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Collapsing metalbestos insulated chimney (C) Daniel Friedman Class A Metal Chimneys & Flues
Installation Manuals, Inspection, Diagnosis, Cleaning & Repair Guide

Metalbestos or Class A Chimney Installation & Inspection:

This article describes how to select, install, and inspect Class-A insulated metal chimneys such as the Insulated Double Wall MetalBestos™ manufactured metal chimney, also called a zero clearance chimney.

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Class A Chimneys - Insulated Double Wall MetalBestos™ type

Collapsing metalbestos insulated chimney (C) Daniel Friedman Metalbestos insulated chimney end view (C) Daniel Friedman

Improper metal chimney installation: As you can see from our photo ( above left), even this superb product can be reduced to shambles when installed by an idiot. The chimney in this photo was not supported, is falling and is unsafe.

See CHIMNEY COLLAPSE RISKS, REPAIRS.

This unfortunate collapsing Class-A type metal chimney was the replacement for the abandoned masonry chimney under the corrugated metal roof we show

at ABANDONED CHIMNEYS, OUTDOORS

and that whose remains we showed

at ABANDONED CHIMNEYS. It looks as if more than one generation of inexpert chimney installers worked in Dover, NY.

Our photo at above right shows an end view of a section of Metalbestos SS All Fuel Chimney™. The fiberglass insulation is not part of the product and was just left stuffed in that opening.

Class-A Insulated double wall all-fuel chimneys such as the MetalBestos model SS All-Fuel Chimney™ are constructed of double walled stainless steel (usually) and include an insulating material (originally asbestos) between the two metal layers. Class A chimneys have been replaced in some jurisdictions with Super Chimneys

or SUPER CHIMNEYS, 629 CHIMNEYS (also called Type 650 C chimneys) which we discuss below.

Class A Chimney sketch (C) Carson Dunlop Associates Carson Dunlop Associates [at REVIEWERS] sketch at left shows a Class "A" chimney, of which our photos are an example.

Metalbestos™ type chimneys are available in different diameters, lengths, and with elbows, caps, and supporting base kits. These chimneys are used often to vent oil fired heating equipment as well as woodstoves and zero-clearance or built-in fireplaces. Our photograph at above left shows an installation leaving the insulated metal chimney exposed on the home exterior.

While these insulated chimneys may be rated for zero clearance from combustibles by the manufacturer, many building codes require at least a one-inch clearance between the chimney and any combustible materials.

Our photo at CHIMNEY CHASE CONSTRUCTION hows a wood-framed chimney chase that might enclose an insulated metal chimney or a Type-B Gas vent chimney.

Class A Chimney Height Requirements

Complete details about chimney height requirements on buildings including Class A Chimneys are now located

at CHIMNEY HEIGHT & CLEARANCE CODE

Class A Chimney Fire Clearance Details

Complete details about Class-A chimney fire clearances indoors can now be read

at FIRE CLEARANCES, METAL CHIMNEYS

Should an Insulated or "Air Cooled" Metal Chimney Be Replaced After a Chimney Fire has Occurred?

Question: What Kind of Inspection Is Required After a Metal Chimney Fire?

I was looking for information on chimney fires, specifically, on how to inspect double wall pipe after such an event. The outside of the outer pipe had paper stickers on it that were not burnt or discolored, so I am assuming the pipe did its job. However the top of the chimney run did catch on fire, presumably from the heat generated from the chimney fire. Anyway, I was looking for a category on how to inspect double wall pipe after a fire. - K.M.

Reply: Inspect for visible flue damage, color changes, movement, cracks; but to be safe: in our opinion you should replace the chimney

After a chimney fire (such as from igniting creosote in a metal flue), a competent onsite chimney inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, we add the following opinion:

In sum: in our opinion after a chimney fire in an insulated metal chimney or an air-insulated metal chimney the chimney should be replaced entirely in order to be safe and to assure that maximum fire protection for the building is maintained. In addition you should inspect for heat or fire damage to surrounding building components. Details follow.

Some chimney repair companies and building codes cite that "metal chimneys must be inspected after a fire for possible need for replacement". We speculate that the chimney inspector will look for deformed, cracked, metal components, movement, color changes in the stainless steel, or signs of overheating or damage to surrounding building components.

If the metal chimney liner and metal exterior of a multi-wall insulated metal chimney are undamaged, one might think that the internal chimney insulation of a multi-wall insulated metal chimney or flue was intact. But that may not be true. In addition there may be subtle damage to surrounding building components such as pyrolysis that could lower the combustion point and increase the risk of a future fire in nearby wood materials.

See FIRE STOPPING in BUILDINGS

Some sources point out that

Stainless Steel turns blue at 900 degrees causing the chemical makeup of chromium carbide to migrate. After this, the stainless steel will rust and cause corrosion. Stainless Steel really should be replaced after chimney fires over 900 degrees. -References or Citations

We have observed steel flues that turned blue and also black after a fire.

A "be safe" approach requires that insulated chimneys are replaced after a fire.

We believe that this makes sense because because during a chimney fire the internal temperatures can be extremely high and thus can damage the flue in both obvious and less obvious ways that may not be visible and no one wants to take a chance.

Furthermore, the "after 900 degrees" rule is impractical: - who is actually measuring chimney temperature during a fire? The 900 deg F number in our OPINION is not helpful.

Most chimney sources point out that it's common for a chimney fire to reach 2000 deg F! Who wants to risk an unsafe flue after a fire when a new fire could still occur and could meet less protection.

A similar example is with earthquake-resistant construction: the construction is designed to keep the building from collapsing and killing the occupants, but it is not expected to prevent any damage whatsoever to the building. So after a quake repairs are going to be needed.

Class A Insulated Metal Chimney Manuals, Codes, Safety References

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-25 - by (mod) -

Yes maybe;

The concern, Tom, is that the 8" chimney may be too cool and too large for a gas-fired water heater to develop sufficient draft; without enough draft such a water heater is unsafe as it risks spilling flue gases and dangerous carbon monoxide into the building interior.

It might be possible to use a power vent or draft booster. Check with the water heater's manufacturer or with the installation manual for your heater brand and model.

Let me know if you have further questions, details, opinions.

On 2020-12-25 by Tom

Can a water heater with a 4" vent be vented into to an existing 8" steel chimney? There are no other structures that would vent into the chimney

On 2020-10-10 - by (mod) -

Gary

Thank you for the interesting fuel conversion chimney question.


I would not change fuels in a chimney without first having the existing flue cleaned of creosote deposits in order to reduce a future fire or corrosion hazard.

At that point if you want to wind the existing chimney with a smaller gas flu that sounds like an interesting possibility. I would not proceed to actually buy the equipment to do that before checking with the manufacturer.

On 2020-10-10 by Gary

I have a class A chimney that I used for a freestanding wood stove. I purcashed a used Osburn free standing natural gas stove and would like to know if I can use the chimney. The Osburn asks for a Simpson Duravent direct vent number DV-GS.

I was thinking of running a 4" chimney up the middle of the class A 6" ,it runs vertical through the center of the house.

So the 4" would vent the waste and the surrounding area would be freach air. Is this possible/?

I talked to a HVAC guy and he didnt think it was a bad idea... but...The class A passed all inspections and I burned wood in it for several years.

Question: ok (safe) to vent a corn furnace (in the basement) with an external steel flue that runs in front of a window

(Sept 17, 2014) Craig Swindell said:
Thank you for posting all this great information!\

(Nov 3, 2014) Anonymous said:
is it ok (safe) to vent a corn furnace (in the basement) with an external steel flue that runs in front of a window on the 1st floor of a home.

Reply: No NFPA 211(2010):8.1.1 does not permit use of single-wall metal chimneys or unlisted metal chimneys inside or outside a one- or two-family dwelling.

(Nov 11, 2014) NH Fire Bear said:

NFPA 211(2010):8.1.1 does not permit use of single-wall metal chimneys or unlisted metal chimneys inside or outside a one- or two-family dwelling.

Even where permitted, an unshielded metal chimney must be no less than 2 ft from any window, door or walkway. 8.2.2.1.4. Locally adopted or amended codes may differ.


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