Medieval heating system of the Malbork Castle
Of all the heating devices that used to be in the castle, the greatest interest was aroused and still aroused by medieval furnaces that heat the chambers with the heat accumulated in the stones gathered above the hearth, and then radiated through channels with an outlet in the floors.
Entering steep and narrow stairs to the basement under the Great Refectory opens the way for us to discover the secret of warm rooms – to a system that allows to obtain a room temperature in winter in such a large cubature room. Undoubtedly, its advantage was efficiency, i.e. fuel economy, especially wood, which was not too much here in Żuławy, and at the same time impressive thermal efficiency. However, not all stoves of this type have survived in the Malbork Castle, and the remaining ones were mostly rebuilt in the 19th century. They were generally identical in structure. They only differed in size. The largest of the stoves consisted of a lower, vaulted furnace chamber, above which there was an accumulation chamber (about 6 m3) filled to half its height with stones. The vault over the hearth was made of six semicircular arches (ribs) made of bricks. There was a brick-width gap between each of them. All the arches in the middle of their height and at the highest point of the arch were fastened with transverse ribs, which looked like a truss in the projection.
In fact, it served as a grate for the accumulation chamber on which the fieldstones were collected. In turn, the accumulation chamber was covered with a cradle with openings for heat channels opening in the hall floor. The grid with their outlets placed on the square had 36 holes. All of them were closed with special, ceramic or metal covers, which could be used to regulate the temperature in the room. The bottom of the combustion chamber was inclined at a certain angle in relation to the actual furnace. It rose towards the rear of the chamber to divert the flame from vertical to horizontal. The main smoke duct was attached to the rear wall of the accumulation chamber. It ran under the refectory floor up a line towards the chimney in the outer wall, reducing its diameter. The chimney in the western wall of the refectory protruded over 3.7 m above the lower edge of the roof. The size of the chimney opening had to be adapted to the type and size of the operated furnace, because too small a cross-section reduced the draft force. The operation of the furnace was regulated by a sliding partition located in the chimney wall. During combustion, the outlet openings of the heat channels in the floor of the room were closed with covers. Then the damper was pulled out, and then the cold air flowed through the main flue over the fuel placed in the combustion chamber. After the required portion of wood had been burned and the gases were discharged outside through the chimney, the damper was reinserted, closing the conduit. This was necessary to avoid unnecessary heat loss. Then, the heat accumulated in the stones of the accumulation chamber and in the walls of the furnace was released by removing the covers from the outlet openings of the heat channels. The described activities were repeated, repeating the whole process in a one- or multi-day cycle, depending on the temperature outside and the parameters of air temperature inside the rooms.
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