The problem is, even if I pre-heat the chimney (i.e., heat, gun, torch of newspaper), I canít seem to create a good updraft without filling the den with smoke. A chimney sweep company suggested building a smaller fireplace ó a ìheating boxî ó in the existing fireplace, which would significantly reduce the size of the fireplace. It was rather expensive (around $750) and didnít sound very attractive. I feel like there should be an easier solution.
Draft problems are rarely caused by the fireplace. The chimney is usually the problem. A masonry flue cross-sectional area (CSA) should be at least 1/10 the CSA of the fireplace opening. So if you have an 8 x 12 flue liner, your fireplace opening should be about 40 inches wide and 24 inches tall. If the flue is too small, you can reduce the fireplace opening with metal side panels. Try sheet metal first, then install masonry or heavy steel panels if the flue drafts well. Glass doors, with gaskets, should be installed to meet building and energy code requirements, anyway.
You may also have a problem with negative air pressure in the rest of the house, or air inversion, drawing air down the chimney more forcefully than you can move air up it. A fan, forced-air heating system or openings elsewhere in the house that draw warm air up and out will cause negative pressure. Glass doors and a fresh-air intake in the firebox, drawing air from an outside source, are required by code and will solve the problem. Even opening a window or door near the fireplace may be the answer. Furnace blower systems often cause negative pressurization in the house as well, if the return air intake isnít perfectly balanced with the flow back into the house through the heat registers. To minimize the vacuum effect, make sure all registers are open.
Chimneys usually draw at least a small amount of air even when thereís no fire below, because the flue extends upward into a lower-density atmosphere than at the bottom. A chimney must extend at least two feet above the roof, but itís possible for a chimney to comply with this requirement but still not extend upward into air of low enough density to establish ambient updraft. This is air inversion, and it creates blockage called a cold air ìplug.î
Try starting a fire using balled newspaper, then gradually building up to kindling. You should see the smoke begin to waft and then rapidly draw up. This is called priming ó developing a heat draft to move the cold plug out of the flue. A fire-starter log wonít have the same effect. You probably donít need to spend $750 on changing the firebox, but you may need to increase the chimney height if the other suggestions donít help. If all else fails, install a draft-inducing cap to draw air upward. Fireplace stores sell such units as the ìVacu-Stack.î Hopefully, youíll be enjoying a nice fire soon. Good luck!
Readers are encouraged to send questions to: HERALD Homes
379 Central Avenue, P.O. Box 220, Lawrence, N.Y. 11559-0220
Attn: Monte Leeper, architect.
