How do I Build a Finnish Masonry Fireplace?

The size of your masonry mass fireplace depends on the size of your room and the heat output you require. For example, a room that measures 22 feet by 15 feet and 8 feet in height has a volume of 2600 cubic feet. You would need to generate approximately 6 kw of energy to heat a room of this size. A masonry mass volume of approximately 9 cubic feet would be required.

Cut smoke channels through the masonry mass with a drill. You can dismantle the mass, then rebuild it later. Follow the same line each time by measuring the central point of the brick with a ruler. Drill each brick individually with a small-bore, masonry drill bit (the exact size is not important, but large drill bits may crack the bricks). Discard broken bricks. Only use complete bricks to build the masonry mass.

Build a firebox around the masonry mass with ordinary bricks and mortar. Apply mortar in between the bricks in a smooth layer with a trowel, before pressing the bricks firmly together. Allow to dry for at least 12 hours before continuing.

Line the firebox with firebrick, a fireproof material which creates an insulating layer to maximize the stove's efficiency. Adhere the firebricks to the inner walls of the firebox and to one another by using refractory grade mortar. According to Energy Savers, refractory mortars can withstand temperatures of more than 1,093 degrees Celsius (See Reference 2).

Burn only dry, seasoned (12-month-old) wood around the central masonry mass. Light the wood with "kindling," made up of small pieces of very dry wood such as aged pine, a lighter or fire starter and a fuel block. Add larger pieces of wood when the fire has started properly. According to Firecrest Fireplaces, a masonry heater can burn 15 pounds of wood twice daily and heat 100 square feet of space (See Reference 3).