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A chimney cowl is a revolving metal ventilator that fits over a flue or chimney pot to encourage updrafts and prevent downdrafts. Most importantly, it prevents wind from blowing smoke back down a chimney and into the room beneath. A chimney cowl also prevents birds and squirrels from nesting in the chimney and often acts as a rain guard. Sometimes referred to as a flue cowl or a chimney cap, it is most frequently called a cowl because it’s shaped like a hood or bonnet, and resembles the cowl worn by a monk. Traditionally, it was manufactured in the same red-colored clay as chimney pots, but today it is usually made from galvanized iron and is available in a variety of styles.
When a fire burns in a fireplace, the smoke is usually propelled along with rising warm air through a flue to a chimney pot on the roof. In windy conditions, however, the force of the wind may be stronger than the updraft of warm air and smoke, and it can push the flow back down the chimney flue. This back draft might result in a poor fire, or it could propel smoke and toxic gases back into the house. A chimney cowl stabilizes fluctuating flue drafts, prevents downdrafts, and increases the draw up the flue, which maximizes fuel burning efficiency and ensures a healthy environment.
The most effective type of chimney cowl is the H-pot or H-style cowl. It has a distinctive advantage over most other downdraft caps because it stabilizes the draft rather than increases it, allowing the fireplace to operate more efficiently. Made from chimney pipes shaped like the letter H, it isolates combustion gases from the winds and turbulence that cause down drafts. The H-cap was originally most prominently used in marine applications because of its bulky appearance, but it has become popular recently thanks to its energy saving functionality.
Today, the term "chimney cowl" is used loosely and may refer to a terminal or vent that fits into an existing chimney pot. A solid fuel insert, like a push-in hood or a push-in top type, may also be called a chimney cowl. Chimneys that are no longer in service may use mushroom hood shaped or bonnet hoods that are also called cowls. These feature round or square base spigots that operate as a flue vents, but they are only safe in unused chimneys.
Is the new type of cowl to which you refer the "Fluecube" which I saw in a video online?
Although controversial, there is a new hypothesis out there introduced with a new type of cowl designed and being manufactured in New Zealand. When a fire is not lighting or burning well, when cowls and flues are getting rapidly blocked up with carbon and creosote, it may be recommended that the flue needs extending. This is an effort to get emissions high enough so as to not sit, blocking the flue or fall heavy to ground level effecting neighbourhood health. It still requires wind to carry the emissions away outside the localised environment only to be dumped elsewhere. No wind - no change in the effect.
The new cowl that not only blocks wind from causing downdraft, but also uses the
cold air currents in its cause and effectiveness. Cold air is heavy and falls downward. Hot air from a heat source rises.
When hot gases from a fire travel up the flue, they are then cooled and the result is visible, particulate filled smoke. The new cowl allows the cold air to travel down four corners. The hot air fans outward, is trapped and circulates inside the cowl's chamber. This turbulence draws a vacuum inside the flue and firebox producing a clean efficient burn - no longer impeded by the cold air above.
If there are any unburnt emissions traveling up the flue they are then trapped in the chamber, circulated and combustion two the three minutes after lighting the result in nearly all cases is no visible smoke.
Tests are currently not available for the device as no emissions tests include cowls scientifically. All emissions tests are done inside the heating appliance or inside the flue, inside a lab. Atmospheric conditions are not even factored into the equation. Standards are based on generalized statistical data that can be debated due to variances in atmospheric conditions.
Wood smoke pollution is the result of hot gases from wood combustion being cooled to early. New wood stove technologies try and sort the issue out by combusting them inside the appliance or filter them inside the flue. In a lot of cases the technology can be worse. Many stories of smoke inside the room or even having to leave the fire door open to produce enough heat. They call them clean and more efficient.
Many have to burn soft woods which are worse for pollution than hard woods. The technology is in the wrong place is all, and instead of paying $3000 for new burners, what most people would, in fact, benefit from is a $300 cowl. P.S. H cowls are not the best cowls. They might stop down draft to a degree when its windy but on a cold, still morning do very little to stop one of the major issues in back pressure - "cold air".
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