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Posted by on Nov 2, 2017 in Tell Me Why |

When We Light Fire in a Furnace Why Does Smoke Go up the Chimney?

When We Light Fire in a Furnace Why Does Smoke Go up the Chimney?

Your chimney–and the flue that lines it–adds architectural interest to your home, but its’ real function is to carry dangerous flue gases from your fireplace, wood stove or furnace safely out of your home.

Smoke will rise up the chimney or through the nearest opening it can find because it is hotter and, therefore, lighter than the air in the room. Before the days of chimneys smoke was allowed to escape through vents or open turrets in the roof.

chimney opening

Chimneys were introduced to induce a draught, thus providing more air for the fire. The hot smoke passing up the shaft made room for cool air, of higher density. Thus a chimney would not only carry away the smoke and gases from a fire but also act as a ventilator enabling a change of air in the room.

Smoke is pulled up a chimney because of draft, which is a combination of pressure, speed and volume. The temperature of the gases contained in the fireplace smoke plays a factor as well. Draft produces a vacuum effect inside the chimney. Because the air pressure inside a chimney is often less that the air pressure inside the house itself, smoke and air are naturally drawn upward. The main reason for the difference in air pressure is temperature. Since the air inside the chimney is warmer than the temperature of the air inside the rest of the house, a vacuum is formed.

winter fire enjoyment

As you relax in front of your fireplace or bask in the warmth of your wood stove, the last thing you are likely to be thinking about is the condition of your chimney. However, if you don’t give some thought to it before you light those winter fires; your enjoyment may be very short-lived.

Why? Dirty chimneys can cause chimney fires, which damage structures, destroy homes and injure or kill people. Indications of a chimney fire have been described as creating loud cracking and popping noise, a lot of dense smoke, and an intense, hot smell.

​Chimney fires can burn explosively – noisy and dramatic enough to be detected by neighbors or people passing by. Flames or dense smoke may shoot from the top of the chimney. Homeowners report being startled by a low rumbling sound that reminds them of a freight train or a low flying airplane.

Content for this question contributed by Richard Crawford, resident of Milliken, Weld County, Colorado, USA