It would be good, at first, to have an idea of what a masonry heater actually is and what it is not. A masonry heater is the combination of a firebox (built from firebrick-šamott), a series of pathways for the smoke from an intensely hot fire to travel to the chimney (with at least one 180 degree turn) built from masonry brick, and usually at least one shut-off damper to capture the heat. There are countless of these in Estonia and they go by the name of ’Ahi’.The šamott firebox is the place where the wood charge is burned, usually at a very high temperature. It is necessary to have a high temperature to achieve secondary combustion.

Secondary combustion is desireable since it is a much more efficient burning of the fuel and also measureably cleaner. A firebox where secondary combustion regularly takes place, has clean walls, meaning that there are no sooty deposits present. As the hot smoke travels through the masonry pathway on the way to the chimney, heat from the smoke is transferred to the surrounding masonry mass (bricks). These bricks then slowly release the heat over a period of time to the surrounding rooms.

A masonry heater can be a very efficient source of heat and is friendly to the environment. The key here is that the wood charge needs to be burned quickly and at a very high temperature. If these conditions are met, most of the energy stored in the wood can be transformed into heat which is then used to heat the home. If the fuel charge is burned correctly, at a high enough temperature, there is very little waste of the wood’s energy and little pollution is released to the outside air.

A masonry heater is not an open fireplace, nor is it a metal insert fireplace. It is important to understand that an open fireplace or metal insert fireplace is no where near as efficient as a masonry heater. It will not burn the wood charge efficiently, nor will the charge be burned cleanly. A fireplace as a rule burns at a much lower temperature and therefore secondary combustion is rarely achieved in any type of fireplace. Fireplaces are also quite a lot less environmentally friendly. This is just a general rule, and of course, may not apply to all metal inserts.