Thread: B vent clearance

The B vent is in contact with the loose fill insulation. I know about the 1 inch rule.

My question is, is the loose fill considered combustible making this installation wrong?

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Originally Posted by John Dirks Jr

The B vent is in contact with the loose fill insulation. I know about the 1 inch rule.

My question is, is the loose fill considered combustible making this installation wrong?

The tags i've seen on the pipe states 1 inch from combustables and insulation

That is the way I state it in my reports, "insulation and combustibles"
The pipe cannot cool itself if buried in insulation.

Thank you for the help gents.

This is an area where I slide on the requirements - I don't report on it. In 16 years I have never seen a resulting problem of B-vent contact with attic insulation. In fact, I've never heard of a problem with insulation igniting from the B-vent passing through the attic. I'll even admit to not reporting on the B-vent in contact of the drywall ceiling above the furnace. This is a very common installation around here and I've never seen scorching as a result. I'm not advocating that others ignore this - it's just my practice to do so.

Eric, I'm not saying the insulation will scorch, burn or even cause a problem with the insulation or the structure but that the insulation prevents the pipe from functioning as designed and listed.
Why else would the manufacturer's require no contact to insulation OR combustibles? The pipe has to cool the outer surface at a given rate in order to keep it from overheating and destroying itself and the structure.
Just because I have not seen someone get electrocuted for lack of a GFCI does not mean I don't call it out.
How many vent pipes have you dug out of insulation and examined the installation and surrounding insulation? A few inches of fiberglass probably won't hurt a thing but how about 18 inches (the Hud recommendation here) or what about if it is a 5' horizontal run buried in cellulose?

I don't have the answers, so I stick to the instructions as best as I know how.

Hi Jim,

I understand where you're coming from. If there's one problem that I do frequently see with B-vents in attics it's that they don't stay warm enough to prevent condensation from forming inside the vent - a cold weather event. So for the vent to be covered in 12" or so of insulation I just kinda of go with it and concentrate on other issues I find.

I have seen one case of what appeared to be melted fiberglass insulation stuck to the outer shell pipe.

A one inch clearance is a one inch clearance *to combustibles. Report it and move on. Note that blown wool (the white stuff that is actually glass not woll) is not combustible but the cellulose in that photo is. As was mentioned, pyrolysis is an issue and when it "becomes" an issue isn't something I want to count on or guess about.

I report it with a photo every time I see it. The proper spacer can be purchased at most local hardware or Home Depot's.

I think Jim hit it on the head. It needs to dissipate heat. I don't think it's because of fire hazard (to and extent) more then a possible hot spot for flue draft purposes. It is my understanding that flue gasses from a 80% unit would be around the 250 degree range. (ballpark) which I also believe is under the combustion temps of what you find in contact with the flue.

There are many homes from the 70's and later that you find Sheetrock, insulation etc, in contact and no harm done. But as Jim said now you have 18" of insulation in attics in contact.

It would be nice to have a rep/tech way in and give the exact reason why?

I concur 100% and write it up every-time, I would like a definitive reason why though other then because it is written. Maybe Bob will weigh in. Inquiring minds want to know.......

I can see three reasons for the one inch gap:

1. In the event of a thermal runaway condition where the appliance's safety controls have failed and the temperatures sky rocket. This would be too rare to imagine.

2. The manufacturer's attorney and head design engineer said to make it one inch.

3. So any rusting can be seen during an inspection.


I have found paper installation manuals tucked in tight against the flue pipe right at the furnace output that had no yellowing or any indication at all of a problem. The oldest house that I have found this on is 7 years and you could tell by the dust that it had been there the whole time.

An insulation shield of no less than 26 gauge sheet metal shall be installed to to provide clearance between the vent and the insulation and extend no less than two inches above the insulation layer in the attic (IRC 2425.4).

For the sake of discussion, would those of you who strictly adhere to "code" write up that these beams are improperly secured?

You guys are giving Eric a hard time and he telling like it is

I think most of you are missing the point of why they use B vent over single walled pipe in the first place..

They could just slip some larger single wall around the B vent to prevent the cellulose from touching the pipe.

To answer the OPs original question: yes, it is wrong. Clearances are air spaces. Even if you filled it with concrete, it would be wrong..

Yes, you can make a ventilated attic insulation shield to comply with the listing. It needs a means of preventing insulation from blowing down btw pipes. You can either 'daisy petal' snip the top and fold the tabs inwards leaving slits to ventilate it or use a storm collar with a nominal gap so it can breathe. If the mfr. makes a listed attic insulation shield, that is what you should use. If they don't, you should fabricate one in the field.

I agree it is rare to see a fire caused by B-vent incidental contact. However, as I have posted before, when treated cellulose comes in contact with metallic venting, moisture tends to condense on the outer wall of the pipe. This moisture will leach out the borate or phosporous based salts used as a flame retardant, rodent/ bug deterrent. I find salt licks on the firestops all the time. The insulation is molded to the pipe and yes, it does begin to pyrolyze. I have seen fiberglass insulation melted if you will, to the pipe. In this case, often the binders form a gooey glue to the pipe. More prevalent on hotter pipes such as solid or liquid fuels.

Bruce made some good points. The one about being able to inspect it without pulling insulation back is kinda negated by an insulation shield but still both are easy to pull back for inspection. The abnormally high stack temp event could be from weak draft or blockage at the flue gas outlet. I've seen B-vent get really hot when plugged with a nest.

Eric, I hear what you're saying and I don't have a list of homes definitively burned down by B-vent in contact with cellulose (maybe Dale Feb does) but...........It is and has been a code requirement for at least 30+ yrs. If you know about it, why not call it out and be done with it? God forbid there was an unfriendly fire, you would be drawn in as a primary target. Even if you prevail, it will cost you a lot of time, money and a piece of your rear end to defend yourself. Just seems too cut and dried to me.

Now, can I go back to sleep? :-)

Is cellulose insulation even combustible?


http://www.nationalfiber.com/uploads...ber_NuWool.pdf

FIRE FIGHTING MEASURES

General Hazard: Nu-Wool® cellulose insulation is not flammable or explosive.
Extinguishing Media: Any fire extinguishing media may be used on nearby fires.
Flammability Classification (29 CFR 1910, 1200): Non-flammable solid.
Unusual Fire Hazard: None. However, material should not be installed where temperatures may exceed 180°F. Adequate clearance should be maintained around non IC rated recessed lights, chimneys, and other heat producing equipment as specified in the National Fire Prevention Code.

PHYSICAL AND CHEMICAL PROPERTIES

Appearance:

Gray, odorless fiber Boiling Point: Not Applicable

Specific Gravity:

0.7 compressed Melting Point: Not Applicable

Vapor Pressure:

Negligible @ 20ºC Flash Point: Not Applicable

Solubility in Water:

Fiber is not soluble; Chemical pH: 7.4 (2.0% solution @ 25°C)

additive is soluble at the rate Viscosity: Not Applicable

Nice picture! Only needing 1" to combustibles is one reason for using B-vent. The other would be to keep the flue gases warm enough to escape before condensing and running back into the furnace. The water is corrosive and will eat away the heat exchanger over time and thats not good either. You can see in the picture that looks to be single walled pipe by the slip connection.( not illegal, but starting the B-vent sooner might have prevented the condensation ) It's not your job to fix it, but having a better understanding of why, helps you document it better.

As I said earlier, I understand where you guys are coming from and you are all correct in your positions. In my area (Chicago burbs) the practice of installing a shield around the B-vent simply is not followed or enforced. Basically, I've given up on reporting it.

Instead I concentrate on other issues. In the attached, the wood surround outside the fireplace has been deemed as safe by the village inspector. I've had to scan NFPA 211 as well as get a PDF manual from a fireplace manufacturer to send to the agent (who knows I'm right) to quell the angry waters over what I wrote. It boils down to picking your battles.

This isn't about report writing - it's about poorly trained/educated muni-inspectors and contractors saying that you're wrong. Most of you guys know the drill.

Craig,

You can document all you want. But if you're talking gibberish to the HVAC guy then it's a real hurdle. He isn't going to believe you - primarily because he doesn't want to look like an idiot. My report has a paragraph about condensation inside B-vents just to educate the client enough to recognize when the contractor has no idea what he's talking about. When you report and have pictures on water stains coming out of the vent joints in the attic the contractor will say it's a roof leak and has nothing to do with the vent or furnace. The second picture is a ice stalagmite on an attic floor right under a 45 degree vent elbow.

Perhaps my area has an exceedingly high number of moronic contractors.

Eric,

Would you write up the foam insulation on this refrigerant line as being too close to the furnace flue? Is it a "combustible" material?

rick

It would depend on if the foam was a petroleum product or not. Some are and some are not. Guess it depends on how much faith you have in the Chinese factory's disclosure on whats in it,,,,,,,,,,

Rick now be fair. The foam is not in contact
Is that electrical conduit on the condensate, Trap?, Secondary?

The attached photo is commented upon at a blog posting, thus:
"Code requires a 3" clearance at the furnace flue, your duct wrap is touching the furnace flue, serious fire hazard."

I rebuilt the ducts to accommodate my attic ladder installation, placing the air-return plenum at the original scuttle hole. I could not imagine a way of pulling the flexible ducting to the furnace, that avoided contact with the b-bent. During furnace operation, the b-vent remained just warm to the touch, maybe 100 degrees maximum. So, I left things as in the photo. I don't consider the flex duct to be flammable. The point contact of insulation can't raise b-vent surface temperature by more than a few degrees.

I suspect by the tone in this thread, that members will not strongly criticize my actions. Yet, I will go back and assemble a one-inch metal stand-off over the b-vent for the criticized job. In time I will cite this thread with new posts, to the responder to my blog. I will criticize the responder for carelessness in citing a three-inch rule, and commanding a "100% rule" on vapor barrier placement. I follow a sensible and accepted two-thirds rule.

I really respect the professionalism already existing in this discussion.