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Sharing Chimney Flues

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


Sweepy 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.


Q: Regarding your article about wood and oil sharing the same flue. I recently installed a high-efficiency wood furnace add-on and I am experiencing all the bad stuff you describe in the article. If I dedicate the original masonry chimney (which is lined) to the newer wood furnace can I re-route my older oil furnace's exhaust straight out through the outside wall like a newer high efficiency furnace? Thus saving me building another chimney. Keeping my costs down is key at this point.

Mark Spetz

Sweepy Hi Mark,
You might consider a power vent system for thru-wall venting of the oil furnace.


Q: I looked all over for the answer to my question and could not get a straight answer. I asked a code inspector and a home inspector and they didn't know. I called around to a few pellet stove and HVAC guys and no one really knew. I found most of my answer on your site. Thank you!

My question is, why can't you vent a wood stove into the same flue with a gas appliance? I read the answer to the question on your site about why you can't vent an oil burning appliance with a wood stove. Would the answer be the same for a natural gas or LP burner?

Thank you for your time.
Mike Krueger

Sweepy Hi Mike,
You'd still be faced with the air intrusion through the gas appliance, its connector pipe or thimble fitting. The corrosion factor would also still apply, as you'd simply be substituting the nitric acid in the gas exhaust for the sulpheric acid in the oil exhaust. Since there is no particulate matter in gas exhaust, the creosote buildup wouldn't be increased, nor would a chimney fire be as apt to "spit" balls of flaming goo, BUT... one of the ingredients in gas exhaust is unburned methane which escapes the primary fire. It only takes a spark to ignite methane, so venting a woodstove into a gas flue could result in the occasional...............BOOM!


Q: Thought you might like to know there are exceptions to the "no sharing the flue" code, at least where wood and oil stoves are concerned. I just read (on a wood stove forum) about a code revision passed by the State Legislature in Augusta, Maine in the fall of 2008 that allows sharing the flue. Evidently, it all started when an influential citizen in the County found out that his woodstove would not be allowed, because it was hooked into the same flue as his oil furnace. Fuel prices were extremely high that season, and there was some concern about people freezing to death, so a certain County Legislator brought this legislation forward:

An Act To Permit the Use of a Common Flue for Oil and Solid Fuel Burning Equipment
Be it enacted by the People of the State of Maine as follows:
Sec. 1. 25 MRSA §2465, sub-§1-A, as enacted by PL 2005, c. 571, §1, is amended to read:
1-A. Routine technical rules. The Commissioner of Public Safety shall adopt rules pertaining to the construction, installation, maintenance and inspection of chimneys, fireplaces, vents and solid fuel burning appliances. Rules adopted pursuant to this subsection may include rules pertaining to maintenance and inspections, except as provided in subsection 1-B.
Rules adopted pursuant to this subsection may not prohibit the continued use of an existing connection of a solid fuel burning appliance to a chimney flue to which another appliance burning oil or solid fuel is connected for any chimney existing and in use prior to February 2, 1998 as long as sufficient draft is available for each appliance, the chimney is lined and structurally intact and a carbon monoxide detector is installed in the building near a bedroom.

Doesn't this set a legal precedent that would allow people in other states to practice flue sharing? Also, not trying to jerk your chain or anything, but I wonder if the findings of the presumably educated legislators in Maine might change your mind about the advisability of sharing the flue?


Sweepy No, and no.

First, as stated at the bottom of this and every code-related page on our website, each code authority is entitled to adopt whatever code specs they want, and amend them as they see fit: the provision enacted by the Maine legislature affects only those who live in that code jurisdiction.

Second, Maine's revision doesn't allow you to go out and buy a wood stove and connect it to your oil furnace flue, even if you live in Maine. As I read it, it only provides a "grandfather clause" for existing installations, stating that if you have already been sharing a flue for over a decade (since before Feb 2, 1998), the Maine Code Authority is not going to make you change your setup.

Finally, a potentially hazardous situation remains a potentially hazardous situation even if the local code authority decides to provide a loophole in the code spec. Should the Great State of Maine enact legislation allowing people who have been burning their trash in open firepits in the middle of their living room floors since before February 2, 1998 to continue to do so, we would still advise against it.


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 211) 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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