Leslie McDowell was nice enough to reply to one of my latest blog posts (Full ArticlePartial Article & Reply here), in which she linked to this document which in part reads

BPI does not recommend drilling B-vent, nor does BPI prohibit it.

Some B-vent manufacturers have specifications for drilling and sealing a manufactured pipe.Local building codes may or may not allow the practice. The BPI Certified Professional is advised to check with the local code official.

The issue with drilling B vent is the inability to properly seal the inner liner. It is recommended that you do not drill B vent unless either the manufacturer or local code official indicates this as an acceptable practice. The code official should also be consulted to provide instruction in writing as to the proper (e.g., approved for your jurisdiction) method of sealing the test hole (outer and inner liners).

In the manufacturer letters, such as those posted on the Bacharach site, they all state that they prefer B-vent not be drilled. They also go on to say, "we don’t believe this would affect the UL list". The question that arises, is do they know for sure?

B vent construction is crucial, because if the seal is lost, it will create its own environment for destruction, and will ultimately fail. There are two dissimilar metals in B vent and two types of each. One is a thin steel lining on the outside with a thin aluminum lining on the inside. The other is a thin steel lining on the outside and a thin stainless steel lining on the inside. This is done for a reason, to prevent the outer lining from rust damage caused by exhaust gas condensation (which will also create sulfuric acid).

Note- Ultimately, it is not the manufactures, code officials or BPI that shall approve the practice of drilling B vent, but rather Underwriters Laboratory. It is UL that places the listing on the product and not the manufacturer, code officials or BPI.

So herein lies my question, is this a practice that should be destined for the scrap heap, or does it need to be changed to where only someone certified & knowledgeable does it? Along those lines, is the indignation of how RESNET performs the training misplaced with this information? I did hear from one BPI trainer/proctor that simply simulates drilling a hole for the training & he gives them the numbers as he knows about the issues.

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I am curious to see where this thread goes. I have been through this before and never was able to reach an acceptable conclusion.

I spoke with some folks at a local supply house and they mentioned using bolts to "seal" the holes. I just heard them out and said that I was on the fence about how to handle B vent situations.

Thanks and that makes two of us. The bolt idea has have never flown for me (for many of the reasons listed above) and if you check out the letters posted on Tru Tech's site they have the recommendations from 4 manufacturers which are all quite similar if not basically the same;

  • You shouldn't drill into said pipe but if done it needs to be done carefully & sealed properly
  • The hole in the inner sleeve should be no bigger than necessary to insert the probe
  • The inner hole needs to be sealed with high temperature non-hardening sealant
  • The outer hole can be slightly larger than the inner one and should be sealed with the same type of sealant used above and then taped over with UL listed aluminum tape

One of the other issues I haveand mentioned in the post, is I reviewed a few of the furnace manufacturers directions (installation & troubleshooting - you know how hard those are to get your hands on?) and not one says to test or diagnose by drilling into the flue. All the tests & fixes are handled inside the box... Unfortunately I could only get my hands on three different models, 2 by the same manufacturer so I am not sure what the remainder have to say

Hello Sean and Bill.

After reading what you two have posted, it still leaves me feeling that I am quite confused as to a proper means to test furnaces with B vent.

I had considered the silicone caulk route and had not yet applied it. I had also thought about what Bill has said, and that seemed appealing, but also seemed to "simple."

I have drilled into a couple of B vents before and was glad I did because one of the tests evidenced CO levels in the flue gasses greater than 250 ppm.

I would like to hear your's (and others') views on Bill's approach.

Thank you gentlemen!


I don't see a problem with drilling B-vent, but I don't feel completely confident squirting silicone blind. Most of the B-vent installations are fan-assist, so you don't have to go 1 foot up on the flue to drill. So, I either drill into the little bit of single wall showing at the appliance connector or if they didn't use one, pull the 1-3 screws and lift the flue off enough to insert the probe and then seal with rags. The same goes for PVC and super-ferritic stainless single wall.

Scrap Heap.  

Devices that can not condense throw away a tremendous amount of latent energy.  Add the likelihood that crappy induced appliances typically have crappy gravity draft water heaters attached, creating sizable health, safety, and envelope improvement impediments, they represent huge added cost not just in energy, but to our process and achievement of our mission.  

Devices that use B-Vents should be outlawed.  

Come on Ted,

That was a bit heavy handed, even for you!

Actually I don't disagree at all with Ted - shoot most green programs already have which includes Energy Star v3 unless you happen to be in climate zones 1-3.

HEY!  I know I'm guilty of that, but I don't think this time.  

The question was "is this a practice that should be destined for the scrap heap, or does it need to be changed to where only someone certified & knowledgeable does it?"

My response was yes, chuck it!  

Devices that use B-Vents should be outlawed.

When I started installing boilers and furnaces back in 1994, all I wanted to install was condensing units, but I noticed some problems with reliability and real world performance advantages. For most installs, I still want to install a mod-con appliance, but... 

There are times when a hi-efficiency appliance is simply not the best choice. Boilers will stop condensing if return temps are too high, so if you have a system with a hi return temp, why use a condensing model with all the problems and extra cost, when you could use a long lasting cast iron boiler and vacation with the money you save. Then there are the old homes have been heated with a coal fired boiler that leaked coal sludge into the secondary to such an extent that even after vigorous cleaning, the high resistance heat exchangers on even the self-cleaning type plug up.

I am not disagreeing with most of what you have said, just the last part.

Reliability has come a long way since 1994, it's a pretty mature technology now. 

But let's back further away.  Look at this from more than simply the heating perspective.   Put your Envelope Professional hat on.  Do you really want to go to extraordinary measures to tighten a building so you can downsize equipment, only to be forced to have a leaky as heck CAZ and relinquish some of your newly gained control?  3 steps forward and 2 steps back?  

wrt sludge - I'm sure your depth of boiler experience is fathoms greater than mine.  What about filtering?  Too much of a service issue?  I suspect if you step back here too, thinking about it creatively may result in solutions. 

There may be situations that require very high temperature water ALL the time (again, out of my depth) in which case the grand slam benefit of condensing goes significantly away.  

But you'll still have gain on the modulating side, and again, potentially a lot of gain on the sealed combustion side.  If you dial in your setup the thing won't cycle much, and you won't require gross amounts of uncontrolled intake to insure proper combustion and draft.  


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