Used in Siberia and Scandinavia for centuries this high efficiency wood burning stove is often referred to as a Russian fireplace.  In climates with temperatures down to -50 degrees F., the designs often incorporated sleeping niches in the masonry where occupants slept with furs.  Here is a paragraph from Wikipedia explaining the principle of operation.

"A masonry heater is defined by ASTM International as "a vented heating system of predominantly masonry construction having a mass of at least 800 kg (1760 lbs), excluding the chimney and masonry heater base. In particular, a masonry heater is designed specifically to capture and store a substantial portion of the heat energy from a solid fuel fire in the mass of the masonry heater through internal heat exchange flue channels, enable a charge of solid fuel mixed with an adequate amount of air to burn rapidly and more completely at high temperatures in order to reduce emission of unburned hydrocarbons, and be constructed of sufficient mass and surface area such that under normal operating conditions, the external surface temperature of the masonry heater (except in the region immediately surrounding the fuel loading door(s)), does not exceed 110°C (230°F)"  Note:  ASTM is short for American Society for Testing Materials.

The following picture shows the Finnish design masonry heater installed in this home.  The picture is taken from the open oak stair landing of the U-shaped staircase, at a slightly downward angle. A track light mounted on the first floor ceiling is shown that illuminates the left side of the fireplace.

The fireplace is approximately 3 ft wide x 7 ft tall x 7 ft deep, 3 bricks thick around the firebox and 2 bricks thick forming the outer shell.  The combined weight, less the detached chimney shown at the rear, is estimated to be 8800 pounds, or 4.4 tons of thermal masonry storage mass.  A diagram below shows its principle of operation.  External air provides complete and efficient combustion, the 4 high temperature plates capture and store heat from the flue gas otherwise lost to the chimney.

As shown, the loading door is in the lower front, opening into a 1ft x 1ft x 2.5ft firebrick lined firebox.  The vertical structure above the firebox is a series of flue loopbacks formed from high temperature boiler liner cement.  The four round black objects are inspection ports to each of the loopback chambers where very dry fly ash can be vacuumed out.  Not shown is a combustion air pipe which routes full oxygen to the firebox for complete wood combustion and no creosote formation.


Complete fuel combustion is an important part of the high efficiency contribution.  However without its massive thermal structure the heater would quickly overheat the home during a firing.  The thickness of the walls however, results in a thermal delay and smoothing effect.  With an initial firing the fireplace exterior will not rise in temperature for at least 5 hours, and then only slowly.  With 3 additional firings the temperature will rise to a measured level of about 130 degrees F., excluding the metal door and other parts which may be hotter.  With two firings a day this is the operating temperature.  Radiant air measured at the side surface will register about 100 degrees F., and only fall about 10 degrees from there until the next firing.

Enhancing the efficiency are the four flue loopback channels which recup the flue gas heat that would otherwise be lost to the chimney.  The experience with fireplace operation, in conjunction with solar heating, has shown that on very cold yet sunny and clear winter days, two firings a day can overheat the home.  Accordingly, on those days only one evening firing will maintain a comfortable interior temperature.

Filled full the firebox holds 2.5 cuft of wood.  One cord of wood is defined to occupy 128 cuft.  Therefore, the maximum fuel utilization rate is 25 days, and two cords would last 50 days.  In actual practice more than two cords has never been required for winter lasting from November to April.  In practice, most days require only one firing of the heater.

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