Expert: James Ball - 2/10/2010

Question
I built a home in 09 with a wood burning masonry fireplace in the basement and one on the first floor.  The work was subcontracted to a brick mason to do the work.  The basement fireplace has never been used, but the one of the first floor has been burned a dozen times.  From the beginning, I get smoke from the lower fireplace that fills the basement when the upstairs fireplace is burning.  Not bad smoke, but enough to irritate the eyes and cause a slight haze.  Both flues are separated by 8" of mortar and/or cinder block.  Both flues are above the ridge line of the roof by several feet.  At the top of the chimney, both flues are separated by 8" and are even or level with each other at the top of the chimney.  Neither is higher than the other at the top of the chimney.  Both have a spark arrestor on top.  I even but a piece of aluminum on one side of each spark arrestor to try to block the smoke from exiting the one flue and entering the other.  It didn't help.  I can't understand why smoke is getting down into my flue and damper in the basement.  I have burned the fireplace upstairs with the furnace and blower off to avoid any suction from the furnace blower.  Both fireplaces have a vent built in the firebox to draw air from the outside into the firebox.  Do the heights of the flues need to be at different heights or do I need to get a special spark arrestor to divert the smoke as it exits the upstairs flue?  I,m just confounded with this problem.  I can't finish the basement room until I get this problem resolved.  Can you help me or give me some advice?  The brick mason doesn't have any ideas.

Answer
Hello Tim, the whole chimney has to be rebuilt....just kidding, this is a very common problem, it is called negative pressure. look at it this way if 10 gallons of air got up the upstairs fireplace then it needs 10 gallons of air to feed it and it will pull that air from the path of least resistance and that is usually down another fireplace. they make a Pop Top Damper cap, an air tight cap with a cable that runs down the inside of the chimney to open and close from the firebox. this will keep all air/smoke/odors from coming down the chimney and stop your problem. which is your house needs make up air. and the small vents in the fireplaces are never enough
keep me posted
James