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Metal chimney at rooftop (C) Daniel Friedman Metal Chimneys, Flues, Flashings
Inspection, Diagnosis, Cleaning, Manuals & Repair Guides

This article describes the types of manufactured metal chimneys, their installation, inspection, troubleshooting & repair.

We explain the difference between Class-A Chimneys, Metalbestos chimneys, Type B Vents, Type L Vents, Triple Wall metal chimneys, and Super Chimneys, 629 Chimneys, and 650-C Chimneys.

Each of these chimney types has specific intended uses, fire clearances, and installation requirements. Improperly-installed chimneys, use of the wrong chimney type, or failure to properly maintain the chimney are likely to lead to unsafe conditions and risk a building fire.

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?

Metal Chimneys & Flues: Types, Codes, Installation Requirements

Metal chimney at rooftop (C) Daniel FriedmanTypes of Metal Chimneys and Metal Vents

Our page top photo shows a remarkable number of metal chimneys (at least 24) on a Duluth Minnesota building. Can you explain it?

The metal faux-chimney top shown in our photo here is being inspected by an ASHI Headquarters staff member during a Chicago IL field trip.

Disassembly would be required to know much about the interior of this chimney, but from the exterior we can inspect for

Without more information we cannot see for sure just what kind of metal chimney terminates in the chimney cap shown by our ASHI Staff person.

Manufactured Metal Chimney Types

Here are the principal types of manufactured metal chimneys. We discuss each of these in the articles linked-to below.

Metalbestos chimney cleanout © D Friedman at InspectApedia.com

Metal chimney and flue: types, clearances, installation, inspection, fire hazards

Metalbestos chimney cleanout © D Friedman at InspectApedia.com

Photo above: a round collar and round chimney flashing have been installed at the penetration through this low-slope EPDM-covered roof of a Poughkeepsie, New York home.

Question: Metalbestos stove pipe fire clearances: roofing mastic was blobbed around our Metalbestos chimney

I have an 8 inch Metalbestos™ stove pipe going through an open beam ceiling, with an Avalon wood stove.

My question is how hot does the outside of the Metalbestos pipe get on the roof in the area of the stove pipe flashing, its a reddish rubber boot style?

The reason I am asking is a roofer installed a new metal roof and rubber boot flashing and cut the back side of boot to get it to fit then patched it with a black sticky flexible flashing, with flash point of 201 degrees. Thanks - R.W.

Our photo (left) shows a Class A "Metalbestos™ type chimney at a low slope EPDM roof penetration - not the chimney penetration in the reader question described above.

Reply: Follow the manufacturer's instructions for air spaces and chimney clearances

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem or identify fire clearance and chimney safety issues including the one you raise. But when workmanship is amateur or incomplete, keep in mind that the same installer may have made other errors.

That said, here are some things to consider:

The temperature that you will find on the outside of the Metalbestos chimney pipe varies from cold (not in use) to warm (in normal use) to potentially higher, depending on what fuel is being burned, distance from the fire source, conditions in the chimney, and other variables.

Example of an insulated metal chimney surface temperature

At a new Class "A" insulated metal flue venting an oil fired heating boiler and located about 35" from the flue vent connector atop the boiler and with the boiler at operating temperature (about 450 deg F inside the flue vent connector at the boiler top) the outside temperature of the Metalbestos chimney was "hot" to the touch but less than 200 deg F

But because of the variables involved that cause variation in metal chimney surface temperatures, I'm not sure anyone could quote a specific number to answer your question.

You will find in the metal chimney installation instructions (and perhaps your local building codes) that the manufacturer wants 2" or 2 1/2" air space clearance between the Class "A" chimney sections and any nearby combustibles.

So if your roofer put combustible roofing mastic against a Class-A chimney chimney s/he has probably violated the instructions and possibly local and national building codes.

Single metal flue through a barn wall north of Duluth (C) Daniel FriedmanDetails about fire clearances for different types of metal flues and chimneys are found

at FIRE CLEARANCES, METAL CHIMNEYS

Readers of this section should also

see CHIMNEY DEFINITIONS.

Also
see FIRE CLEARANCES, METAL CHIMNEYS

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.

This article series explains the types, uses, characteristics, and installation requirements for different classes of metal chimneys.

All of these metal chimneys use multiple-walled metal pipe; some products add a fire-proof insulating material between the walls of the chimney pipe while others rely on air space.

Single-wall metal pipes used to connect heating appliances to chimneys (metal or masonry) are discussed separately

at FIRE CLEARANCES, SINGLE WALL METAL FLUES & VENTS where we provide details about single wall metal flues and their fire clearance requirements.

The installer of the single wall metal chimney (probably venting a woodstove) observed on this Minnesota barn on Highway 61 north of Duluth had to go to some trouble to clear the edges of the barn roof. This chimney made me nervous.

Metal Chimney / Flue Installation Manuals

Oliver MacLeod chimneys & woodstove logo - cited & discussed at InspectApedia.com

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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 2020-06-24 by Sherwood Botsford

I have a combined radiant heating system/domestic hot water system. This unit is a middle effiiciency unit rated at 87% The boiler is connected to a B-vent chimney with single wall vent connector. The boiler is also equiped with an electric damper to reduce air going up the chimney when not operating.

The B-vent was replaced 15 years ago and is in good shape. The vent connector is so corroded that last night it collapsed. It was replaced at the same time.

I have shutdown the unit until I can get it vented properly again.

A: Why is my vent connector pipe corroding so badly?

B: How can I prevent this from happening again?

***

My suspicion is that the boiler is oversized (90,000 BTU) and is short cycling. Even on cold winter days it rarely runs more than 2-3 minutes at a time with 10-20 minute off times. This results in hot humid air in the vent and chimney which is kept there when the electric damper closes. Water condenses in the exhaust system, and never really dries out.

My thoughts:

* Disable the damper in the open position.

* Punch a hole in the damper so that it doesn't shut all the air off in the off position.

On 2020-06-24 - by (mod) -

Sherwood

I don't know for sure what's going on with your system from just a brief e-text, but it's possible that the system wasn't venting properly, producing corrosive condensate, corroding the flue vent connector.

A short cycling boiler could, as you wisely suggest, be blamed: if we never run long enough to heat up the flue then condensation is more likely to be a problem.

And condensate in a gas flue is indeed corrosive.

Worse, the system could be unsafe; be sure you have properly located, tested CO & Smoke detectors.


On 2019-06-22 by Jean

We have a metal stove pipe to our wood stove, how far above our peeked roof must the stovepipe be and does it have to be triple wall

On 2019-06-23 - by (mod) -

In the article index you'll find an article in chimney height that gives all the details for above roof and other clearances. Yes your chimney needs to be rated for use with the type of fuel that it burns, such as wood.


On 2017-05-23 by Jim Osekowsky

@Anonymous,Galvanized is only for INSIDE the house use. As an aside, galvanized metal gives off toxic fumes at 500 degrees, use ss.

On 2017-05-23 - by (mod) -

Thanks for the remark, Jim.


On 2015-12-06 by Anonymous - Installing a 5790 Vogelzang pellet stove

Can someone tell why a pellet stove can't vent into masonory chimney with a clay liner.I live in VA and just bought pellet stove always had wood stove wife tired of mess bugs cutting of wood so on you get my point.

Installing a 5790 Vogelzang pellet stove in basement want to use existing thimble mad me a piece up 8 x 4 reducer.My manual came with stove says use simpson dura vent or metal fab pipe say don't use type b or galvanized pipe.

Can some one explain why can't use galvanized or type B its much cheaper and its already gonna be inside clay liner.

I have access to both liners cleanout doors.Whats the most economical way to vent both stoves and chimneys.Basement stove VG5790 its about 25ft. run to he top of liner clay liner is 8 1/2 by 12 1/2 .

Upstairs stove is older stove very well maitained Avalon 900 pi insert stove putitng in fireplace clay liner is 12 1/2 by 12 1/2 about 11ft to 12ft run at the most top of liner could someone please please help me trying to figure most cost efficent way to vent these and get the most out of my stoves HELP PLEASE!

On 2015-12-06 - by (mod) -

Ron:

I think the reason you find people using stainless steel chimney liners is its durability against corrosion and its inclusion of flexible material that can pass chimney bends with best draft performance.

You should never us a chimney product or material in application for which it has not been certified safe - or in the parlance "listed" - the risk is death by carbon monoxide poisoning or building fire.


Question: rust stains on the wall of a cottage in Wales

(Aug 12, 2012) Mark Whitaker (UK) said:

The following information may be useful. I experienced problems with rust stains on the wall of a cottage in Wales beneath the outlet of a flue from an oil-fired boiler. The stains were due to rain falling on the outlet and dripping down.

I totally solved the problem by installing a protective cowl which I made from thin sheet aluminium fashioned into an inverted angular 'U' form (a bit like a metal paper staple in profile) to shield the outlet from the rain. This has saved me continually repainting the wall to erase the unsightly rust stains.

Reply:

Thanks for the idea and comment, Mark. Send us some photos if you can. It may assist other readers.

 

Question: bulged metalbestos chimney dangers

(Jan 17, 2013) Diane said:
We have a metalbestos II chimney that's been in use for 18 years. We use it with our Harmon wood boiler. We've noticed some bulges/buckles in the interior of the chimney for the past couple of years while cleaning it out. Are these dangerous? Has anyone had experience going back to Selkirk for a replacement?

Reply:

Diane:

a bulged metalbestos chimney sounds unsafe - as if its internal layers have been damaged by frost or some other expanding force. I'd have a professional chimney sweep inspect the chimney for safety. Let us know what you're told and we can comment further.

 

Question: what's inside of old chimneys?

(Jan 27, 2013) Kristina said:
I have opened up a closet wall to expose the original lath and plaster and discovered a metal (over ceramic?) tube full of old old ashes that i was able to clean out by opening a plate at the bottom and scooping out about 2 grocery bags full of old ashes.

I would like to remove or cap this off under the ceiling and then recover the wall with sheetrock AND I am unclear if this is something I should leave alone or if I can do this removal.

I have been told that this must have been the inner chimney for wood burning stove/cooking from the original house (built in 1926)I now have a gas stove and a hood that I dont think is attached to this circular chimney thing.

Do you have any idea what this is or how I can get more information on how to remove it or if I should just leave it alone? (asbestos?) thansk so much!

Reply:

Kristina

If you are confident that the old chimney is no longer in use, its blockage or abandonment are less of a concern - if it were in use the situation would be quite dangerous - enough that I'd shut off anything venting into that flue.

A second concern is possible presence of asbestos - or cement asbestos - if the flue were transite - an asbestos cement material. If the chimnney is really metallic it's not transite but if it's a multi-walled chimney and is an older product its internal insulation could be or contain asbestos.

So removal could raise safety concerns. If it can be safely sealed in place and left alone that'd make the least disturbance.

 

Question:

(Nov 9, 2014) Wes said:
I have an external masonry brick chimney (with an internal clay flu, that has began to separate from the home about 70% up. The bottom half is fine, and is still aligned well. Is it possible for me to remove the top 30%, and connect steel chimney pipe to replace the masonry? wesgull@hotmail.com

Reply:

Wes:

Indeed having just traveled throughout New Zealand where earthquakes have damaged many masonry chimneys, we saw a number of installations that did just what you suggest.

The damaged or unsafe upper portion of the masonry chimney was removed entirely, an insulated metal chimnney was constructed to provide safe above-roof venting and was installed, connected, and sealed to the intact lower masonry flue by a chimney professional.

A detail that's important is the treatment of the "flat" masonry top - to be sure that there is no leakage into the masonry.


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