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CHIMNEY INSPECTION DIAGNOSIS REPAIR
Abandoned Chimneys - Indoor Inspection
Abandoned Chimneys: Outdoors
Angled Chimney Flues
ARCHITECTURE & BUILDING COMPONENT ID
Attic Chimney Inspection
BACKDRAFTING HEATING EQUIPMENT
Blocked Chimney Flues
Bracket Chimney Collapse & Fire Risks
B-Vent Clearances Table
CARBON DIOXIDE - CO2
CARBON MONOXIDE - CO
CHIMNEY CAP & CROWN
CHIMNEY CHASE Construction & Defects
Chimney Cleaning Advice, Procedures
Chimney Cleaning Fraud Warning
CHIMNEY CLEANOUT DOORS
Chimney Components Definitions
CHIMNEY COLLAPSE Risks, Repairs
Chimney Crack Detection & Diagnosis
Chimney Draft & Performance
CHIMNEY FIRE ACTION / PREVENTION
CHIMNEY FLASHING Mistakes & Leaks
CHIMNEY HEIGHT & CLEARANCE CODE
Chimney Height Extensions
Chimney Inspection Checklist
Chimney Inspection: Flue Interiors
CHIMNEY INSPECTION CAMERA
Chimney Inspection Indoor Procedures
Chimney Inspection Outdoors From Ground
Chimney Inspection Outdoors at Rooftop
Chimney Leaning, Separation, Movement
Chimney Repair Fraud Warning
Chimney Repair Methods
CHIMNEY SAFETY - CPSC Alert
CHIMNEY SHOULDER LEAKS
CHIMNEY SHROUD, Decorative
CHIMNEY STAINS & LEAKS
Chimney Types & Materials
COMBUSTION AIR DEFECTS
COMBUSTION AIR for TIGHT buildings
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
COMPLETE COMBUSTION, Stoichiometric
CREOSOTE FIRE HAZARDS
Curved Brick Chimneys - Sulphation
Dead End Chimney Flue Hazards
Definitions of Chimney Types & Parts
DIRECT VENTS / SIDE WALL VENTS
DRAFT HOODS - gas fired
DRAFT MEASUREMENT, CHIMNEYS & FLUES
DRAFT REGULATORS, DAMPERS, BOOSTERS
EFFLORESCENCE, Salts & White / Brown Deposits
EMERGENCY RESPONSE, IAQ, GAS, MOLD
FIRE CLEARANCES INDOORS
Fire Clearances for Masonry Chimneys
Fire Clearances for Metal Chimneys
FIRE CLEARANCES, Single-Wall Metal Flues
FIRE CLEARANCE WOOD & COAL STOVE FLUES
FIREPLACES & HEARTHS
FIRE STOPPING in BUILDINGS
FLAME COLOR, BLUE vs YELLOW COMBUSTION
FLASHING, CHIMNEY Mistakes & Leaks
FLUE SIZE SPECIFICATIONS
Flue Separation Requirements
Flue Tile Damage in Chimneys
Flue Vent Connectors - Boilers, Furnaces
Fuel Changes for Heating Appliances
HEATING COST FUEL & BTU Cost Table
HOME HEATING SAFETY
INDOOR AIR EMERGENCY RESPONSE
Lennox SAFETY WARNING
Metal Chimneys & Flues
Moisture / Frost Damaged Chimney
NOISE / SOUND DIAGNOSIS & CURE
ODORS & SMELLS DIAGNOSIS & CURE
ODORS FROM HEATING SYSTEMS
OIL HEAT SAFETY INSPECTIONS
PLASTIC HEATER VENTS
ROOF STAINS from CHIMNEYS
Safety Recalls, Chimneys, Vents, Heaters
Shared Chimney & Shared Flue Hazards
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAINS on/near CHIMNEYS
Three-Sided Chimneys: Problems
Transite Pipe Chimneys & Flues
UNLINED FLUE INSPECTIONS
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Unlined chimney flues: are they safe? How do we inspect and evaluate the safety of older masonry chimney flues? This document describes safety issues and building code requirements for unlined masonry chimney flues. Proper flue thickness, lining, cleaning, rain protection, and design are important to avoid building fires and potential escape of dangerous flue gases into the building.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
For a complete and very detailed photo guide to inspection of all types of chimneys, please also see CHIMNEY INSPECTION GUIDE. See Chimney Cleaning Advice, Procedures for help locating a chimney professional.
In Canada, single wythe brick flues are accepted. In many United States locales, single wythe brick flues remain in use but several standards require or recommend either re-lining (and other safety measures) or the confirmation that 8 inches of solid masonry exists - i.e. a double wythe or greater flue.
Periodic inspection of all chimneys and flues is important for fire and gas safety. Gas safety includes carbon monoxide hazards, especially where gas-fired equipment is in use in a building. The level of inspection you request may depend on the level of probable risk involving the chimney.
First let's look at the interior of the chimney shown at the top of this page.
This view of the interior of an un-lined brick chimney, combined with the initial outside view (at the top of this page) shows even the casual on-roof building inspector that the chimney was constructed as a single wythe chimney flue.
"Single wythe" means that the chimney wall was constructed using a single thickness of brick running in stretcher or "long-way" for construction of the chimney wall. The significance of this construction is that there is only a single thickness of masonry (brick and mortar) of roughly 4" forming the chimney wall.
Any defect such as a cracked brick or lost mortar risks sparks or flue gases entering the building - a potential fire or gas hazard. A more-safe construction used at least two bricks to form the thickness of the chimney wall, and staggered masonry joints so that even if some mortar is lost there is not a direct path for sparks to enter the building structure.
And of course most modern brick or masonry chimneys use a high-temperature fired clay flue tile or chimney liner (not shown in these photos).
This view of the bottom of interior of this un-lined brick chimney, shows an added condition which is not permitted in modern construction. Near the bottom of this large single-brick-thick chimney flue you can see that the chimney splits into two sub-flues as it passes further down in the building.
Each of the sub-flues supports a different use: a fireplace in one case and a gas-fired furnace in the second case for this building.
Shared-flues passing between floors in a building present several safety hazards including potential dangerous gas leaks, fire passage among floors, and difficulties in establishing proper draft.
This chimney needs to be re-lined with two separate flues from the chimney to all the way down to the appliance.
Chimney flue re-lining alternatives include complete reconstruction using conventional masonry and clay flue lining tiles, use of a lightweight chimney lining concrete such as Permaflue™ or Supaflue™ which is pumped into the flue from the top, and stainless-steel chimney liners.
These options are listed in rough order of cost. When a chimney is sound enough for pumping a masonry flue liner I prefer the middle option for safety and durability. Any of these options can be safe if properly installed.
Our inspection found that this unlined chimney was unsafe for these reasons
In fact the roof deck in the attic had been charred, probably by prior appliances, but was no longer in danger as the chimney now vented directly into the basement by spilling all of the gas-fired boiler combustion products out of the boiler's draft hood and into the living space!
Where states such as NY have gone to a performance code we are probably jeopardizing our clients if we are not aware of what some reasonable benchmarks are for acceptable chimneys, and if we don't make people aware of telltale signs and conditions in which further investigation is warranted.
Safety Lessons from this Unsafe Chimney Story
Thanks to Alan Carson, Carson Dunlop, Toronto, for suggesting these clarifications.
CHIMNEY INSPECTION GUIDE contains detailed suggestions for inspecting building chimneys including the detection of blocked chimney flues or indications that a chimney may be blocked.
Because single thickness (wythe) brick flues may involve extra risk of fire and gas hazards, and to address the obligations of home inspectors, in 1993 an email discussion of the safety and other chimney concerns occurred between Daniel Friedman (ASHI Technical Committee Chair, Poughkeepsie, New York) and Alan Carson (ASHI President and principal of Carson Dunlop, Toronto, Ont.) to find the determining building regulations and advice for this matter.
Arlene Puentes (October Home Inspections, Kingston, NY) has sent me the following code citation regarding unlined chimney flues:
RR1001.8 Flue lining (material). Masonry chimneys shall be lined. The lining material shall be appropriate for the type of appliance connected, according to the terms of the appliance listing and manufacturer's instructions. RR1001.8.1 Residential-type appliances (general). Flue lining systems shall comply with one of the following: 1. Clay flue lining complying with the requirements of ASTM C 315 or equivalent. 2. Listed chimney lining systems complying with UL 1777. 3. Factory-built chimneys or chimney units listed for installation within masonry chimneys. 4. Other materials that will resist, without cracking, softening or corrosion, flue gases and condensate at temperatures up to 1,800F (982C).
DF to AC: since the [New York] state backed off of quantitative code, many inspectors continue to require something explicit, particularly when examining older building to which older codes pertained - and I am probably not the only one who keeps two older generations of code manuals around as "interpretation" aides for the current more vague writing.
Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys, refers to Chapters 23, 29, and 37 of the Building Code.
Gas venting into existing masonry chimneys.
Existing lined masonry chimneys and unlined chimneys with not more than one side exposed to the outside may be used to vent gas appliances provided:
There are other restrictions, getting a bit far out here, except for requiring checkout for blockage, cleaning old creosote, providing cleanout or capped tee, etc.
Unlined chimneys with more than one exposed side (outside) have to be lined per this paragraph.
The UMC has nice details and tables on what devices can be vented through what types of chimneys, clearances, shared flues, etc. They don't address (far as I can see) masonry thickness questions, except as follows
Table No. 9-D--Chimney Connector Systems and clearances from room wall combustibles for residential heating appliances
System A, 12" clearance - A 3 1/2" brick wall shall be framed into the combustible wall. A 5/8" thick fire clay liner shall be firmly cemented in the center of the brick wall maintaining a 12" clearance to combustibles. The clay liner shall run from the outer surface of the bricks to the inner surface of the chimney liner, but it shall not protrude into the chimney liner.
The above was not changed by amendments as of 1993.
This, of course, is discussing thimbles, not flues. We need to take a look
at Building Code 23 29 and 37 - which I don't have - Douglas H. might, or
perhaps you do.
Also see Separation of Chimney Flues - Chimney Top for a discussion of the need for solid masonry separation between multiple flues in a masonry chimney.
Flues venting oil-fired appliances: Summarizing our discussion of risks to clients, I agree that oil and solid fuels are probably more risky of fire than gas - for the obvious reasons of operating flue temperatures and combustible flue deposits.
Flues venting gas-fired appliances: Gas in turn seems to do more damage to old soft bricks - precisely what were used in single-wythe old unlined flues. OTOH, if there's an opening in the old flue the risk of venting CO into the house (excepting blocked flues) is probably less than the risk of reduced draft due to infiltration *in* to t he leaky chimney.
Frequently Asked Questions (FAQs)
Questions & answers or comments about un-lined masonry flues: safety, inspection, diagnosis, repair or replacement: is a new chimney liner really necessary? Not always.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.