Of all the heating systems that ever existed in the Castle, the medieval stoves were and still are a source of great attention. The stoves warmed chambers with the heat accumulated in stones place above their ovens. This heat was radiated through channels that had openings in the chamber floors.
We now call this system the Medieval hypokaustum. Without a doubt the advantage of this system was efficiency, in other words heating effectively whilst conserving fuel. Fuel consisted mainly of wood, of which there is relatively little in the Żuławy region. Nevertheless, not all the stoves of this variety remain in the Malbork Castle. Those remaining were in large part modified in the nineteenth century. The stoves were generally identical In build, varying only in size.
Stove under the Great Refectory
The largest of these stoves was located underneath the most beautiful hall, the Great Refectory Hall. It consisted of the lower vaulted chamber of the oven, above which lay the accumulation chamber of about 6 m3 filled half way with stones. The vaulting above the hearth was built of six semicircular saddlebows (ribbing), which were built out of common brick. Between each bow was a free space of a brick's width. All of the saddlebows were connected in their middles and at their highest points by cross ribbing which, viewed from above, was reminiscent of latticework.
This served as the framework of the accumulation chamber, on which fieldstones were placed. A cradle housing the heating channel openings roofed the accumulation chamber. The heating channel exits were in the refectory hall floor and they comprised a grid, whose 36 exits were laid-out in a rectangular shape. All these openings were closed with special metal covers, with which it was possible to regulate the hall's temperature. The bottom of the burning chamber was placed at a certain angle in relation to the actual hearth. The bottom rose toward the back of the chamber so as to change the flames' direction from perpendicular to horizontal. The main smoke channel came out of the back wall of the accumulation chamber. Decreasing in width, the channel ran under the Refectory Hall floor toward the chimney located in the outer western wall, across from the refectory entrance. The chimney rose 3.7 metres above the lower edge of the roof and had a wind indicator, thanks to which it was possible in due time to regulate the size of the flames in the burning chamber. The size of the chimney opening had to be adjusted to the type and size of stove, for if it was too small it could reduce the necessary draught. A damper located in the chimney's wall was used to regulate the size of the stove's fire.
Whilst burning in the stove the heating exists in the hall floor were closed with their covers. Next, the damper was opened and cold air would enter via the main chimney channel to the fuel located in the burning chamber. After burning the necessary portion of wood in the stove and releasing the resulting gases to the exterior via the chimney, the damper was closed, sealing off this exit. This was necessary to avoid the unneeded loss of heat. Next, the heat gathered in the accumulation chamber's stones and stove wall was released by removing the covers from the heating channels' exits.
The activities described above were repeated as part of a one or several day cyclical process. The frequency of the cycle depended on the exterior temperature and on the parameters established for interior air temperature.