Short  metal chimney (C) Daniel Friedman Installation & Inspection of Type L Chimneys & Vents for Oil Fired Appliances
     


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L-vent chimney specifications: this article describes the installation and inspection of L-Vent chimneys used to vent oil-fired heating systems. We illustrate both safe and unsafe L-vent metal chimney installations, including chimneys that are too short, too close to other structures, are damaged, or are missing components.

Our page top photo shows an L-vent chimney that is too short above the roof, and too short in total rise - the oil fired boiler it vents never worked properly and always had marginal draft until this chimney was replaced with a taller unit.

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Type L Vents - Double Wall Metal Vents

Type L Vent (C) Carson Dunlop AssociatesWe use Type L-Vents, double-walled metal chimneys for venting heating appliances not only for fire safety but also because the insulation provided by the doubled walls improves draft - an important safety requirement when venting oil or gas fired appliances.

A Type-L vent is used for oil-fired appliances but can also be used for venting natural gas fired appliances.

But as we explain at TYPE B VENT vs L VENT DIFFERENCES, the reverse is not true - that is, you can substitute an L-vent chimney for a B-type vent, but you cannot go the other way: a B-type vent chimney cannot be used in high temperature applications where an L-vent chimney is required.

As Carson Dunlop's sketch indicates, Type-L vents are tested to 1000 degF. [Some products list lower listing temperatures, as we explain here.]

Some manufacturers such as Metal-Fab, Inc. provide installation manuals for L-Vents indicating that their L-Vent systems are listed for gas and oil furnaces that do not exceed 570 degF at the outlet temperature [of the furnace or boiler], and for 1700 degF. for a 10-minute "safety overfire" situation."

L vent and B vent height requirements (C) Carson Dunlop Associates

Chimney Height Requirements for Type L Vents

Short  metal chimney (C) Daniel FriedmanHeights for both L-Vent and B-Vent metal chimney vents are shown in Carson Dunlop's sketch. You will see that the L-Vent needs to be at least two feet above anything within ten feet of the chimney, and it should be at least two feet above the roof surface.

The sketch notes add that a metal chimney venting a wood burning appliance like a woodstove must be at least three feet above the roof surface.

Here is a metal chimney which is obviously too-short, violating both of the height specifications cited just above. The chimney is less than two feet above the ridge and less than three feet above the roof surface.

You may also notice that it was improperly installed - the roof flashing sides are open to leaks.

More complete details about chimney height requirements on buildings is at CHIMNEY HEIGHT & CLEARANCE CODE

L-Vent Chimney Fire Clearance Details

More complete Details about L-Vent chimney fire clearances indoors can be read at FIRE CLEARANCES, METAL CHIMNEYS.

L-Vent Type Chimney Defects, Goofs, and SNAFUs

In addition to the "too short" L-vent type chimneys already describe above, here are some other troubles to look for.

How about that rusty L-type chimney at below left?

Type L chimney defects and hazards (C) Daniel Friedman Type L chimney defects and hazards (C) Daniel Friedman

At above right the metal cap has rusted and blown off of this Type L vent serving an oil fired heating system. Rain down the flue invites corrosion, chimney damage, and unsafe flue, and a damaged heating appliance. At below left we can see stains showing that water has been running down inside this chimney, leaking out inside the building. In some installations, depending on how the flue vent connector joins the chimney, water runs on into the heater itself.

Type L chimney defects and hazards (C) Daniel Friedman

Readers should also see METAL CHIMNEYS & FLUES and then FIRE CLEARANCES, METAL CHIMNEYS. This article series on chimneys, chimney construction, and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects.

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