Type B gas vent installation specifications (C) Daniel Friedman Type B-Vent Chimney FAQs
Type B gas vents

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Metal B-Vent flue installation requirements & inspection questions & answers:

This article answers frequently-asked questions about B-vent metal chimneys used for gas-fired appliances.

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Type B-Vents for Gas Fired Appliances

Type B gas vent installation specifications (C) Daniel FriedmanThese questions & answers about B-vent chimneys & vents were posted originally at the topic home page: TYPE B-VENT CHIMNEYS.

On 2017-02-15 by (mod) - chimney cap blew off of B-vent: ok to run the heater?

I'm a bit scared to bet your life and safety on a "yes" for a home and system about which I know not a darn thing, but in concept, the heat will work; the risk is downdrafts or inadequate draft - that could produce fatal CO - carbon monoxide.

If you're running the system while waiting for replacement of the missing cap be SURE you have working carbon monoxide detectors properly located, installed, tested.

On 2017-02-13 by C Stuart

We had a windstorm today and the vent along with its oval tpye B gas vent cap blew off the roof. Is it OK to turn on heat until it's repaired?

On 2016-10-28 by (mod) what to do about a hole in the inner metal pipe of a multi-wall metal chimney

For safety, and considering the risk of a fatal carbon monoxide poisoning or a fire at a building of which we know so little, if it were my job I'd replace the damaged section of B-vent.

If nothing else we're unsure how much remaining life there is in the inner pipe that you found as "undamaged"

On 2016-10-27 by Larry

my B. chimney has a 1 inch hole in the outer pipe, the inner pipe is undamaged. Is it safe to patch the outer pipe, then use metal tape to seal it? I await your advise thank you

On 2016-09-29 by (mod) trying to replace parts of the b type vent pipe from the attic thru the roof

I think the manufactuer question was asked because newer replacement sections of B-vent may not clip properly, nor safely, to older B-vent parts from a different manufacturer.

To replace parts and keep parts of the existing system, why not bring along a section of the existing B-vent when you're shopping? That way you can compare features and connections.

On 2016-09-28 by MIKE

My house was built in 1962 and I am now trying to replace parts of the b type vent pipe from the attic thru the roof. when I went to buy the materials they asked what manufacturer?

They told me it was stamped on the pipe? I can't find any stamping on any of the sections?

How do I replace this vent pipe?

On 2016-02-13 by (mod) For flat and very low slope roofs the above-roof chimney height may in some cases be less than two feet


You're right. For flat and very low slope roofs the above-roof height can be less than two feet, as shown in the TYPE B-VENT ROOFTOP CLEARANCE TABLE given in TYPE B-VENT CHIMNEYS

Keep in mind that on these low slope or flat roofs, at least in some buildings, you'll need to consider the adequacy of draft - a too-short chimney may not provide enough draft, so its above-roof height may need to be extended for that reason.

I've clarified the text in the article. Thank you for careful reading and for taking the time to comment.


On 2016-02-13 by AaronBrauer

I'm confused. The article TYPE B-VENT CHIMNEYS states,"type-b vents should be at least two feet above the roof's surface."

But the table shows that they can be as short as one foot above the roof's surface. Huh?


(Oct 14, 2011) Chris said:
what is the definition of vertical wall regarding b-vent termination? Interior wall or exterior wall continuing higher than the roof


Chris, as we are terminating the B-vent outdoors and above the roof line, the "vertical wall" pertains to outside conditions.


(Oct 26, 2011) anthony pace said:
I have a Carrier mod. 58pav090-16 (Dec.1999)-the bottom of the flue pipe (cap) is corroded and has accumulated (sand like) debris. The pipe up thru the attic looks good and there is a raincap on the top.



When a gas flue vent or chimney is corroded it is potentially unsafe as it may perforate and leak. It most likely needs replacement.

Further, corrosion of a metal flue on gas-fired equipment is a red flag warning to look out for improper and thus unsafe heating appliance operation.

For example, insufficient combustion air, leaks into the chimney, improper chimney installation (too cold to vent properly) can increase the rate of flue corrosion while at the same time those conditions can threaten the production of dangerous, even potentially fatal carbon monoxide gas.

To be able to sleep safely while waiting for your chimney or HVAC expert, be sure you have working carbon monoxide detectors properly installed.

Question: elbows in vents

(Mar 22, 2012) Gettinitright said:

Is there any code regulation against installing 90's (elbows) 1 or 2ft from the protrusion of the roof? Updating a unit from 1990 and the new code says I need to have 1" clearance around all wood framing. I would like to install 2 elbows instead of cutting a new hole in the roof to effectively relocate it.

(Oct 30, 2012) Tom Painter said:

I have a two store house built in 1923. I was thinking of closing up the fireplace and walling it off. A new water heater and gas furnace vent a flue in the basement that by-passes the first floor fireplace. The top of the chimney leans a little and needs to be repaired. With the fireplace walled off, and its very existence hidden behind drywall, can I have the top rows of chimney bricks removed down to B vent height requirements?

(Nov 30, 2014) George Drexel said:
How far can I run b-vent horizontally from a furnace before turning upward?


George I can't guess at an answer on horizontal B-vent limits - as I think it depends on the appliance being vented. Smaller BTUs, for example will probably permit only shorter runs. An easy solution is to check the installation instructions for the appliance being vented - or tell us the appliance brand and model and we can help research the question.

(Apr 27, 2015) George said:
I have a B vent extending up thru the roof of a commercial building. We added a parapet wall 12" away from the vent.

It presently extends up 24" Do in need to extend up above the new parapet wall ?

I neglected to say the new parapet wall is 48"


George I think we've answered this in the clearances above and in a separate article that should be helpful:

B-Vents should be at least two feet above the roof surface as well. See the detailed table below and also see TYPE B-VENT CHIMNEY CLEARANCES - separate article compares with other chimney types.

If a vent terminates next to a wall it may fail both fire clearance and suffer draft interference. Your local building code inspector is the final authority so that's whom to ask. If I were inspecting (and I"m not) I'd want the vent to terminate 2 feet above roof components including a nearby parapet wall.

Question: code on passing a B- Vent chimney through a "sheetrock" wall

(July 23, 2015) woody said:
what is the calif.code requirement for furnace (type B-vent) passing thru a sheetrock wall or can i use single wall



Please see the B-Vent clearance distances from combustibles given at


There you'll see that for gas appliances you'd need 6-inches from combustibles (including drywall and wood framing)

Do not run a single wall flue vent connector to pass through building walls. You'd need (varying by location and local codes), typically 18" to combustibles.

Question: a rafter is in the way of the direct route for my B-vent chimney

(Aug 13, 2015) Robert said:
My gas furnace uses a B-Vent, I have a rafter in the way. Does the B-Vent have to be straight or can I use 2 90 degree with a short straight piece to go around rafter?


Robert, depending on total flue length and the free space available you may be able to turn around the rafter, but you may want to use 45's rather than 90's for better draft. I'd sketch the whole flue length and dimensions and then check once more with the appliance manufacturer to be sure they agree that it will vent adequately. A mistake could be fatal.

Question: why do B-vents rust near the chimney top and at the rain cap?

(Aug 30, 2015) gregh said:
Many experts seem confused as to why some b vent pipes tend to rust near the top under the rain cap. The rust has absolutely nothing to do with the age or condition of the gas appliance. The rust is a result of the flue gas coming in contact with the outer shell of the b vent which is a type of galvanized metal.

This outer shell is generally cool to the touch , so when the warm flue gasses contact the outer shell and condense on it over a long period of time rust will result as the flue gasses are highly corrosive. Several manufactures became aware of this 40 years ago. Selkirk designed a rain cap around 1971 which incorporated a lower ring shield to direct flue gas away from the b vent pipe. Both Selkirk and Ecco manufacturing sold a painted termination pipe from 1973-1985 .

By this time most cap designs used some type of shield to keep flue gas from drifting down onto the b vents outer surface. Also mid efficient furnaces are terrible for creating more corrosive vapor in flue gas as these appliances don't have a draft hood to introduce dilution air which would help dry the flue gas somewhat.


Interesting details, Greg, thank you. Indeed condensate from gas fired appliances is quite corrosive. Understanding just where the corrosion shows up is both diagnostic and interesting. We'll edit your comments into the article on Chimney wet time over at



(Oct 17, 2015) Ralph rangel said:
There is a machine screw sticking into the inner B-vent wall (flue.
it sticks in approx. 1/3".

Is this permitted by code (Chicago)

On 2015-12-30 at RID:11 by jim H. [delete]

wanted to find out, what are the requirements for installing a chimney in a half round building. i have a building im going to use as a shop to tinker in, and need some heat. any ideas. its getting cold and winter is coming.


Ralph: I don't know but I doubt that it's an issue (except on dryer ducting).

Jim you'll want to meet the same roof clearances and fire clearances as for other buildings, keeping in mind that I can't answer a question like this: we don't know the fuel, type of heater or what the heck you're venting.


Continue reading at TYPE B-VENT CHIMNEYS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Or see CHIMNEY WET TIME & CORROSION for a discussion of causes of corrosion in B-vents and other metal chimneys

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