Posted by: Mr. Chimney Cap on August 14, 2001 at 14:42:53

In Reply to: Chimney Cap Cover Clearance Posted by: Harold Kestenholz on August 14, 2001 at 11:07:45

You want the clearance of the chimney cover to be sufficient to permit the flue products to exit the chimney when all flues are in use, so there is no back pressure. The cap perimeter is 140 inches. If you raise the cap 1-inch you will have 140 square inches of opening area. Raising 2 inches gives 280 square inches, etc.

To prevent constriction the opening should be at least 1-1/2 times the total of the square inch cross-sections of all flues. That height should be 1-1/2 times more than each individual flue as well, so if one flue is 6 x 8 or 48 square inches in cross-section, the perimeter is 28 inches and the cap should permit 72 ,(1.5 times 48), square inches of area around that flue so the cap should be a minimum of (72/28) 2.6 to 3 inches above that individual flue.

The cover should not overhang the chimney sides, as prevailing winds can be trapped under an overhanging lip to make a pressure zone to pressurize the cap interior unless a base plate is made to prevent wind from entering under the cap as is done with replacement caps. The negative pressure zone on the leeward side of the chimney is an assist.