Old-house and the requisite fireplaces chimneys that exhaust them are misunderstood fixtures. Learn more for better comfort and safety.
CHIMNEYS & FIREPLACES ARE INTRINSIC FEATURES OF OLD HOUSES, AND IMPORTANT TO KEEP UP FOR YOUR SAFETY. GET THE BASICS.
Nothinga fire on a like cold, the damp flickering evening. flames That andis, if cozythe fire warmth actuallyof catches and draws without smoke billowing into the room. Chimneys work on the principle that hot air rises because it is less dense than cold air. When a chimney is filled with hot gas, the gas tends to rise because it is less dense than the air outside. The rising hot gas creates a pressure difference called draft, which draws combustion air into the firebox and expels the exhaust gas outside. Most old-house fireplaces draw well because they were built by masons skilled in the art of chimney and firebox construction. Over time, however, conditions that affect draw may change. “The house has to be in a pressure situation that allows the chimney to draw,” says Tyler McClave of Superior Clay, a maker of Rumford fireplaces.
In a conventional wood-burning fireplace, the hot gases formed by combustion create turbulence that swirls around and eventually finds its way through the throat—a tight spot near the bottom of the flue—and out through the chimney. In the more compact design of a Rumford chimney, named for the physicist who invented the style in the 18th century, the throat is narrower and more streamlined, so that the turbulence and smoke evacuate more quickly. “It’s sort of like squeezing the end of a water hose to make it flow faster,” McClave says.
The hotter the gases in the chimney are compared to the air temperature outside, the better the chimney will draw. That’s why a fireplace that draws well in cold weather is sometimes smoky or fitful when it’s not cold outside. The turbulence created during combustion has a harder time finding its way up the flue.
Taller chimneys tend to draw better than those built to the minimum requirements, as do chimneys built within the exterior walls of a house rather than outside.
That said, houses draw air just like chimneys do, according to the Wood Heat Organization (woodheat.org). Warm air tends to push up towards the top of the house, creating higher pressure near the roof and lower pressure on the ground floor and basement. The difference in pressure at different points in the house is called the stack effect.
A house with two or three storeys produces more stack effect than a one-storey Ranch or an Arts & Crafts Bungalow because it produces a taller column of warm air. Houses with more leaks in the upper levels—and old houses typically fall into that category—produce more stack effect. [ text cont. on page 44]