Sooty Boilers, Furnaces, Water Heaters: Cause, Problems, Cure
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How to inspect the oil fired heating equipment for soot build-up
Dangers of carbon monoxide poisoning if there is soot production at gas fired heating appliances
How soot build-up causes higher heating costs on oil fired heaters
Why is soot build-up potentially dangerous on heating equipment?
Cleaning & maintenance guide for heating systems
Questions & answers about sooty heating equipment: causes, problems, cures

Sooty oil fired heating equipment: this article explains the significance of soot on, around, or inside oil fired heating boilers, furnaces, and water heaters. We explain how much oil burner soot is normal and when soot production is a problem. Thick soot build-up inside of a flue, the chimney, or inside of the boiler or furnace heat exchanger or inside of a domestic water heater is a problem that causes higher fuel bills, equipment operating problems, and potential fire and safety problems.

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How Soot Causes Problems with Heating Boilers, Furnaces, Water Heaters

A discussion of our page top photo and those rusty sooty fragments in view is found at How to Inspect a Barometric Damper. Separately, diagnosing black stains on indoor surfaces in the living space, possibly caused by oil fired equipment sooty operation or puffbacks, is discussed at THERMAL TRACKING & HEAT LOSS.

This website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Watch out: Safety warning about soot: Sooting can occur with both gas fired and oil fired systems.

Soot coming from a gas fired heater is probably indicating a very dangerous condition risking carbon monoxide poisoning. If a gas fired appliance is producing soot, it should be turned off and Immediate service is needed. See COMBUSTION AIR DEFECTS and also SOOT on OIL FIRED HEATING EQUIPMENT and GAS FIRED WATER HEATERS for safety warnings about soot at heating appliances.

Soot coming from an oil fired heater warns of improper operation and risks a destructive puffback. See OIL BURNER SOOT & PUFFBACKS

Significance of Sooty Debris visible in the flue vent connector or on top of equipment around the heating boiler

If you see soot, rust flakes, and debris in the flue vent connector (photo at page top) or chunks of black and brown sooty crud on top of horizontal surfaces near your oil fired heating equipment, this means that the heating system needs to be cleaned and serviced.

Our photo at page top shows what was probably several years of accumulated soot, rust flakes, and debris in the flue vent connector of an oil-fired horizontal furnace in a wet moldy crawlspace. The owner thought that his system, which was almost impossible to access, had "just been cleaned".

Debris in a stack pipe (C) Daniel Friedman

Our photo (left) shows soot and crud that has blown out of the boiler flue vent connector and sprinkled itself on every horizontal surface in the boiler room and in a nearby garage.

We wiped these surfaces clean and even vacuumed every thing in sight each time the boiler was serviced. That made it easier to convince our heating service tech that the system was running "dirty".

A little oil burner soot after service is normal: Because cleaning an oil fired heater disturbs soot and some of that leaks into the utility room, it's normal to see a very light coating of soot dust in the 24 hours or so after your heating system is cleaned and serviced.

But if you clean that stuff up it should not quickly reappear. If the heating system has "just been serviced" and you see soot and crud blowing around, especially if it's new soot production, this debris means that service was incomplete.

Watch out: don't use your household vacuum cleaner to vacuum oil burner soot unless you're willing to risk sacrificing that vacuum cleaner - it may become so dirty you won't want to use it in the home for general cleaning. Worse, if the vacuum cleaner is leaky you may end up blowing soot into the living area.

A proper service procedure for oil fired heating equipment includes removal of the flue vent connector and thorough cleaning of all debris from these components as well as a visual inspection of the condition of the chimney to which the flue vent connector joins to send combustion products outside.

Opening this damper and looking inside meant we literally "hit pay dirt". The dirt meant that the system needed to be cleaned, that the owner was paying for heat, but the heat was going up the chimney, not into the home -- as we explain a bit more below.

A hard to access heating system in a cramped nasty area rarely receives thorough cleaning and service. That was the case for this system.

Why is soot or crud in the oil fired boiler or furnace flue a problem?

A flue vent connector ((also called the "stack pipe" or "flue pipe") is the metal pipe that connects the oil fired heating appliance to a chimney in order to safely vent combustion gases outdoors. That's a place where we can see some helpful clues about how the oil fired heating equipment is operating without disassembling the oil fired boiler, furnace, or oil burner itself.

How to open and inspect a barometric damper and flue vent connector (C) Daniel Friedman

Soot in oil burner flue damper tee (C) Daniel Friedman

When we look in to the flue close to the heating boiler or furnace, such as at the barometric damper shown at above left, it's normal to see a thin coating of soot on the interior of the metal flue pipe.

Because soot acts like an insulating coating, too much soot in a heating system causes problems. So we do not want to see soot accumulating in depth in the flue vent connector or chimney (above right) nor blowing around the boiler room, accumulating on top of the boiler, or blowing into and staining other areas of the building.

Technical note: the seasonal efficiency of oil fired heating equipment declines between service calls - that's the normal result of soot accumulation. If we run oil fired heating equipment too hot, to produce "zero" soot, we're sending too much heat (and money) up the chimney. So a modest amount of soot accumulation in the boiler, furnace, or flue vent connector is normal between annual service calls.

More examples and photos of soot in and on oil burners and oil fired heaters are at How to Inspect a Barometric Damper and at Chimney Height & Clearance

Watch out: if your oil fired heating equipment can't make it from one annual service call to the next one without blowing soot into the building, sooting up, clogging, and becoming noisy or smoky, something is wrong. Just how much soot is ok? We answer that just below.

Soot layers too thick in boilers or furnaces means we spend more to heat the building:

Soot inside the furnace or boiler reduces the transfer of heat into the heating system's water (or air if it's a furnace). Thus the transfer of heat into the building is reduced by soot in the heating equipment. As the soot layer gets thicker less heat is transferred and more of the heat simply continues to go up the chimney instead of into the building.

When a heating service technician measures the "efficiency" of a heating system, the number, say 85%, means that for each dollar you spend on heating oil, about 85 cents is coming into the building as heat, and the remaining 15 cents is going up the chimney as wasted energy.

Soot layers too thick in a boiler or furnace could be unsafe, even a fire hazard:

Soot layers in boilers or furnaces that are too thick means that the equipment is running "hot" and could even be unsafe.

Where a heating boiler was nearly blocked solid with soot, we've measured flue temperatures close to the boiler of over 1000 degrees F.!

Any combustibles too close to a hot metal flue or chimney could catch on fire at this temperature.

So how thick can the soot layer be before we have to clean the furnace or boiler?

1/8" of soot is the limit

So if the thickness of the soot you see in the flue vent connector, looking in at the barometric damper is 1/8" thick or more, the system should be cleaned and tuned.

Warning: even if the soot layer is thin and fine where you're looking, don't rule out other possible boiler, furnace, flue, or chimney problems. For example, debris could be blocking the chimney, or the flue pipe could be blocked with debris further on closer to the chimney.

But seeing that the flue looks clean where we can inspect is good news. BOILER OPERATING PROBLEMS discusses the signs of improper oil fired hot water at operation. Warm air furnaces are discussed at FURNACES, HEATING, and problems with loss of heat are discussed at NO HEAT - BOILER / FURNACE DIAGNOSIS.

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SOOT on OIL FIRED HEATING EQUIPMENT

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Dirk Faegre, Camden, Maine (207) 232-9494 is a certified BPI energy auditor and certified Envelope technician who kindly suggested draft regulator and flue vent connector inspection defect additions 6 Sept 09

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Sooty Boilers, Furnaces, Water Heaters: Cause, Problems, CureInspectAPedia® -

How Soot Causes Problems with Heating Boilers, Furnaces, Water Heaters

Significance of Sooty Debris visible in the flue vent connector or on top of equipment around the heating boiler

Why is soot or crud in the oil fired boiler or furnace flue a problem?

Soot layers too thick in boilers or furnaces means we spend more to heat the building:

Soot layers too thick in a boiler or furnace could be unsafe, even a fire hazard:

So how thick can the soot layer be before we have to clean the furnace or boiler?

1/8" of soot is the limit

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Sooty Boilers, Furnaces, Water Heaters: Cause, Problems, Cure
InspectAPedia^® -