Re: wood stove puff backs
Chimney is Too Short
Chimneys must meet specific requirements for height compared to the roof of the house. But briefly, there are two factors that relate to wind conditions. If the chimney is too short compared to the roof, wind moving across the roof will either blow directly into the chimney, possibly forcing smoke into the house, or create a high pressure system around the chimney, also causing the potential for smoke spillage.
If the overall height is inadequate, the chimney will be prone to spillage from wind, since a short column of hot air produces a weaker and more easily disrupted draft, and more likelihood of draft reversal than a tall column of hot air.
Often, simply adding a few feet to the chimney will solve the problem. But consult with a chimney professional first, to determine the practicality and cost of increasing the chimney height. A chimney professional will also check the system for other possible causes, some of which might be more critical than the chimney height.
Very large flues, which sometimes have problems even on calm days, are often more prone to wind induced downdrafts, since the draft is often weak to begin with. Especially when coupled with a short chimney, an oversized flue can be a serious problem on windy days.
The best solution is to re-line the flue with the correct liner size.
A chimney cap helps deflect wind. Some are specially designed as downdraft-deflectors. If you don't have a cap, get one, even if you don't have a draft problem . While a standard chimney cap – a lid of some sort on posts or a mesh base – is not specifically designed to deflect wind, merely having that lid above the opening will help reduce some wind-related smoking problems.
But if your downdraft problem is severe, a downdraft deflector cap might be a better option. These caps have a baffling system, generally wide bands of metal curving up over the chimney top that re- direct the wind and in some cases actually create a slight increase in chimney draft when the wind strikes the cap. Before you buy a specialized cap, have a chimney professional check the chimney to determine if your chimney will likely benefit from a special cap, and which type is best for your chimney system.
A chimney built on the outside of the house rather than up the middle of the house is more prone to wind- induced draft problems, since the chimney itself is more vulnerable to the elements. Wind draws heat away from the chimney, reducing the temperature of the smoke and gasses in the flue, thereby reducing draft. It is generally not practical to rebuild the chimney in the center of the house. But if your chimney is an exterior one, it is even more critical that the rest of the system be correct – such as flue size, liner insulation, and chimney caps – to provide the best possible conditions in spite of the poor chimney location.
Pressure Conditions in the House
Sometimes it is not the chimney that's to blame for a wind-induced smoking problem, but rather, the house. The chimney could be adequately tall, properly lined and capped, and the stove properly installed, and yet a wind-induced smoking problem could occur due to the construction of the house.
Example: Wind whipping around an unevenly insulated home can actually draw air out of one side of the house, creating a depressurization problem inside the house (in much the same way as an exhaust fan draws air out). Depending on the location of the chimney, the house might actually pull air in through the chimney (and smoke with it) to make up for the air being sucked out by the wind.
The specific science of wind pressure is beyond the scope here. But the common element in wind-pressure problems is the wind induced movement of air into and out of the house.
Nutshell summary -----
If wind factors create an area of low pressure in the room where the stove is located, then air (and smoke) may be drawn down the flue into the house to compensate. If wind factors create an area of high pressure in the room where the stove is located, then the flow of smoke and gasses up the flue might actually be increased.
As a general rule, well insulated sealed homes are less prone to pressure changes inside the house from wind than poorly insulated or unevenly insulated sealed homes, since less air is driven into or out of the house.
Without proper instruments and training, you won't be able to pinpoint this type of problem in your home. Your best bet is to tackle the other more obvious possibilities (above) first, such as chimney height, chimney caps, etc.
If you continue to experience wind- related problems, take note of wind direction and intensity and whether or not the stove smokes. Consult a chimmney professional or a venting expert, and discuss the possibility that the house might be the culprit.
"" an ounce of perception -- a pound of obscure "