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Puffbacks: Cause, Cure, Prevention of Oil Burner Puffbacks on Boilers, Furnaces, Water Heaters

Here we explain 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 heaters. Also, diagnosing black stains on indoor surfaces in the living space, possibly caused by oil fired equipment sooty operation or puffbacks.

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 and may cause the flue vent connector (stackpipe) to become disconnected, and 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 and can include:


Oil Burner Furnace or Boiler 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.

If you see black soot and debris on top of your boiler, furnace, or water heater, or black soot stains on ceilings or walls in the boiler room, furnace room, or living area, the cause could be a poorly-operating heating system that needs prompt attention.

Our photo (above) 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 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 and the cause of the noises diagnosed, and repairs made.

Guide to Preventing Oil Burner Puffbacks in Buildings and Homes


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. Have 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.
  • Stay alert for odors, noises, soot in the boiler / furnace room or elsewhere in the building or home. Then call Kevin’s Plumbing and heating 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) 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.

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. If the heating system has not been serviced, then this clue means service is needed.

Lack of regular inspection and service of oil fired heating equipment poses risks like loss of heat and related building damage, or sometimes, a messy and dangerous puffback.

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