; however, too little air will lower combustion efficiency and produce soot. Soot can build up in the chimney and cause a fire. This will not happen with a properly fired masonry heater, which underlines the importance of the heater’s operator.


A briskly burning fire is the goal as shown in this masonry heater. Also, notice the decorative tile work at the top of the oven door. This is the Socorro with side chimney.

 

In milder climates, one burn a day is generally sufficient. On the other hand, in colder climates two burns a day may be desirable, or not. How long you fire depends on the size of the heater, whether it was cold to begin with, and how much heat is needed. A cold heater may need a second firing to get the necessary temperature. If the heater is already warm, it will take less time and fuel.

Fuels
Actually, just about anything that burns can be used.  In other lands weeds, straw, brush and twigs are used effectively.  In North America wood is generally used but it needs to be of uniform size, preferably no larger than three to five inches in diameter.  “Limb” wood is ideal for a masonry heater and generally costs a good deal less.   Small pieces of wood create good combustion conditions.  Large pieces of wood tend to burn slower and cooler and that is not what you want for efficient combustion.

Care should be taken not to burn items that produce toxic fumes and gases.  Plastics are great offenders, as well as some new inks.  Coal, which would seem a logical choice in some areas, does produce toxic gasses and is generally to be avoided.

Chimneys
Chimney fires should not occur in a properly fired masonry heater even though they do occur in airtight iron stoves.   With that in mind some attention needs to be given to chimney design and safety.

  • Is the chimney adequate to contain a 2,100°F creosote fire and keep it from spreading to the structure?
  • If the chimney is adequate, does the chimney in fact survive the fire in the sense of remaining sound for further use?

Many existing chimneys are not adequate for wood burning as evidenced by the increased incidence of chimney fires.  With this is mind, it would be prudent to consider the following.

  • If you intend to use an existing masonry chimney, have an expert look it over with care.  The chimney should have a tile liner or its equivalent.  It should meet code requirements.  The mortar joints should be sound.  Replace or repair the chimney if it fails on any count.
  • If you intend to use a prefabricated chimney, make sure it meets Canadian and U. S. standards.  Do not use the air-cooled variety.
  • A tile liner requires space to expand, especially when subjected to a chimney fire.  If the liner then cracks during a flue fire, it makes the chimney unsound.
  • Insulating the space between the flue liner and the chimney’s outer wall has the effect of keeping the flue warm and improving draft.  Insulating materials that can be used are six parts of expanded clay to one of Portland cement.  The mix is poured into the space between the liner and the chimney’s outer casing.  Other mineral based insulations that can be used are vermiculite, perlite, expanded shale, or expanded slate.
  • Use one flue per heater.  Use one as large as you need but no larger.
  • Locate the chimney entirely within the house.  You will benefit from whatever heat it radiates and it will reduce draft or condensation problems that can develop when chimneys are built outside.

For more technical data, please see the Technical Data section.

Call or email today to get more information and to schedule an appointment with your local soapstone showroom.

303.919.3706 or email:
info@greenstoneheat.com 

Click here for Technical Data on our Masonry Heaters


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