In Wood Heat 101 we discussed the shortfalls of the heat recovery of wood burning Stoves. Wood Heat 102 is dedicated to a review of Masonry Heaters.

It’s easier to include an image to describe the concept of the masonry heaters. Of the different types, the concept is the same, to channel the heated exhaust air from the combustion chamber through a maze of masonry prior to exhausting out the flue. The waste heat would impart upon the masonry whose thermal mass would retain and radiate heat for hours afterward. Many heaters would incorporate a sealed oven within the core of the unit. If the ovens are exposed to the direct heat they were called ‘black ovens’ whereas ‘white ovens’ only relied on radiant heat from the heated core. The amount of mass (in the form of masonry) affects the radiant properties which varies with each design.

We are uncertain how these ovens were fired. With wood burning stoves today we seem to fire them and add wood until your desired warmth is achieved. Modern Masonry Heaters are meant to fire with one hot fire without adding any more fuel. Owners claim they can still on the morning of the 3rd day after firing-there is still enough heat to bake a batch of muffins. We expect the traditional stoves would have been used similarly. Because of the ability to scrub more heat off the combustion of wood, masonry heaters use surprisingly less wood. Some users claim as little as 1/5 of the amount. The wood used however is required to be fully seasoned.

The trade-off for this efficiency is the shear size these units require. They take up considerably more room. The smallest of which was the round sweedish tile heater. The second trade off is the structure required to support some of the units. In new construction the structural implications are not insurmountable, but as a renovation it’s potentially onerous.