It's the time of year for a jolly, round fellow dressed in red to make his way down your chimney.
There is another, and far less welcome, visitor that sometimes makes its way into your home via the fireplace. That visitor is smoke -- the very smoke that should be going up and out of the chimney.
The condition is called "backpuffing" and can occur for a number of reasons. It is a signal that one or more factors is adversely affecting your fireplace's performance.
One of the primary reasons for this condition is wind-induced downdraft. This is the downward flow of air into a chimney. It can be caused by the deflection of wind by obstacles higher than the chimney, such as neighboring tall trees, hills and buildings.
A home's location relative to hills and the dynamics surrounding the direction and velocity of the wind off those hills can make it nearly impossible for a chimney to do its job. The same is true for neighboring buildings. This frequently is the case in side-by-side homes when one is a single story and the other higher. The wind will whip across the roof of the taller structure and across the top of the chimney of the smaller one, blowing smoke down the chimney.
Dynamic wind loading can also contribute to backpuffing and smoke. It may occur in buildings when the wind causes negative inside air pressure. Negative inside air pressure occurs when the home is drawing air in rather then exhausting it.
Negative air pressure and consequent tell-tale backpuffing may be particularly noticeable when a door or window is open on a non-windward side of the building. To restore normal pressure in the building, make-up air is apt to enter via the chimney and down into the living area. If the appliance is operating, smoke is then carried into the room.
This can be remedied by cracking open a window on the side of the house where wind strikes and closing windows on the other sides.
Another common cause of smoke and backpuffing is wind loading on a multistory building. As the wind flows against the second-story vertical wall, a high pressure zone develops at the top of the adjacent single-story chimney.
The occurrence of smoke and backpuffing when the air is calm may be the result of a number of structural factors that relate to the heating system. For example, the chimney might be too short or the flue opening too large or small for the heating appliance. Smoke may also occur if the chimney is blocked with soot, creosote or debris.
With modern, strict energy codes, homes today are tighter than ever. That's good and bad. It's bad to the extent that in a relatively air-tight house, negative pressure problems can be caused by appliances that exhaust the inside air, such as a range hood, clothes dryer vent, furnaces and other devices. It is for this reason that most of the prefabricated "zero clearance" fireplaces come equipped with outside air kits, designed to assist with proper draft.
Various steps can be taken to correct the problem, short of bricking the fireplace in. One of the first is to thin out and top dense or tall trees in the area of the chimney.
With wind-induced downdraft, there isn't much that can be done about a tall building next door. However, a solution might be to extend the chimney in an attempt to divert the air stream produced by the adjacent building. This should also be considered in the case of wind loading from an adjacent second-story wall or roof structure.
Before you run out to buy bricks and mortar to extend the chimney, we have a less expensive and simpler solution to try. It's called Vacu-Stack. The Vacu-Stack Chimney Cap prevents wind-induced downdraft and is also a partial solution for dynamic wind loading. Rather than blowing into the chimney, the wind is deflected around the Vacu-Stack, even if it is blowing downward. The resultant accelerated air flow around the Vacu-Stack creates a venturi effect, causing flue gases to flow up and out of the chimney.
The Vacu-Stack is made of stainless steel and can be installed by most handy homeowners. Professional installation can be done by a stone mason, roofing contractor, fireplace installer, sheet metal contractor or chimney sweep.
Though the product is designed to fit into round flues, there is an adapter for square and rectangular masonry chimneys.
James Carey and Morris Carey are nationally recognized experts on home building and renovation.