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CHIMNEY INSPECTION DIAGNOSIS REPAIR
BACKDRAFTING HEATING EQUIPMENT
CARBON MONOXIDE - CO
CHIMNEY CAP & CROWN
CHIMNEY CLEANING PROCEDURES
CHIMNEY COMPONENT DEFINITIONS
CHIMNEY CRACK DETECTION & DIAGNOSIS
CHIMNEY DRAFT & PERFORMANCE
CHIMNEY FIRE ACTION / PREVENTION
CHIMNEY HEIGHT & CLEARANCE CODE
CHIMNEY INSPECTION, FLUE INTERIOR
CHIMNEY LEANING, SEPARATION, MOVEMENT
CHIMNEY REPAIR METHODS
CHIMNEY STAINS & LEAKS
CHIMNEY TYPES & MATERIALS
DIRECT VENTS / SIDE WALL VENTS
DRAFT REGULATORS, DAMPERS, BOOSTERS
FIRE CLEARANCES INDOORS
FIREPLACES & HEARTHS
FLUE VENT CONNECTORS
MASONRY CHIMNEY GUIDE
METAL CHIMNEYS & FLUES
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOOT AT CHIMNEY TOP
STAIN DIAGNOSIS on BUILDING EXTERIORS
WOOD, COAL STOVES & FIREPLACES
Chimney size rules of thumb & sizing factors: this article describes both some simple rules of thumb that used to be used for guesstimating the required chimney size in diameter, height etc. and continues with a list of the considerations taken by engineers who calculate the tables of chimney flue sizing requirements given in our separate article FLUE SIZE SPECIFICATIONS. Page top sketch of masonry chimney flue separation is provided courtesy of Carson Dunlop Associates.
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Before taking a look at the various metal and masonry chimney and vent sizing guidelines listed below, it is useful to understand that there are a number of critical factors affecting the permitted (that is safe and functional) chimney and vent sizes including at least those listed below.
Chimney Sizing Rules of Thumb
An old sheet metal workers rule of thumb was to size the chimney flue by taking the
i.e.: a 3" plus a 4" appliance vent will require a 5" flue. ( 32 + 42 = 25. The square root of 25 = 5.)
NFPA 1992 (7.5.3) states that draft hood appliances, Category I appliances and other appliances that use Type B vents must have a
The following calculation works out to be the same answer as the "Rule of Thumb" sizing, i.e.: (4/22 x 3.14 = 12.56) + (3/22 x 3.14/2 = 3.53) = 16.09 / 3.14 = 5.1251/2 = 2.26 x 2 = 4.52" or 5".
The 1992 GAMA vent sizing tables for single-wall metal vent connectors attached to a tile lined masonry chimney uses Table 8. The result of those calculation using a 38,000 BTU water heater with a 3" draft hood and a 37,500 BTU boiler with a 4" draft hood connected to a 20' high chimney is to use a common flue with an area of 28 square inches or a 6" flue vs a 5" flue in the previous examples.
These tables also indicate that a 3" vent is not capable of venting the 38,000 BTU water heater.
A 4" would be required. Also the flow area of the chimney must not be more that 7 times the area of the smallest vent area. Therefore, 7 x 7.065 = 49.455 vs 28, a 6 inch flue pipe is ok. The maximum size flue that could be used is 8 inch at 50.3.
These Factors Affect the Required Chimney Size
Watch out: you cannot safely choose a chimney flue diameter based only on the BTU input of the appliance(s) being vented. The required size or diameter of a metal chimney such as a B-vent depends on at least the following factors:
Generally, taller chimneys, larger diameter chimneys, and chimneys that are larger in diameter can support a greater total BTU input load. The UMC table includes data for chimneys up to 12" in diameter and 50 ft. in height but we have restricted our version to typical residential chimney diameters and heights.
Watch out: venting a small, low-BTUH appliance into a chimney of any type (masonry, metal, insulated metal, B-vent, etc) that is too large can be dangerous - the small appliance may be unable to heat the chimney to sufficient temperature to generate sufficient draft to vent the appliance safely. We find this problem in older homes where a residential gas-fired water heater is the sole appliance vented into a large, tall masonry chimney. You will see this restriction illustrated by the blank squares in the lower left corner of the table below.
For typical residential heating appliance BTU capacities, the chimney sizing tables given below provides examples of B-Vent Chimney (or "smoke pipe") sizing for common single-appliance BTU capacity and is adapted from the 1991 Uniform Mechanical Code.
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