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

Q&A on B-Vents for Gas Fired Appliances

These questions & answers about B-vent chimneys & vents were posted originally at the topic home page: TYPE B-VENT CHIMNEYS. Please be sure to see the advice and fire clearances discussed there.

Photo: Chris's damaged B-Vent chinmney with multiple safety concerns.

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How do I support my chimney B-Vent flue properly where it passes through the ceiling?

My flue has recently separated at the ceiling. It appears that the installer simply anchored a pipe collar directly to the drywall via four sheet metal screws which have since given way.

I want to repair and support the flue properly by installing framing that can support the flue. I'm fairly handy with tools and general carpentry and whilst I know that this type of thing is best left to professionals, I'm unable to hire an expert at the moment.

The piping is mostly made by MTL-FAB with a couple of bends by Simpson near the furnace itself.

My problem is that where the MTL-FAB pipes have separated there is a gap of ~1". Furthermore, it appears that these pipes don't join together properly, it's almost as if they are two female fittings. There is old aluminum tape at the interface suggesting that the two pipes were simply taped together before the collar fell from the drywall.

These two pipes do not seem to use the twist-lock mechanism that I see in the company's online literature and I am unable to find a short "coupler" available from them. They offer a "Ceiling Support" model 6TGCSP which looks like it may include a short piece of pipe that would join the two sections but the drawing in the installation literature is not clear on this.

I found your site and figured I should post my issue before buying something unecessary that may not fit.

I will try calling the company as well but I think they will simply refer me to a HVAC company. On 2019-12-06 by Chris

Answer by (mod) -

I'd like to see some photos of the problem area, Chris;

Beware that making a mistake can kill you or burn down the house, so take care.

It sounds as if the original installation did not use all proper parts;

When I know more I'll be able to comment further. Let's be sure we are on the same page about fuel and chimney materials:

What's the fuel?

What's the chimney type?

What are your fire clearance distances?

Do you have working smoke & CO detectors properly installed and located?

Below is a typical chimney support used at a wood framed ceiling.

by Chris

Thank you for the quick response Dan.

Htmlcommentbox went down just as I posted a response - I wonder if the included photo was too large.

I assure you that I'm aware of the risks and fully intend to install detectors as soon as I figure out which parts are missing and repair the damage.

The furnace is gas and the chimney type is a typical metal roof exhaust. From what I've read in product literature and in my locale, the fire clearance should be 1".
I can only add one photo per comment but I have more. Is it possible to communicate over email? I hope my comments can aid others in the future.

Please forgive the insulation and damaged sheetrock around the pipe, I haven't cleaned the attic area yet. The collar has been removed for the photo.

You will note that there is a 1-2" gap between the sections. Eyeballing the ceiling support I linked earlier, I doubt I will have enough clearance to fit it in between the sections (even if it fit pipes).

Reply & Suggestions by (mod) -

No you didn't break it, Clark's server re-booted and left us off the air;

Gas flue = B-Vent so we want to find embossing or labeling confirming that all of the chimney parts are so-rated. Links above at Continue reading give B-vent clearances (starting at inspectapedia.com/chimneys/B-Vent-Chimneys.php) as also will the manufacturer'sinstructions on any replacement parts you need.

Typically that 1" clearance to combustibles is stamped right into the metal. Separate rooftop clearances are given in another of the links above.

There are ceiling support kits that connect to joists or rafters and clamp around the B-vent section. (Using foil tape as in the photo is not proper nor sufficient).

The leak stains in the ceiling make us want to look carefully at the rest of the installation from rooftop and chimney cap down, to find the leaks - it could have all been condensate but don't assume that.

I suspect you're going to want to remove and replace most or all of what's there. Fortunately B- vent sections are not the most-expensive.

It looks to me as if this vent is installed upside down, with the "male" end facing up towards the ceiling. That means that any condensate running down the flue interior wants to leak out at the joints. If that's the case then the whole installation is suspect (safety, correctness, etc) as it would have been installed by someone who didn't know the best procedures.

In the example of a B-vent from Hart & Cooley www.hartandcooley.com/chimney-and-vent/type-b-vent-gas shown below you can see the mating sections in proper orientation. Notice that INSIDE the flue the inner lip of the innermost-liner of the upper section laps over the up-facing lip of the lower section so that any condensate runs in the flue not out of it.

As we note in our article link given above, "Amerivent stamps or embosses [an arrow] right into the steel surface as well as marking it again on a printed label, giving us two chances to get it right."

by Chris

Yes, all the parts are B type double walled vents. I have inspected the roof sheeting and attic area and it doesn't appear to be leaking from outside.

I suspect the water damage was caused by the condensation in the exhaust itself when the pipes separated.

These must've been separated for quite some time (venting into my house!) before I noticed it. Luckily, we rarely use heat at my location.

This is the interior of the attic section. [Below, notes added by InspectApedia DF]

Both the section of venting in the attic and the lower section are stamped with UL 5M MTL-FAB Type B Gas Vent and 1" clearance ratings.

Both sections of pipe also have arrows pointing up which leads me to believe that they are in fact oriented properly. I'm enclosing a close-up photo of the inside of the two mating pipes.

by Chris

This is the interior of the furnace side

by (mod) -

OK right, thanks for clarifying; yes arrow up = correct.

So why were the sections taped? A mis-fit or a leak perhaps;

Watch out: The danger is fatal CO poisoning from flue gas leaks or (less likely) a fire.

by Chris

I'm not sure why it was taped. Perhaps roofers did a shoddy reinstallation at some point? I can't imagine that the furnace installer did this. This experience has been a real wake-up call for me - I will certainly be installing a dedicated detector near the furnace as well as a couple inside the home after this ordeal.

I think I was mistaken when I stated that they both seemed like female ends. I need to clean up the old tape but I think that it is likely these two sections will join together properly but I still have the issue of the gap. Do you have any idea if MTL-FAB has always used the same twist-lock securing mechanism?

Since my vents appear to have no model/part numbers beyond a generic '5M', I'm wondering if the existing vents will mate up properly to new vents made by MTL-FAB. They have a different pattern/color to the metal as compared with the new versions. I suspect these pipes date back to the early 1970's.

I came across this dual-walled flexible (accordian-styled) B vent named 'B-Flex' by the same company.
https://www.mtlfab.com/media/L1940.pdf

This seems like a better idea than trying to jimmy the large ceiling support "box" I posted earlier into place and hope that the height is perfect. I can use the thinner SureGrip support plate as well. It's quite tempting for a several reasons: Ease of assembly, lighter-weight (aluminum), rust-free, and less joints/bends.

The only downside I can think of (assuming this will mate to my existing vent) is that it likely less sturdy and robust than the original rigid steel vent.

What do you think?

by (mod) - dangers of retro-fitting metal chimney sections that do not connect properly

So perhaps someone was retro-fitting new-to-existing chimney sections and the lengths didn't match up sufficiently to make a proper and safe joint.

That might happen, for example, when connecting a new furnace and chimney in the building up to a pre-existing metal chimney at the rooftop, if the new equipment were slightly shorter or lower (or taller would give the opposite problem).

Just taping a "not quite fits" joint with metal tape is in this case unsafe.

Proper would have been to

- adjust the rooftop chimney up or down or replace it

- adjust the in-building system up or down by lifting the entire heater the necessary fraction of an inch during installation

- use an adjustable-length or cut-to-length- or flexible B-vent

Watch out: in one of your photos (shown above) I see what looks like

• Damage: chimney interior bent inwards: a likely leak that may make that section unsafe.
• Also that sooting is not common in a gas-fired heating chimney and may indicate improper combustion - itself unsafe risking fatal carbon monoxide CO poisoing;
• And of course using foil tape to span gaps in B-vent chimney sections is improper and unsafe, risking fire or CO poisoning.
• Also there are signs of condensate or external leaks along the right side of your image
  
  

Should I fill an abandoned chimney with vermiculite?

I had my gas furnace replaced recently & changed to a hi-eff & also the gas water changed to a power vent type.

So the B-vent chimney is no longer needed. The bottom is capped & I will cap the top in the spring. Before I cap it, should I fill it with a loose insulation like vermiculite ? On 2018-11-23 by Al

by (mod) -

Al

Thanks for an interesting and somewhat subtle question.

I am not aware of any reason to fill the abandoned vent with loose fill insulation; If you cap both top and bottom there will indeed still be some convection currents that circulate air within the chimney but in my OPINION that should have no effect on the building UNLESS the chimney is exposed and passes through conditioned space.

That is to say, a capped, unused metal flue passing through a chimney chase framed around and enclosed ought not affect the structure.

Contractor installed a "turtle vent" instead of a chimney cap

Hi - I had a contractor replace the roof the home I am buying. My worries is that he put a turtle vent where there use to be a type B vent - is this okay? On 2018-02-21 by Ces

by (mod) - not OK

Ces

If we're talking about replacing the chimney cap on a b vent, replacing that with an unlisted alternative device would be improper and unsafe.

Do I need to center the B-vent between rafters when penetrating the roof?

I have my B-Vent securely anchored to a rafter (maintaining a 1" clearance using AmeriVent Vent Pipe Hangers). Should I relocate it directly between 2 rafters instead before I have a roofer cut the hole and install flashing, extension pipe, storm collar, and cap? On 2018-11-11 by Scott

by (mod) -

Scott

As long as you are meeting the B-vent manufacturer's fire clearance distances including from the nearest rafter, I don't know of any reason that it needs to exit the roof precisely in the middle between two rafters.

Photo: Scott's B-vent during installation, before cutting the hole in the roof.

However looking at your photo I see a few items worth mentioning:

1. You will need to remove the B-vent up close to the roof underside or otherwise protect it from both damage and from sawdust and roof debris that otherwise will fall into the vent during the cuttting of the roof exit opening.

2. Your roof decking looks liike two or more generations of pine boards, possibly on an older home that originally was roofed with wood shingles. Or were those nailers overlaid with plywood? I can't quite make that out.

In any case, if the roof opening cuts through one of the original roof nailer boards, you may need to add blocking to support free end of the board.

3. The roof framing looks a bit hodge-podge to me with incomplete rafters tacked together to the left of your B-vent, abutting what may have originally been a lower ridge board with almost no visible nails - the connections could be questionable.

If I were a roofer asked to work on this roof from above, and if I had any experience with old roofs (and workers falling through them or of later being blamed for subsequent roof failures) I would want to inspect the condition of the roof framing and support from the attic side before going on the roof itself.

I might then have some suggstions.

On 2018-10-07 by Chuck

Do b vent fireplaces need a glass front?

by (mod) - requirement for a glass door on a fireplace

Thank you for the question Chuck.

I think you'll find that the requirement for a glass door on a fireplace is going to come from the fireplace design, or on occasion from energy codes or fire codes in new construction, not the chimney to which it is connected.

How do I tell the manufacturer of my B vent

How do I tell the manufacturer of my B vent as I hear the different manufacturers are not compatible On 2018-09-28 by RogerVicory

by (mod) - How do I tell the manufacturer of my B vent

Look for a data tag or raised metal stamping on the vent.

And you're right that usually we cannot mix B-vent parts from different manufacturers because the patented interlocking parts between chimneyt sections are usually unique.

Trying to replace parts of the b type vent pipe from the attic thru the roof - who made mine?

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 2016-09-28 by MIKE

by (mod)

I think the manufactuer's 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.

How do I extend up the vent for my gas fired water heater?

I live in a newer manufactured home. The gas hot water ventpipe from factory is too short. Snow gets in every time.

Would like to add two feet? And with back of the elbow facing where the snow and rain comes from. And put factory capback on. I do not think i need a guy with a masters degree lol to charge me 300 to do this.

So appreciate ideas, assurances,etc. Much thx. I lovvvvvvvve this site.:):) On 2018-06-19 by John

by (mod) -

Why don't you start by picking up a copy of the installation manual for your heater. That will give you the specifications on venting and you can be sure that you're using the right materials.

You should also be able to identify the type and brand of metal vent / chimney already installed and then simply buy another section that you insert at the current top, moving the chimney cap to the top of the new section.

If you need help finding the vent for your gas fired water heater, see WATER HEATER AGE & MANUALS - home

Also check out TYPE B-VENT ROOFTOP CLEARANCE  - Dan

How to calculate base or plate length and width for b- vent flashing

How to calculate base or plate length and width for b- vent flashing for any shape. Is there any thumb rule? On 2017-03-09 by VIn

by (mod) -

Stunning question, Vin. I've never considered calculating the flashing dimensions because I buy pre-fab vent flashings.

The vent flashing I buy has to have a boot that is properly sized for the diameter of the vent pipe. Some of these pre-fabs use a boot that can be trimmed to fit larger diameter pipes but take care that the fit is snug and uniform or there will be a leak.

For gas venting, there is typically a collar that mounts around the flue above the vent flashing. I seal that collar with a high-temperature silicone caulk/sealant as insurance against water traveling down through the metal boot. (Plumbing flashings use a rubber boot).

Width typically extends half of a shingle tab under at sides and 6" above, and below far enough to bring water out atop the next shingle course below, typically 4-6".

You're right to infer that on a low slope roof you might need a larger extension of vent flashing but I'venot encountered problems with factory pre-fab units.

I would not - DF

Chimney cap blew off of B-vent: ok to run the heater?

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 2017-02-13 by C Stuart

by (mod) -

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.

 

What to do about a hole in the inner metal pipe of a multi-wall metal chimney

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 2016-10-27 by Larry

by (mod)

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"

 

What is the definition of vertical wall regarding b-vent termination?

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

Reply:

Chris, as we are terminating the B-vent outdoors and above the roof line, the "vertical wall" pertains to a nearby exterior surface of a building wall. On other than the simplest "single box" building construction there are often multiple building sections or wings not all of which extend up to the same height or same number of floors.

So we may have a chimney coming out through a lower roof over a lower building section in a case where other parts of the building extend up one or more additional floors - providing a nearby "vertical wall" for which we need to respect fire clearance distances.

 

Question:

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. (Oct 26, 2011) anthony pace

Reply:

Anthony,

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.

 

Code on passing a B- Vent chimney through a "sheetrock" wall

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

Reply:

Woody,

Please see the B-Vent clearance distances from combustibles given at inspectapedia.com/chimneys/Metal_Chimney_Clearances.php

FIRE CLEARANCES, METAL CHIMNEYS

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.

 

A rafter is in the way of the direct route for my B-vent chimney

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?
thanks (Aug 13, 2015) Robert

Reply:

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.

Watch out: A mistake could be fatal.

 

Why do B-vents rust near the chimney top and at the rain cap?

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 manufacturers 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. (Aug 30, 2015) gregh

Reply:

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

CHIMNEY WET TIME & CORROSION

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Continue reading at TYPE B-VENT CHIMNEYS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• ADJACENT METAL CHIMNEY SEPARATION - separation between metal chimneys and their rain caps
• CHIMNEY WET TIME & CORROSION for a discussion of causes of corrosion in B-vents and other metal chimneys
• HEIGHT REQUIRED for L VENTS & B VENTS
• METAL CHIMNEYS & FLUES - home
• METAL CHIMNEY INSTALLATION MANUALS
• TYPE B-VENT CHIMNEYS - home
• TYPE B VENT vs L VENT DIFFERENCES
• TYPE B-VENT CEILING FLOOR WALL CLEARANCES
• TYPE B-VENT ROOFTOP CLEARANCE

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Questions & answers or comments about B-vents chimneys & flues: installation specifications.