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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.

Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

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

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.

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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HEATING SYSTEMS
AIRBOUND HEAT SYSTEM REPAIRS
ANODES & DIP TUBES on WATER HEATERS
ANTI SCALD VALVES
BOILERS, HEATING
ZONE VALVES
CARBON MONOXIDE/DIOXIDE
CARBON MONOXIDE WARNING
CHIMNEY INSPECTION DIAGNOSIS REPAIR
CHIMNEYS & Flues - Asbestos Transite Pipe
COOL OFF HEAT Thermostat Switch
COMBUSTION AIR
COMBUSTION AIR for TIGHT BUILDINGS
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
DEFINITION of Heating & Cooling Terms
DRAFT HOODS - gas fired
DRAFT REGULATORS - barometric dampers
DUCT SYSTEMS
DUCT INSULATION, Asbestos Paper
DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
ELECTRIC HEAT
ELECTRICAL POWER SWITCH FOR HEAT
ENERGY SAVINGS in BUILDINGS
ENERGY SAVINGS PRIORITIES
ENERGY SAVINGS RETROFIT CASE STUDY
ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE
ENERGY SAVINGS RETROFIT OPTIONS
FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
FURNACES, HEATING
GAS PIPING, VALVES, CONTROLS
GAUGES ON HEATING EQUIPMENT
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HOUSEWRAP AIR & VAPOR BARRIERS
HEATING COST SAVINGS METHODS
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING OIL CLOUD WAX GEL POINT
HEATING OIL EXPOSURE HAZARDS, LIMITS
HEATING OIL SLUDGE
HEATING SMALL LOADS
HEAT PUMPS
HEATING SYSTEM INSPECTION PROCEDURE
HOT WATER HEATERS
HOT WATER IMPROVEMENT
INSULATION & VENTILATION INSPECTION & IMPROVEMENT
MIXING VALVES
NO HEAT - BOILER / FURNACE DIAGNOSIS
OIL BURNERS
OIL BURNER INSPECTION GUIDE
OIL BURNER NOISE SMOKE ODORS
OIL BURNER NOZZLE & ELECTRODES
OIL FUEL TYPES & CHARACTERISTICS
Oil Odors: Leaky Oil Tank Piping
OIL SAFETY VALVES
OIL TANKS
OIL TANK GAUGES
OIL TANK LEAKS
OIL TANK SLUDGE
OIL TANK TESTING
OIL TANKS, BURIED
PLASTIC HEATER VENT
RADIANT HEAT
RADIANT HEAT Floor Mistakes to Avoid
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES
RADIATORS
RELIEF VALVES - TP Valves on Boilers
Relief Valves - Water Heaters
SAFETY DURING HEATING INSPECTION
Safety Recalls
SAFETY DURING HEATING INSPECTION
SOOT on OIL FIRED HEATING EQUIPMENT
SPILL SWITCHES - Flue Gas Detection
STAIN DIAGNOSIS on Indoor Surfaces
STEAM HEATING SYSTEMS
TANKLESS COILS
THERMAL EXPANSION of MATERIALS
THERMAL MASS in BUILDINGS
THERMAL MASS in UPSTAIRS
THERMAL TRACKING & HEAT LOSS
THERMOSTATS
Transite Pipe Chimneys & Flues
WATER HEATERS
ZONE VALVES

DRAFT REGULATORS - barometric dampers
  Purpose of Barometric Draft Regulator what is a barometric damper and why do we need them
  Measure Draft - how when where and why we measure draft and how we set the draft regulator
  Barometric Damper Defect List
  How to Inspect a Damper - how to look into the flue, what you'll see, what it means about heating system economy and condition
  Automatic Vent Dampers - how an automatic vent damper saves heating cost
  Define flue damper, duct damper, & fire damper
  Draft Inducer Fans - for solving draft and chimney problems

• 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.
• ...

More Information on Building Diagnostic Inspections and Repairs

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• 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,
• 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

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