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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 REGULATOR, DAMPER, BOOSTER
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 & flue sizing tables by type, BTUH, building codes, and other factor: 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.
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Sizing of Metal & Masonry Chimney Flues, Flues, Vents - Category I Draft Hood and Fan Assisted Appliances
Watch out: Check with your local building code officials for local building code requirements when designing, building, or installing chimneys and vents.
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 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.Details about factors that go into calculating the size of a chimney or flue are discussed separately at CHIMNEY SIZE FACTORS.
Tables of Chimney, Flue, or Vent Size Requirements vs. BTUs of Appliances being Vented
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 Effects 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.]
Continue reading at CHIMNEY FLUE SIZE FACTORS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: chimney size vs. btuh ratings
(Aug 30, 2011) david said:
David I don't quite understand the question. But in general the chimney venting requirements are set by the fuel type and the BTU rating of the appliance.
Question: standard chimney heights
(Sept 9, 2011) Graham said:
Graham please see CHIMNEY HEIGHT & CLEARANCE CODE
Question: reduce furnace flue vent connector size below furnace outlet size?
(Oct 13, 2012) Brent said:
My question is that my looad calculation calls for a 136,000 btu furnace, in this range I'm looking at a 142,000 furnace.
My chimney (30 ft tall) has a stainless 6 " liner but the new furnace outlet is 7"
Can I safely reduce the outlet from 7" to 6" ? I also have a gas hot water heater on the same flue in a "y" connection.
Question: ok to share chimney flue if heaters don't run at the same time?
(Jan 27, 2014) DanJoeFriedman (mod) said:
"My coal boiler and oil boiler vent into the same chimney. I never use both at the same time. The chimney is about 30 feet high. The stove pipes are 6" single wall. The coal boiler enters below the oil boiler and on the opposite side of the chimney. There is no rise to the coal boiler's stove pipe, it enters straight into the chimney and runs slightly down hill. I'm burning bituminous coal get smoke billowing out the door when I open it even though I have lots of draft. I'm thinking it may be due to the way it has been piped causing a limitation in flow.
Before thinking about sharing a chimney flue - generally prohibited as a safety concern but permitted in some jurisdictions, particularly with gas and oil fuels provided proper venting arrangement - I'd check with your local building department about what they permit. Permission may also depend on the chimney type and design.
Safety issues aside for a moment there can be draft control and operating difficulties - any solution has to prevent automatic draft from one appliance from fouling up the other.
You might find draft conflicts (can't optimize for both appliances), fire hazards (one heater igniting deposits from the other), code violations, and fire spread safety hazards.
Question: shared Y connectionfor chimney connection?
(Apr 2, 2014) sal avanzato said:
You have 175,000 + 75,000 BTUH = 250,000 BTUH venting required
You don't give the horizontal distances of your 7" flue vent connector(s) between the appliances and the chimney, nor do you say if the appliances are natural draft or forced draft, so we don't have the total answer to your question, nor am I confident commenting on "safety" for an unknown system since a mistake can be fatal.
At the start of the article above we give some help choosing which tables to use to answer chimney questions.
If we are venting multiple appliances into a single chimney flue we use Table C-9-F
Your chimney cross section area is 8 x 13" = 104 sq.in. Your chimney height is 30 ft. (which gives good draft).
But an important limit here is the input 7" flue diameter. And some jurisdictions (see Seattle WA code for example) limit the horizontal run distance (to about 10 ft. for 7" flue vent connectors OR you have to reduce the assumed venting capacity of the system)
There are also assumptions about outdoor temperatures that affect the chimney draft conditions.
Not shown anyway in our table (I'll add some data) is that per the AGA NFPA code, if I assume the horizontal flue vent connector distance is 10 feet or less and the chimney height is 30 feet,
assuming an outdoor temperature range of 27 to 36 degF or higher (colder ranges give better draft once the chimney has heated-up so this is a cautious assumption) your 104 sq. in. chimney (assuming that's *internal* area, falls between two standards:
a 78 sq. in. 30 foot masonry chimney in this outdoor temperature range can vent 445,000 input BTUH
So as your chimney is in between those two and even the smaller internal size chimney can vent more BTUH than you are connecting the chimney itself meets the standards as I read them. (Check with your local building code officials for safety and local code compliance and of course check that the chimney is in fact intact, not leaking, damaged, blocked, that there is safe combustion air etc. or ALL of this discussion would be nonsense since there could be very unsafe conditions, even fatal ones, present).
Adding about outdoor chimney temperatures vs. working chimney height, there is a second concern that in cold conditions it is more difficult to get draft working in a taller exterior masonry chimney since the flue gas from the appliance has to overcome the weight of a tall column of cold air.
Question: moving a wood heater insert into a shop
13 Jan 2015 Steve Said:
I took a 20 year old wood heater insert out of my fire place. Im putting it in my shop.The heater does not have a pipe collar .It has a 7 x 8 inch opening where i can put a collar.I dont know what size to put 6 inch or 8 inch? Inside measurements of heater are 24 inch wide by 19 inch deep by 18 inch height 24x19x18 inside
Height from proposed collar to ceiling 13feet 4inches through metal roof and insulation 2inches roof has a 1 by 12 pitch.. What size stove pipe do i use and why
Generally a larger vent is functional and safe as long as the chimney size and height are not so large that the heating appliance won't develop adequate draft. It's not just stove pipe size you need to consider, but proper fire-rating of the flue or chimney materials, clearances from combustibles, cleanout access and code.
Watch out: But keep in mind that most likely you need to obtain a building permit and inspections for installing any new heater anywhere - don't view that as a stumbling block but rather as an extra inspection that might help avoid a fire.
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