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

  • SHARED CHIMNEY FLUES OK - CONTENTS: Examples of cases where some jurisdictions permit sharing of a single chimney flue with multiple appliances or fireplaces - What is a "shared flue" in a chimney and why are they dangerous - multiple heating appliances, fireplaces, woodstoves all using the same chimney flue
  • POST a QUESTION or READ FAQs about cases when shared chimneys or chimney flues may be permitted.
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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.



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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. Here are some additional examples of commonly occurring shared chimney flues, courtesy of Carson Dunlop.

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.

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's 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

Multiple Fireplaces 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.

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