Shared chimney flue (C) Daniel FriedmanShared Chimney Flue Exceptions
Examples of (sometimes) Permitted Chimney Flue Sharing

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When is it acceptable for appliances or heaters to share a single chimney flue?

This article describes exceptions to the general case of prohibiting shared chimney flues, multiple heating appliances, fireplaces, woodstoves all using the same chimney venting path through a building and to outside.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Shared Chimney Flues - Legal or not?

Wood and oil fired appliances sharing a chimney (C) Carson Dunlop AssociatesShared flues can create fire and smoke spread hazards and carbon monoxide hazards in buildings. In some communities or jurisdictions for certain cases, it is permissible to share a single chimney flue among more than one heating appliance or fireplace.

But important safety constraints still apply.

[Click to enlarge any image]

Our photo (page top) shows two gas-fired appliances sharing a single flue.

In Carson Dunlop Associates sketch shown here, a wood fired heating appliance is sharing a flue with an oil fired heater, both entering the shared flue on the same floor (an essential reqirement).

Watch out: this arrangement is prohibited in many jurisdictions. For example per NFPA 31 and NFPA 211

Although two openings are permitted into a single chimney flue; NO combination of solid fuel-fired appliances are permitted to be vented into an oil-fired or
gas-fired flue.

This article reviewscommonly occurring shared chimney flues. Sketch above provided courtesy of Carson Dunlop Associates a Toronto home inspection, education, and report writing tool company.

Article Contents

Combination Wood / Oil Fired Appliances on Same Floor May Share a Common Flue

Wood Oil combination boiler (C) Daniel Friedman Wood oil combination fuel boiler controls combustion air when burning wood (C) Daniel Friedman

A wood-fired heating appliance may be vented into the same chimney flue as an oil-fired heating appliance provided that they are on the same floor - in some jurisdictions.

Combination wood-oil fired heating boilers join wood and oil burners in the same physical unit, making this exception necessary if the boiler is to be used at all.

However we found some difficulty in tuning the oil burner to work at its best when a combined fuel unit was installed, because on the models we serviced the draft requirements were different depending on which fuel was in use, and because it was not the case of these systems working in an "either-or" mode - that is exclusively burning only heating oil or only wood at a particular time.

More common, the wood fire might be burning down low and no longer providing enough heat, requiring the oil burner to turn on.

We also opine that it is very important to keep the chimney well cleaned in a combination unit or a shared wood and oil device chimney.

A slow-burning woodstove can deposit thick combustible creosote on chimney walls, especially if the users are burning wood that is still green, or if the wood heater is not run at a sufficiently hot temperature.

We worry that the oil burner exhaust or the exhaust from a later and hotter wood fire might set the creosote afire - causing a very dangerous chimney fire.

Be sure your chimney is inspected and cleaned at least as often as the manufacturer of your appliances and your local fire marshal recommend.

Pertinent Code Citations:

Two Gas Fired Appliances Venting into a Single Flue permitted from Different Floors in Some Jurisdictions

Two gas appliances venting in one flue (C) Carson Dunlop Associates

Two gas-fired appliances, such as a water heater and heating boiler may be vented into the same flue if your local jurisdiction approves of that installation.

In some locales fire officials also permit two gas appliances to vent into a single flue when the heaters are on different floors, as we see in Carson Dunlop Associates [at REVIEWERS] sketch.

Watch out: Since there is a potential of increased risk of fire or gas leak spreads between floors in this shared flue arrangement, additional installation details may be required, if this is permitted at all.

Check with your local fire officials and building code enforcement officers.

Multiple oil-fired devices may be vented into the same flue from the same utility area and same building floor provided the flue capacity is adequate.

Also see safety and functional notes about draft balance and regulation at  DRAFT REGULATORS & DAMPERS.

In some jurisdictions it is permitted to vent a gas fired appliance into a chimney where oil fired appliances are also vented, with special design details needed to prevent back-drafting out of the gas fired appliance.

Watch out: venting gas fired heating equipment into an older masonry chimney may be unsafe. The lower exhaust gas temperatures produced by gas fired appliances using many clay lined chimneys may mean inadequate draft and risk fatal carbon monoxide poisoning of building occupants.

Below are three gas fired heating appliances sharing a single flue. Is the chimney size able to safely accomodate all of the possible operating scenarios?

Three gas appliances venting into a single chimey flue (C) Daniel Friedman at

Multiple Fireplaces on Different Floors Sharing a Single Flue

Fireplaces sharing a flue (C) Carson Dunlop Associates

Older homes were often constructed with two fireplaces sharing the same flue on the same floor or even on different floors.

Although this arrangement is sometimes found still in use today, this installation may not be permitted in your locale, and there are fire and smoke spread hazards as well as possible draft adequacy questions.

Watch out: Be sure you have your fireplace flues inspected for safety, and where a shared flue is discovered check with your fire marshal and local building code official before attempting to use either fireplace.

Sketches courtesy of Carson Dunlop Associates, a Toronto Home Inspection & Education firm.

Can Oil & Gas Heaters Share a Single Flue?

Yes, with some provisions and restrictions an oil and a gas fired heating appliance may be vented into the same chimney flue.

In some jurisdictions an oil fired heater and a gas fired heating appliance, such as an oil fired heating boiler and a gas fired water heater may be permitted to share the same flue provided

Photo below: oil exhaust stains around the draft hood of a gas-fired water heater provide evidence of unsafe back-drafting of exhaust from the larger heating appliance back out at the smaller gas fired appliance.

Water heater back-draft burns (C) Daniel Friedman at

Pertinent Chimney & Venting Code Citations


Gas Appliance Vent Categories (ANSI Z223.1)

International Fuel Gas Code (2012) Chapter 5 Chimneys and Vents

Appliance Category Exhaust Vent Pressure Appliance Type Flue Vent Material

Category I

Gravity furnace

Induced draft furnace Plus-80


Natural Draft

Non-condensing B-Vent

Category II

Boilers & wall-vented heaters

No shared flues


Natural Draft

Condensing Special from Manufacturer

Category III

Tankless water heaters

Cannot share flue with natural draft appliances

Positive Non-Condensing Stainless Steel

Category IV

High-efficiency water heaters

Cannot share flue with natural draft appliances

Positive Condensing Plastic


Continue reading at SHARED CHIMNEY & FLUE HAZARDS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SHARED CHIMNEY FLUES OK FAQs - questions and answers posted originally on this page.

Or see CHIMNEY CODES for downloadable copies of common chimney and appliance venting codes and standards




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