Old-house and the req­ui­site fire­places chimneys that ex­haust them are mis­un­der­stood fix­tures. Learn more for bet­ter com­fort and safety.

Old House Journal - - Restore - BY MARY ELLEN POL­SON


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 (wood­ 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]

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