Sooty Boilers, Furnaces, Water Heaters: Cause, Problems, Cure InspectAPedia® -
How to inspect the oil fired heating equipment for soot build-up
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
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Here we discuss soot on or inside oil fired heating boilers, furnaces, and water heaters. 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. 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.
How Soot Causes Problems with Heating Boilers, Furnaces, Water Heaters
Debris visible in the flue vent connector (stack pipe) visible through the barometric damper draft control opening. If you see soot, rust flakes, and debris in the flue vent connector this means that the heating system needs to be cleaned and serviced.
If the heating system has "just been serviced" this debris means that service was incomplete.
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.
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.
Our photo 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".
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?
When we look in to the flue close to the heating boiler or furnace, such as at the barometric damper shown here, 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:
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.
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Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, 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 design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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,
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
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