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CHIMNEY INSPECTION DIAGNOSIS REPAIR
BACKDRAFTING HEATING EQUIPMENT
CARBON MONOXIDE - CO
CHIMNEY COMPONENT DEFINITIONS
CHIMNEY FIRE ACTION / PREVENTION
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
FLAME COLOR, BLUE vs YELLOW COMBUSTION
HEATING SYSTEM INSPECTION
HOME HEATING SAFETY
ODORS GASES SMELLS, DIAGNOSIS & CURE
SAFETY RECALLS CHIMNEYS VENTS HEATERS
STAIN DIAGNOSIS on BUILDING EXTERIORS
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
This article describes the size requirements for chimney flues for different categories of heating appliances. We include excerpts from UMC tables along with additional commentary to assist in sizing metal or masonry chimneys according to chimney diameter, height, materials, and the input BTUH of the appliance(s) to be vented. .
As with most building nspection and safety topics, this material may be incomplete. Check with your local building code officials for local building code requirements when designing, building, or installing chimneys and vents.
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The 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.
Table of Chimney, Flue, or Vent Size Requirements vs. BTUs of Appliances being Vented
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 following table 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.
A Guide to Chimney Sizing Tables for Venting Heating Appliances: Which Chimney Sizing Table to Use
Here is a quick guide to the UMC's chimney sizing & BTUH venting capacity tables when sizing a metal or masonry chimney diameter to match the total input BTUH of the heating appliances (boiler, furnace, water heater) being vented. Note that different chimney sizes are required for venting fireplaces and woodstoves. Masonry fireplace chimney size requirements are at FIREPLACES & HEARTHS.
Table C-9-A (Excerpts): Single Appliance Venting, B-Vent Chimney Capacities for Venting a Single Heating Appliance where a Single Wall Metal Flue Vent Connector is Used Between the Appliance and the Chimney
BTU Color Coding Note: in the tables below we use colors to indicate where an example-approximate appliance BTUH vent capacity rating falls across various chimney parameter combinations. The Gray color indicates that this chimney height and diameter combination cannot be used.
Table C-9-B (Excerpts): Single Appliance Venting, B-Vent Chimney Capacities for Venting a Single Heating Appliance where a Double-Wall B-Vent Flue Vent Connector is Used Between the Appliance and the Chimney
Table C-9-C (Excerpts): Single Appliance Venting, BTU Capacity of Masonry Chimneys where a Single Wall Metal Flue Vent Connector is Used Between the Appliance and the Chimney
Table C-9-D (Excerpts): Multiple Appliance BTU Vent Capacity of Type B Double-Wall Vents where a Double Wall Metal B-Flue Vent Connector is Used Between the Appliance and the Chimney
Watch out: the first portion of Table C-9-D specifies the maximum input BTUH capacity of any individual heating appliance that is to be vented along with other appliances out through a single chimney. The continuation of C-9-D below gives the chimney specifications for the total input BTUH of all of the appliances, combined.
Table C-9-D (Excerpts Continued): Multi-Appliance BTUH Venting Capacity of Type-B Double-Wall Vents [Chimneys] with Type-B flue vent connectors
This table gives the chimney specifications for the total input BTUH of all of the appliances combined and joined into a single vent when the flue is a Type-B double wall metal chimney and when the flue vent connector (between the appliance and the chimney entry point) is also a Type-B double wall unit.
Table C-9-E (Excerpts) Multi-Appliance BTUH Venting Capacity of Double-Wall B-Vent Chimneys where a single-wall metal vent connector is used between the appliances and the chimney
This two-part table gives the chimney specifications for the total input BTUH of all of the appliances combined and joined into a single vent when the flue is a Type-B double wall metal chimney and when the flue vent connector (between the appliance and the chimney entry point) is a single wall metal component.
Table C-9-F (Excerpts) Multi-Appliance BTUH Venting Capacity of Masonry Chimneys where a single-wall metal vent connector is used between the appliances and the chimney
This two-part table gives the chimney specifications for the total input BTUH of all of the appliances combined and joined into a single vent when the flue is a masonry chimney and when the flue vent connector (between the appliance and the chimney entry point) is a single wall metal component.
Details About Chimney Cross Sectional Areas and Effecs on Venting Capacity of Masonry Chimneys - Translating Rectangular Chimney Flues to Round Flue Equivalents
The UMC Appendix C Table C9-G, "Masonry Chimney Liner Dimensions with Circular Equivalents" translates square or rectangular chimney flue inside dimensions into an equivalent "round" flue diameter and into equivalent square inches of venting capacity.
Why does this translation matter? The venting capacity of a rectangular or square masonry flue, if translated directly into actual or measured square inches of cross section, is less than a round chimney flue of the same square inches of venting capacity.
For example an 8" x 8" square nominal clay chimney flue tile liner has inside dimensions of 6 3/4" x 6 3/4". Just muliplying these dimensions together (6.75 x 6.75) = 45.56 sq.in. But chimney experts and draft measurements show that in a rectangular flue not all of the cross-sectional area is equally effective in developing draft. The effective square inches of venting area, translated to an equivalent round flue, would be less - or aboutd 42.7 square inches.
So when calculating the venting capacity of a square or rectangular clay chimney flue tile or liner by looking at its cross section, we use the UMC's Table C9-G of "round chimney flue equivalents" to get the effective chimney flue vent cross sectional area before we enter the masonry chimney BTUH venting capacity tables 6-9-C (single wall flue vent connectors and single heating appliances) or 6-9-F (single wall flue vent connectors and multiple appliances).[Note: pending clarification, we have not provided excerpts of Table C-9-G because, frankly, some of its data is confusing. - Ed.]
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