Thread: B vent height above roof

Hi all. Been a long long time since posting here.

I'm looking for some clarification on B vent requirements and hope I can get a clear answer.

The photo shows a B vent from a fan assisted furnace flue on a steep sloped hip roof. I know about the 2 feet taller than the roof when a vertical wall is within 8 feet as in illustrated in my code check books. However, I have searched archives on this and another site and have come across various interpretations of the height requirement. Also, I have a Carson Dunlop book that states that B-vents need to be 2 feet taller that anything within 10 feet. (Doesn't specify natural or mechanical draft or appliance type) Can I get a definitive answer on if this install is O.K.? What if it was a natural draft vent from a water heater. Any difference?

My interpretation based on reading and talking to other inspectors was always similar to the illustration in the Carson Dunlop Book. I should be able to spin a 10' stick from the top of a chimney/vent and not hit anything.

Thanks in advance for any help.

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Hi Matt,

Look at figure G2427.6.3 in the 2006 IRC Here's Oregon's version (same): 2008 Oregon Structural Specialty Code

It's not based on the 3-2-10 rule for chimney's, it's based on the roof slope............

It does not matter what the B vent is venting (as long as it's allowed for that use), it depends on the slope of the roof.

Diagram attached.

Dang this gets confusing.

I just talked to the code enforcement supervisor for the town this house is located in. He states that a B-vent must be 2' taller than ANYTHING within 10' which was my original write up in the report. I just sent an e-mail with the code enforcement officers phone # to the kind HVAC rep that belittled me this afternoon. Hopefully I can now get on with my afternoon.

Anyone else run into this problem?

If your code man is basing it in the IRC, he's wrong............. I'd ask for a code cite.

If that B vent is within 8' of a sidewall, etc. it needs to be 2' above that point. That is not the case in your photo.

That's why I made the B vent rule of 1*, 2-8 to remember it from the chimney rule of 3, 2-10.

The * at the 1* means 1 foot minimum for 6/12 and under, and then goes up according to that chart.

Not expecting anyone to remember the chart, just that B vent is 1*, 2-8.

The 2-8 means 2 feet higher than any vertical obstruction (like a wall) within 8 feet.

The two rules put the same components together:
3, 2-10
3, = minimum height ...... 2-10 = 2 feet higher than any part of any structure within 10 feet

1*, 2-8
1*, = minimum height ...... 2-8 = 2 feet higher than any vertical obstruction within 8 feet
*for 6/12 and lower slope, goes up from there

I thought I was right.

I thought I was wrong.

I thought I was right again.

Well I gave the name and contact number to the code supervisor so hopefully he will either stand behind the claim or admit that he gave me bad info.

I asked him at least 4 times. "Are you sure that a b-vent from furnace or WH needs to be 2 ft taller than ANYTHING within 10 ft?"

"yes" he replied.

I gave my stick description as in the earlier post.

"Yes" he replied.

"Are you sure, because I'm looking at this chart online and in my code check book that says" (cut off)

"Yes" he replied.

No I can't decide whether I should call this out if I see it again. I hate going through the back and forth.

Anyone else in NC have a similar experience?

Hi Matt,

What I recommend that you do is get ahold of your states residential building code book (probably based on the 2003 or 2006 IRC with amendments). I have read/ re- read Oregon's version many times. There is pretty much no way to memorize everything, but it gives you an idea where to look for things, and you will know when something looks fishy that you need to research. Your states version (OR's is) may be available on- line for free (haven't checked). That way, you are not relying on someone else for info. It is often much easier and faster to look some of this up on your own. Also, nobody is perfect, nor does anyone know everything. The code guy you contacted could have been having a bad day, may not inspect this kind of stuff, may have sorta listed to you but been picturing something else, etc.

I have contacted several AHJ's in my area regarding questions/ concerns I had. I have been recently told by AHJ's that staples were just fine on a new comp. roof installation, that drywall was not required beneath an enclosed stairwell, and a couple of others. I gave them my code references, and some changed their minds, the others well............

Matt,

I also helps if you click on the 'User CP' at the left of the top menu bar just below the inspectionnews logo, then click 'Edit Profile', then enter your city and state in the 'Location' 'Where you live' box.

That helps us help you, and helps you help us.

Here is why I get confused. This is from an A.O. smith natural draft gas water heater. In my interpretation, this clearly states that the B-vent must be higher than anything within 10 feet.

I have also seen this requirement on furnace and fireplace installation instructions as well. I guess the table attached to Brandon's post is good unless otherwise stated by the manufacturer?
Wouldn't all water heaters be the same? I don't think an A.O. Smith drafts any better or worse than a Bradford White......

Any thoughts?

The installation must comply with the manufacturers installation instructions. If those are the instructions for that specific water heater, they must be met if more restrictive than the code specified clearances.

Originally Posted by Brandon Whitmore

The installation must comply with the manufacturers installation instructions.

In that case (the A. O. Smith drawing) you have three competing requirements: 1) the code; 2) the water heater manufacturer's installation instructions; 3) the Type B gas vent installation instructions.

The most restrictive applies, provided it also meets code (i.e., the code applies unless it would violate the MII, in which cast the MII take precedence over the code. "Doing something better than code" does not violate the code, so the MII would rule.