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Here we discuss the cause and prevention of potentially dangerous and sooty oil fired heating equipment puffbacks that can occur at an oil fired boiler, furnace, or water heater. 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.

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What Causes Sooty Puffbacks at Oil-Fired Heating Boilers, Furnaces, Water Heaters

A puffback at oil fired heating equipment is literally an explosion of un-burned oil in the combustion chamber of an oil-fired boiler, furnace, or water heater. Depending on the quantity of oil that is ignited, the puffback can damage the boiler itself, may cause the flue vent connector (stackpipe) to become disconnected, and may blow soot throughout the building.

The cause of this puffback explosion is the ignition of un-burned oil lying on or in the bottom of the heating appliance combustion chamber. Common causes of the presence of this un-burned fuel is improper oil burner operation that leaves incompletely-burned heating oil at the end of one or more on-off cycles of the oil burner include:

• Leaks in the equipment oil supply piping, often visible as oil drips or leaks that occur when the equipment is not running, become air-leaks into the oil piping and oil burner when the burner is running. Our photo (left) shows heating oil drips on the floor near an oil burner. An alert home owner, home inspector, or heating service technician should recognize the meaning and importance of this clue.
  
  To atomize and burn heating oil, the oil burner's fuel unit compresses incoming heating oil to 100 psi or more. Because any extra air in the heating oil piping is also compressed to high pressures (often more than 100 psi) during oil burner operation, any air bubbles present in the system when the oil burner stops its run cycle is exposed to a pressure drop inside the system.
  
  The 100 psi air bubble expands, pushing additional un-burned heating oil out of the end of the oil burner nozzle and into the combustion chamber where it may accumulate until, in a dramatic "BANG!" the unburned oil is ignited at the beginning of a future oil burner on-cycle.
• Oil burner shutdown problems: Incomplete heating oil combustion may also occur if the "shut-down" phase of oil burner operation is not working properly. Inside of the fuel unit of most oil burners is a spring-loaded valve whose job is to quickly and securely stop oil flowing to the oil burner nozzle the moment that the oil burner's fuel unit RPM's begin to slow - when the oil burner is turning "off" at the end of a burn cycle. Even a small amount of dirt on the seat of this oil valve can cause it to leak oil into the combustion chamber instead of making a clean oil-flow shutoff.
• Poor oil burner maintenance: if an oil burner is running poorly, for example with a dirty oil spray nozzle that has become partly blocked, the result is a poor flame and possibly incomplete combustion of the heating oil being sprayed into the fire chamber. This, too, can lead to accumulation of un-burned heating oil and a subsequent ignition and puffback.

Puffback Warning Signs

Before a catastrophic puffback occurs, most heating equipment will give plenty of warning in the form of heating oil or combustion gas odors and soot in the building. See OIL BURNER NOISE SMOKE ODORS for details.

If you see black soot and debris on top of your boiler, furnace, or water heater, or black soot stains on ceiling or wall surfaces in the boiler room, furnace room, or living area, the cause could be a poorly-operating heating system that needs prompt attention. But also see THERMAL TRACKING & HEAT LOSS as other things can cause black stains indoors.

Our photo (left) shows an incompetent and potentially dangerous "fix" to an oil fired heater that was blowing soot all over itself and the furnace room.

You can see the remains of soot on top of the furnace (lower right), and the fix - someone sealed the opening from which soot was leaking - the draft regulator. This goofy repair fails to recognize and fix the underlying problem - a draft or blocked chimney and perhaps other troubles plague this heater.

If your oil burner continues to make a soft rumbling sound (or if you actually see combustion continuing at a reduced rate inside the combustion chamber) right after the oil burner has stopped "running" then there is an oil burner shutdown problem.

If you hear noises when your oil burner starts each run cycle, perhaps a more modest "PUFF" or a small "bang" sound, unburned oil is probably being ignited.

In either case the system needs to be inspected, the cause of the noises diagnosed, and repairs made.

Preventing Oil Burner Puffbacks in Buildings

The best ways to avoid an ugly and dangerous oil burner puffback are:

• Have all of your oil fired heating appliances serviced regularly, normally once a year. Encourage the service technician to do more than a superficial cleaning (oil filter and nozzle change) - pay extra if necessary to have the equipment opened, cleaned, and inspected thoroughly. You will get a better cleaning job if you schedule this work in the spring when the oil company is less swamped with service calls and the technician may be allowed more time to do a proper job.
• Stay alert for odors, noises, soot in the boiler / furnace room or elsewhere in the building, or rumbling at the oil burner, and call your heating service company for inspection, diagnosis, and repair if needed.
• Watch for oil leaks anywhere on the oil piping system or at the oil burner - since oil leaks are also air leaks in many cases. Our photo (above-left) shows a dripping flare connector at oil piping entering the fuel unit on the oil burner.
• If the problem is a dirty oil shutoff valve inside the oil burner's fuel unit, an easy "fix" is to install an oil-delay valve on the oil burner - this extra valve is designed to shut off cleanly the moment oil pressure begins to drop at the end of an oil burner run cycle.

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
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DUCTS, Asbestos Transite Pipe
DUST FROM HVAC?
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ELECTRICAL POWER SWITCH FOR HEAT
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ENERGY SAVINGS RETROFIT CASE STUDY
ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE
ENERGY SAVINGS RETROFIT OPTIONS
FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
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GAUGES ON HEATING EQUIPMENT
HEAT LOSS in BUILDINGS
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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 SOOT & PUFFBACKS
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
  BLUERAY Recall
  CHIMNEYS & Flues - Asbestos Transite
  Goodman HTPV RECALL
  Heat Recovery Ventilator RECALL
  Lennox Furnace Manuals
  Lennox WARNING
  Weil McLain RECALL
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

• 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.

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.
• 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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