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.

Type L Vents - Double Wall Metal Vents

We 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 Associates' [reference] sketch indicates, Type-L vents are tested to 1000 deg F.

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 deg F. at the outlet temperature [of the furnace or boiler],

and for 1700 deg F. for a 10-minute "safety over fire" situation."

In the U.S. in the 1940s - 1960s, Type E vents, such as Condensing Engineering's Vitroliner were UL listed for use with multiple fuels such as gas and oil, and are in our opinion a predecessor of modern L-vent chimneys. See details at

TYPE E VITROLINER CHIMNEYS

Chimney Height Requirements for Type L Vents

Fire safety minimum heights for both L-Vent and B-Vent metal chimney vents are shown in our sketch just above.

You will see that the L-Vent needs to be at least two feet above anything such as a roof surface or building wall within ten feet of the chimney, and it should be at least two feet above the roof surface, three feet for a metal chimney serving a wood burning appliance like a woodstove. We note that some jurisdictions may permit a lower minimum chimney height.

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.

My photo (DF) shows 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 nearby roof 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?

At the prefab metal chimney with faux brick surround shown below, 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.

In my photo below we can see stains showing that water has been running down inside this chimney, leaking out inside the building. Actually water is coming down the chimney and exiting at three points as shown by the yellow arrows.

In some installations, depending on how the flue vent connector joins the chimney, water runs on into the heater itself, causing serious damage that can cause the boiler, furnace or other heating appliance to become unsafe or to stop working entirely.

Type E Vents: the Vitroliner Insulated Chimney

A predecessor to modern L-vent chimneys were E-vents, [shown above] an insulated metal (and other materials) chimney flue listed in the U.S. by Underwriters Laboratories as suitable for multiple fuels, such as the venting of natural gas and oil in the same chimney.

Watch out: nevertheless there are flue venting and fire hazards if you combining a wood burning appliance or some other solid fuel burning appliances with gas and/or oil fuel combustion gases into the same flue.

Photos below of a Vitroliner Type E chimney top and of a Vitroliner's UL label, provided courtesy of home inspector Lawrence Transue who found this Virtoliner type E chimney in a Pennsylvania home constructed in 1957.

Watch out: in our OPINION, considering that the chimney is now 60 years old, even though it was a listed (approved) product at the time of original manufacture and installation, it would make sense to inspect the flue interior for signs of damage or corrosion.

That rust at the chimney top is typical and doesn't address the flue itself.

Hidden from us are the condition of the chimney's interior surface, its insulation, and also the condition of the chimney's aluminum liner layer that in our OPINION is not necessarily very-long-term durable if exposed to corrosives from a combination of flue product deposits and condensation or leaks.

Really? An abundance of caution is appropriate where fire and life-safety questions pertain, but old doesn't necessarily mean "no-good" (the InspectApedia.com editor is ... old.)

In follow-up Mr. Transue reports:

The home buyer hired a chimney inspector who found that this chimney was is in nice shape. Just had a new furnace in 2016.

Type E Vent Chimneys and Vitroliner Type E Research

• Condensing Eng. VITROLINER TYPE “E” FLUE BROCHURE [PDF] 1948, Condensation Engineering Corp., 122 S. Michigan Av., Chicago IL, USA produced vitreous enameled pipe, produced the Vitroliner prefabricated chimneys or “flues” consisting of a steel liner coated with an acid-resistant vitreous enamel, a “special non-combustible insulation” (asbestos?), and an outer aluminum casing and an over flue housing made of galvanized sheet steel.
  
• Arts & Architecture, August, 1950, John Entenza, Ed., 3305 Wilshire Blvd., Los Angeles CA. (Est. 1911) provided in the magazines list of available manufacturer’s literature:
  
  Excerpt: “( 25a) Prefabricated Chimney : Fold· Pr entitled "Vitroliner Type 'E' Flue"; functions as a complete chimney for all home heating equipment; individually designed to fit the particular roof pitch of house with tailor-made roof flushing and flue housing;
  
  made of heavy-gauge steel, completely coated with acid-resisting porcelain; low initial cost; installs in two hours, light weight, saves floor space, improves heating efficiency, shipped complete in two cartons; listed by UL for all fuels; good product, definitely worth investigation . - Condensation Engineering Corporation , 122 South Michigan Avenue, Chicago 3, Ill. “ - p. 15
• The Vitroliner Type E prefabricated chimney was UL-listed for all fuels and was approved or added as approved by some individual municipalities in their local codes. E.g. Bremerton Washington Government ordinance NO. 1970 - http://lfpublic.bremertonwa.gov/WeblinkPublic/DocView.aspx?dbid=0&id=21949&page=1&cr=1
• Gas Appliance Merchandising, (January 1949) Vol. 21, p. 28, listed the Vitroliner type E as among just two chimneys or linings that were UL listed and approved for use in chimneys where combustion gases from multiple fuels (gas and oil) were to be vented.

Special thank you to:

• Lawrence Transue is a Pennsylvania building scientist and consultant, a certified ASHI home inspector, a Licensed Pesticide Applicator, a BPI Building Analyst & Envelope Professional, with 18 Years of Home Inspection Experience. He can be reached by Telephone: 610.417.0763, by Email: lawrence@lawrencetransue.com as well as at his WEBSITE and at FACEBOOK.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question:

I have changed from an old Anthes furnace from 1970s to mid efficiency 60K BTU gas furnace. The furnace guy says that because it is less than 80K BTU therefore it needs a Type B venting. This does not make sense, as Type B has lower resistance to temperature than Type L.

Thus having Type L is more safe just in case there is over firing etc. I suspect this guy is trying to make money in addition.

Also, the old furnace has a gas connection where the gas pipe coming down vertically has slight bent like 5 degrees max. This has been since I bought the house.

He says this bent is not right and needs a modified gas connection. Please advise what should I do regarding these issues.

Also, previous gas connection had a street elbow instead of a regular one and he says street elbows are not allowed. (Nov 26, 2012) Ric

Reply by InspectApedia moderator (DF):

Ric I think you're right that as long as the size dimensions & clearances are correct you can use a safer, better-insulated type chimney. Did you ask your local building department what they'll approve?

I'm not sure what to make of the bend in your gas piping, but I do warn that any gas leak is of course very dangerous, risking an explosion, fire or other hazards. Ask your licensed plumber to take a look.

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Continue reading at FIRE CLEARANCES INDOORS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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• METAL CHIMNEYS & FLUES - home
• METAL CHIMNEY INSTALLATION MANUALS
• TYPE B VENT vs L VENT DIFFERENCES
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