Masonry Chimneys and Fireplaces
There are advantages and disadvantages of capping a masonry chimney or fireplace. Let's list negative and positive points and then we'll discuss the different models that are available.
Possible Disadvantages:
1. Draft Restriction - An improperly designed or installed chimney cap can possibly reduce the draft of a chimney.
2. Clogging of Screen in Cap - Some chimney caps have small screening which can become clogged with soot and creosote after heavy use. This will cause draft reductions and poor performance of your stove/fireplace.
3. Discoloration of chimney exterior - In some cases, creosote and tar can collect on the chimney cap and then run down the outside of the chimney structure. This can discolor the stucco or brick. This is usually a problem only with older woodstoves and/or the use of green (unseasoned) wood.
4. Replacement/Reinstallation - Chimney caps can blow off in high winds. This can destroy the cap or at the very least require another trip on top of the home to reinstall it.
Possible Advantages:
1. Keeps Animals, especially Raccoons and Birds, from setting up home in your chimney or fireplace.
2. Keeps Rain and moisture from building up inside chimney. This can help reduce the chimney smell that often invades homes in the warmer weather.
3. Sheds ice and rain from the chimney crown, which is the cement cover on the top of the chimney structure. This will extend the life of the crown and eliminate some causes of cracking of the flue tile.
4. Increase draft of chimney, especially in areas with constant wind.
Note about chimney sizes - The three most popular sizes for masonry chimneys are 8" x 8", 8" x 12" and 12" x 12". Many other stock sizes are available. Custom sizes and styles are also produced by many manufacturers.
Cap Types for Masonry Flues
Standard Cap with screen - Black or Stainless Steel Draft Increasing Caps Caps with integral Damper Mechanisms Custom Caps - Metal and Masonry
Standard Caps
Standard caps are made from a number of materials including steel, stainless steel, copper or aluminum. The most popular styles are stainless steel and black painted steel. Most models feature side screening which keeps most birds and small animals out of the chimney and also keeps larger sparks from existing the chimney.
These caps are mounted onto the chimney in various ways, the most popular being screws which are spaced around the base angle of the cap and tightened against the flue tile. If the chimney is unlined or the flue does not protrude far enough out the top, inside mounting models can be used. If neither of these methods work, manufacturers also provide mounting angles or other fasteners which can be lag bolted or cemented down to theta of the chimney structure.
Stainless Steel is a preferable material since it can better handle constant exposure to rain, wind, smoke and the other nasty conditions on top of your chimney. It also is less susceptible to rust, which will reduce the chance of getting stains on your chimney from cap runoff. Example caps at:
http://www.chimcapcorp.com/meshsingleflue.html
According to wind tunnel studies, standard caps have good properties for increasing draft in windy situations. In fact, standard caps can often function as well as, if not better than, certain models marketed as "draft increasing".
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