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Hearth Pad
The hearth pad is the lowest part of the fireplace on which the firebox is constructed. The pad should be reinforced with 318 inch reinforcement rods (rebar or rerods) on 18 inch centers in both directions. The rods should be centered under the firebox (firebrick) and the ornamental brick safety wall to avoid cracking the
structure due to slight shifts in the substructure. Hairline cracking of the pad can be reduced by the use of 2" x 4" welded wire fence mesh, wired to the reinforcement rod with soft steel wire or baling wire.
The pad should always rest on well tamped ballast for new construction. The base must be of sufficient size to carry the load of the masonry stove as determined by soil type at the construction site. The load is approximately 7 to 8 pounds per square inch or 1008 to 1152 pounds per square foot. The size of masonry stove featured in this publication, including the pad, will weigh about 11,000 pounds and contains approximately 400 firebrick and 550 ornamental bricks plus the mortar. Tile or additional ornamentation will increase this weight. This amount of material is adequate to build a masonry stove for a small single story residence with eight foot ceilings and a five foot attic to gable. Note: a pad is not necessary if the stove is set on a reinforced slab on grade. Firebrick must protect the pad or the slab floor. A pad does add to the decor, making the unit easier to load and or clean the masonry stove, and also simplifies the intake air ducting, unless a new floor slab, including air ducting, is being poured.
In the concrete hearth pad, use 112 inch diameter clean silicon gravel and clean silicon sand, drinking clarity water and enough type A1 portland cement (available at lumber yards) to handle the weight of the structure.
Do not use limestone aggregate in the hearth pad, because it expands from heat and pops out of the slab. Five or six bags of cement per cubic yard of concrete mixture is more than ample for strength. Use the concrete mix as dry as it possibly can be worked; excess water reduces concrete strength. Tamp or work the mixture well to
remove voids and air pockets. Tap against the side of the pad forms to remove voids in the side wall.
CHIMNEY SUPPORT MASONRY STOVE WITH OVEN
RIGHT SIDE ELEV. SEC. E-24
Combustion Air:
Whenever possible, outside combustion air should be delivered directly into the firebox by insulated air ducts that have full flow valves which will totally block air flow when the masonry stove is not in use. Any combustion heat device will consume oxygen, which normally comes from outside air. If the outside air is not
supplied by ducting, the air must be drawn through the house by way of air infiltration around doors, windows, reverse flow in other vents and any other cracks that exist in a house. Air infiltration is especially high where
the walls meet the floors. In the winter, any air moving through the house will be colder and dryer than normal house air and since it is moving air, it will cool body temperatures faster than calm air and one feels uncomfortably cooler, even at a warmer inside temperature. A primary reason that the English style fireplace is so
inefficient, is because it consumes 7 to 10 times more air than is necessary for maximum combustion.
Combustion Air Intake
Combustion air can be supplied using three 1% to 1% inch galvanized water pipes. Water pipes have been used in a fireplace for two heating seasons without visible problems. Lighter duty pipe could probably be used. While this small water pipe has worked, the air volume supplied is not adequate for the size masonry stove that is dictated by the use of standard 4% x 9 x 2% inch firebrick which results in a firebox of 18 x 27 x 12% inches. Ro 3 x 3 inch square metal downspouts total 18 square inches and should provide adequate intake air.
The downspout metal projecting into the firebox will eventually corrode out. The extension of the metal spout into the firebox serves to help keep ashes from entering the air tube. Short insert nipples could be adapted later for replacement if proven desirable. Electric conduit tube could be used for air ducting and also costs less than water pipe.
Cross-Sectional Area of Selected Size Pipe and Ducts:
3 x 3 inch square pipe -- 9.00 square inches
3 inch round pipe -- 7.00 square inches
2 inch round pipe -- 3.14 square inches
2% inch round pipe -- 4.90 square inches
1% inch round pipe -- 1.76 square inches
Maximum suggested air intake area is 18 square inches via outside ducting. Each duct should be ten feet or less in length, longer lengths produce friction of the air flow reducing air volume in the firebox. If a longer run is required, increase the duct size from outside up to the air distribution box referred to in the next section.
During the fire start-up period, the air damper on the fire door can be left open so that some inside (house) air can supplement the combustion until the fire dies down to a level that can be supplied by the outside air tubes. This time span is an experience factor and depends on fuel moisture, outside air temperature and volume of fuel in the firebox but usually takes less than one hour.
Hearth Pad
The hearth pad is the lowest part of the fireplace on which the firebox is constructed. The pad should be reinforced with 318 inch reinforcement rods (rebar or rerods) on 18 inch centers in both directions. The rods should be centered under the firebox (firebrick) and the ornamental brick safety wall to avoid cracking the
structure due to slight shifts in the substructure. Hairline cracking of the pad can be reduced by the use of 2" x 4" welded wire fence mesh, wired to the reinforcement rod with soft steel wire or baling wire.
The pad should always rest on well tamped ballast for new construction. The base must be of sufficient size to carry the load of the masonry stove as determined by soil type at the construction site. The load is approximately 7 to 8 pounds per square inch or 1008 to 1152 pounds per square foot. The size of masonry stove featured in this publication, including the pad, will weigh about 11,000 pounds and contains approximately 400 firebrick and 550 ornamental bricks plus the mortar. Tile or additional ornamentation will increase this weight. This amount of material is adequate to build a masonry stove for a small single story residence with eight foot ceilings and a five foot attic to gable. Note: a pad is not necessary if the stove is set on a reinforced slab on grade. Firebrick must protect the pad or the slab floor. A pad does add to the decor, making the unit easier to load and or clean the masonry stove, and also simplifies the intake air ducting, unless a new floor slab, including air ducting, is being poured.
In the concrete hearth pad, use 112 inch diameter clean silicon gravel and clean silicon sand, drinking clarity water and enough type A1 portland cement (available at lumber yards) to handle the weight of the structure.
Do not use limestone aggregate in the hearth pad, because it expands from heat and pops out of the slab. Five or six bags of cement per cubic yard of concrete mixture is more than ample for strength. Use the concrete mix as dry as it possibly can be worked; excess water reduces concrete strength. Tamp or work the mixture well to
remove voids and air pockets. Tap against the side of the pad forms to remove voids in the side wall.
CHIMNEY SUPPORT MASONRY STOVE WITH OVEN
RIGHT SIDE ELEV. SEC. E-24
Combustion Air:
Whenever possible, outside combustion air should be delivered directly into the firebox by insulated air ducts that have full flow valves which will totally block air flow when the masonry stove is not in use. Any combustion heat device will consume oxygen, which normally comes from outside air. If the outside air is not
supplied by ducting, the air must be drawn through the house by way of air infiltration around doors, windows, reverse flow in other vents and any other cracks that exist in a house. Air infiltration is especially high where
the walls meet the floors. In the winter, any air moving through the house will be colder and dryer than normal house air and since it is moving air, it will cool body temperatures faster than calm air and one feels uncomfortably cooler, even at a warmer inside temperature. A primary reason that the English style fireplace is so
inefficient, is because it consumes 7 to 10 times more air than is necessary for maximum combustion.
Combustion Air Intake
Combustion air can be supplied using three 1% to 1% inch galvanized water pipes. Water pipes have been used in a fireplace for two heating seasons without visible problems. Lighter duty pipe could probably be used. While this small water pipe has worked, the air volume supplied is not adequate for the size masonry stove that is dictated by the use of standard 4% x 9 x 2% inch firebrick which results in a firebox of 18 x 27 x 12% inches. Ro 3 x 3 inch square metal downspouts total 18 square inches and should provide adequate intake air.
The downspout metal projecting into the firebox will eventually corrode out. The extension of the metal spout into the firebox serves to help keep ashes from entering the air tube. Short insert nipples could be adapted later for replacement if proven desirable. Electric conduit tube could be used for air ducting and also costs less than water pipe.
Cross-Sectional Area of Selected Size Pipe and Ducts:
3 x 3 inch square pipe -- 9.00 square inches
3 inch round pipe -- 7.00 square inches
2 inch round pipe -- 3.14 square inches
2% inch round pipe -- 4.90 square inches
1% inch round pipe -- 1.76 square inches
Maximum suggested air intake area is 18 square inches via outside ducting. Each duct should be ten feet or less in length, longer lengths produce friction of the air flow reducing air volume in the firebox. If a longer run is required, increase the duct size from outside up to the air distribution box referred to in the next section.
During the fire start-up period, the air damper on the fire door can be left open so that some inside (house) air can supplement the combustion until the fire dies down to a level that can be supplied by the outside air tubes. This time span is an experience factor and depends on fuel moisture, outside air temperature and volume of fuel in the firebox but usually takes less than one hour.
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