Maybe this seems like a strange topic for a blog post, but I’m sitting here by my nice warm fire, typing into my laptop, while it’s about 15 degrees outside (F). I am not a medieval person, but I hate being cold and get grumpy if my house is below 68 degrees (and with the fire, I can get it a lot warmer than that).
Saxon huts did not have chimneys. They had fire pits in the center of the room and a hole in the roof for the smoke (ideally) to leave. In the winter, then, people lived and worked in smoke-filled rooms, with a fire that constantly had to be stoked and was also a danger to the whole household. Can you imagine raising a toddler in a hut like that? Or the constant danger of a spark lighting the thatch? “Buildings were square, rectangular, and round in plan. A central fire pit provided warmth and light, with smoke making its imperfect escape through a hole in the (typically) thatched roof above.” http://www.octavia.net/anglosaxon/earlyEnglishArchitecture.htm
Typically, the Romans did have the knowledge of chimneys (for baking), and otherwise had a hypocaust venting system. “During the Roman era, some houses were warmed with interior pipes laid under floors and within walls, and bakeries had flues that piped smoke outside the building.” http://www.apartmenttherapy.com/ny/retrospect/quick-history-chimneys-162580 They also used braziers, which could be moved from room to room and to which you could sit closer than a real fire.
“With the Norman Invasion (in 1066) came a new concept: two-story houses. An upstairs meant that you couldn’t have a fire in the middle of the floor anymore, and you needed to draw the smoke outside instead of straight up, so the fire was moved to a niche in the wall. (In stone houses, walls were so thick that the excavation of a fireplace did not effect the external appearance at all.) At first, holes were poked in the exterior wall to allow smoke to escape; eventually, flues were constructed to help control the downdraft. During the Gothic era, up to the end of the 14th century, some grander homes installed stone hoods to facilitate ventilation.” http://www.apartmenttherapy.com/ny/retrospect/quick-history-chimneys-16258
The first chimneys were constructed in castles. The first one we know of is from the 12th century.
“If the later Middle Ages had made only slight improvements in lighting over earlier centuries, a major technical advance had come in heating: the fireplace, an invention of deceptive simplicity. The fireplace provided heat both directly and by radiation from the stones at the back, from the hearth, and finally, from the opposite wall, which was given extra thickness to absorb the heat and warm the room after the fire had burned low. The ancestor of the fireplace was the central open hearth, used in ground-level halls in Saxon times and often into later centuries. Such a hearth may have heated one of the two halls of Chepstow’s 13th-century domestic range, where there are no traces of a fireplace. Square, circular, or octagonal, the central hearth was bordered by stone or tile and sometimes had a backing of tile, brick or stone. Smoke rose through a louver, a lantern-like structure in the roof with side openings that were covered with sloping boards to exclude rain and snow, and that could be closed by pulling strings, like venetian blinds. There were also roof ventilators. A couvre-feu (fire cover) made of tile or china was placed over the hearth at night to reduce the fire hazard.
When the hall was raised to the second story, a fireplace in one wall took the place of the central hearth, dangerous on an upper level, especially with a timber floor. The hearth was moved to a location against a wall with a funnel or hood to collect and control the smoke, and finally, funnel and all, was incorporated into the wall. This early type of fireplace was arched, and set into the wall at a point where it was thickened by an external buttress, with the smoke venting through the buttress. Toward the end of the 12th century, the fireplace began to be protected by a projecting hood of stone or plaster which controlled the smoke more effectively and allowed for a shallower recess. Flues ascended vertically through the walls to a chimney, cylindrical with an open top, or with side vents and a conical cap.” http://www.wtv-zone.com/Castle_Keep/castle_castle/Great_Hall/Hall-2/1.html