Defining Furnace Puff Back and Preventing It

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Furnace puff back occurs when your oil-burning furnace misfires. In layman terms, if the burner fails to ignite upon triggering, fumes can build up. Then when the furnace finally ignites, an explosion occurs.

“Furnace puff back is similar to a car backfiring, but it is more dangerous and damaging,” said Mark Lamphear, Senior Director of Restoration at Cleantec Services. “The pressure pushes soot and fumes through your ducts and into every room of your house. Not only does the residue cover furniture, walls, linens and everything in between, but inhaling soot can cause serious health issues. Extensive cleaning is needed when furnace puff back occurs to restore your home.”


What Causes Furnace Puff Back?

In most cases, oil leaking from the burner nozzle into the combustion chamber causes furnace puff back. When not in operation, oil accumulates in the chamber until the furnace finally ignites causing an explosion. Causes include:

  • Leaks in the oil system

  • Bubbles in the oil line, which expand and press oil out of the burner nozzle

  • Malfunctioning shutdown valve on burner nozzle

  • Clog in the burner nozzle


How to Prevent Furnace Puff Back

The first step in preventing furnace puff back is recognizing the warning signs. These include:

  • Black soot on the furnace, walls or ceiling. This indicates oil is burning improperly in the furnace and may be leaking out of the unit.

  • Continued burning after furnace shuts off. If there is excess oil in the chamber, burning will continue after the furnace has stopped. Once the furnace has shut off, listen for a noise similar to the one made when the combustion chamber is lit.

  • Banging noise. If unburned oil has pooled and ignited, the furnace will make a banging noise at the beginning of the burn cycle.


By performing regular maintenance on your furnace, you can prevent furnace puff back. Your HVAC system should be cleaned and serviced annually - this will ensure optimal operating. Make sure your HVAC professional opens the unit inspecting for signs of problems or damage. You can also inspect the furnace periodically, looking for signs of oil leaks and paying attention to any odd smells or soot in the furnace room. You should also look for debris in the flue vent connector, which could also be a sign of problems with the furnace.

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