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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
Chimney draft troubleshooting: this article describes how the location of the chimney on an exterior wall, imbedded in the wall, or located inside of the building affects chimney draft and performance. These articles on chimney construction, design, troubleshooting, cleaning & repair include description of how to perform a thorough visual inspection of chimneys for chimney safety, draft, chimney fire hazards, chimney collapse hazards and other defects. Our sketch of types of chimney placement on a building is courtesy of Carson Dunlop Associates.
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As Carson Dunlop's sketch shows, a warm chimney works best at developing good draft which in turn helps assure that the appliances or fireplaces being vented by the chimney will perform properly.
Construction of the chimney running through the interior of a home was originally done to get the most heat out of the chimney in cold weather.
Even though it is easier to build the chimney on the outside wall of a building, a central chimney provided heat through its masonry to the building interior on all floors.
[Click to enlarge any image]
A chimney's thermal performance provides the "draft" by maintaining a warm interior lining. The draft is the pressure difference between ambient air and the less dense flue gases within the chimney. The lighter gases are buoyant and rise to be displaced by heavier ambient air.
The chimney must contain the hot gases and protect the surrounding materials against combustion. Residential masonry chimneys must protect the building while under exposure to 1000°F continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about 300°F and oil burners with a flue gas temperature of about 500°F.
The oil fired boiler has blown soot into the utility room and garage throughout its' life, a constant source of annoyance that probably stems from inadequate total draft even when the oil burner, boiler, and chimney flue are up to full operating temperature.
We could address this short chimney with a draft inducer fan, but a taller flue would be smart anyway, to get the chimney top higher than the roof surface. We discuss draft inducer or "draft boosting" fans for heating systems (and maybe for some fireplaces) in detail at DRAFT INDUCER FANS
The articles listed below assist in diagnosing other causes of poor chimney performance.
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Frequently Asked Questions (FAQs)
Question: poor chimney draft, smoke from first floor fireplace comes out of basement fireplace.
Hi, I am a builder in Georgia. Last year we completely demolished a house except for the basement walls and the existing fireplace. We rebuilt the first floor, added a second floor and extended the existing fireplace. The house is sprayed foam and it is cooled and heated by a geo-thermal system.
The chimney is shared, two separate flu stacks, one coming from basement and the second from the first floor.
When a fire was started on the first floor fireplace, smoke was coming through the basement fireplace. I am guessing we have a negative air pressure causing this.
My fireplace contractor has recommended installing a fan on top of the chimney, my question is
1. Is this a good solution?
2. Do we need to install some kind of system to bring in fresh air to balance the air being taken out? - H.K., Georgia, 1/16/2014
Reply: serious red flags on chimney safety are raised by draft and smoke observations
I'm unsure what you meant by "... extended the existing fireplace" and I'm worried that the "extension" means someone added a fireplace without giving it its own flue.
And I am more confused by "... The chimney is shared, two separate flu stacks," given that you observed that "...When a fire was started on the first floor fireplace, smoke was coming through the basement fireplace"
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone, but from your description and observation some serious safety questions and possibly building code violation questions are raised that I'm sure you'll want to get clarified.
Regarding your use of the phrase "shared chimney" - A chimney structure may contain one or more individual flues or passages to vent combustion products. Each of those flues must be properly constructed and intact throughout its passage and cannot communicate with other flues, as such communications or inter-flue leakage is unsafe and also risks draft and fireplace or heating appliance performance and other safety problems.
Heating appliances between floors can never share a chimney flue.
Some building codes in some jurisdictions permit two or more individual heating appliances to be vented into the same flue if those appliances are on the same floor, and some codes/jurisdictions permit oil and gas fired appliances on the same floor to vent into the same flue provided the connections are properly located and installed with respect to one another.
Watch out: When you saw smoke coming out of the basement fireplace in response to starting a fire in the upper floor fireplace this is a significant red flag - as you doubtless recognized. But the problem is far more serious than just a draft defect for the upper fireplace chimney flue.
For smoke to come out of the lower floor fireplace when a fire was ignited on the first floor there must be some flue gas and smoke communication between the two fireplaces - which is a prohibited condition that is unsafe as well as not functional. Such leaks mean that draft is uncontrolled as well as defective, they invite fire spread between building floors, and depending on what other chimney flues exist and appliances are connected to them there is also risk of dangerous flue gas or even carbon monoxide poisoning of building occupants.
So your first order of business is to have an expert, certified chimney inspector examine the chimney and flues to find the defects and hazards.
Now with respect to poor chimney draft and down-flow of smoke between floors, beyond the unacceptable cross-flue leakage I've already cited, I add that cold air falling down a chimney can cause downdrafts but normally as the fire is ignited and chimney is warmed this condition quickly switches to updraft and proper drafting for the fireplace. But a leak between flues, such as an opening between a basement fireplace and first floor fireplace flues that are supposed to be isolated from one another can also cause inadequate draft for both fireplaces.
Once you have found and repaired the unsafe and cross-leaking chimney flues, if draft is still inadequate, a last resort is a chimney top draft inducer fan. I am afraid of fans in fireplace chimneys because of a concern that in event of a chimney fire or other unsafe condition the fan may add to the hazard. (We do see draft inducers on oil fired heating equipment connected to inadequate chimneys on occasion.)
A better solution is to provide outside combustion air - a design that is required for wood burning (and possibly other) fireplaces in modern construction codes.
Finally, with the flues properly intact and separated and isolated from one another, with an assurance that no other chimney or fireplace design or installation safety hazards remain, and with outside combustion air provided for the fireplaces, and with a check of chimney height and clearances, you will want to assure that a properly designed chimney cap is installed both to protect the flue from weather damage and to reduce site-induced downdrafts.
Fireplace ash pit door safety & draft check
Don't forget to include a check on ash pit doors in fireplaces: often I find these doors open to a common ash pit for fireplaces between floors - a possible source of communication between fireplaces if the fireplace ash pit opening doors are not properly constructed, installed, located, and normally closed.
Example Building Code Specification for Fireplace Combustion Air
Chapter 10 of the 2009 IRC Section R1006 defines combustion air requirements for masonry fireplaces.
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