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Sharing Chimney Flues

Q: Why do they say not to run both wood and oil in the same flue?
Thanks,
John

Hi John,
A wood-burning stove or fireplace is always required to have its own, unshared flue. This is largely because there is no practical way to connect another wood, gas or oil appliance to that flue without causing air infiltration into the flue through the second appliance, its connector pipe, or thimble fitting. Air infiltration slows the rate of exhaust travel up the flue while simultaneously cooling the wood exhaust gases, causing excessive creosote condensation and an increased incidence of chimney fires. The codebook is quite clear on this point: every wood-burning appliance must have its own flue.

When dealing with oil exhaust intrusion into a chimney venting wood exhaust, other factors come into play. The sulpheric acids contained in oil exhaust blend with the aldehydes found in wood exhaust to create an extremely corrosive mixture inside the flue. This mixture attacks both the bonding agent in the mortar and the actual chimney structure itself, drastically reducing the usable lifetime of the chimney.

Finally, the combination of oil soot and wood creosote in the flue presents the most dangerous of chimney fire hazards. Oil soot ignites at extremely low temperatures, and wood creosote burns at extremely high temperatures. When ignited, this mixture "spits" burning balls of oily creosote out the top of the chimney in every direction. Thus, a chimney flue that is coated with a combination of oil exhaust and wood exhaust deposits is much more likely to experience repeated chimney fires, and those fires are much more likely to destroy the chimney and/or burn down the neighborhood.

The bottom line: The only way you can safely vent an oil-burner and a wood-burner into the same chimney is if you install two stainless steel chimney liners, one for each appliance.

Note: Each Code Authority chooses the standards that regulate appliance and chimney installation and usage in their jurisdiction, and may modify code specs as desired. The above-referenced specifications are interpreted from the National Fire Prevention Association (NFPA) and Washington Association of Building Officials (WABO) Code Books, and may not reflect the code requirements in your area. Contact your Code Authority for local regulations.

Manufacturers who submit an appliance to a recognized laboratory for safety testing may receive a specific listing for that appliance which may be accepted by your Code Authority in lieu of the standard code requirements. These listings are unique to each model, and can be found in the appliance's installation manual.

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