Thread: Hi/Low vents terminate in the attic?!

Take a look at the picture. I found these 2 in the attic. The buyer was present and he went to the basement where the furnace and the water heater are located. Behind the units out of view there were hi/low vents. They run inside a chase. Any reason they cannot terminate in a well vented attic. I have always seen them terminate to the exterior of the home.

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Ray, I see it done that way very often. No problem.
Is that the B vent next to your foot?

Originally Posted by neal lewis

Ray, I see it done that way very often. No problem.
Is that the B vent next to your foot?

Under my foot is air exchange for the room below me. To the right is the furnace and water heater shared flue pipe.

If I remember correctly, the top of both must be >=12" above the insulation.

Raymond, please tell me you wrote up this B-vent.

Bob,

Does that B vent look like its coming apart? It seems to get all weird shaped as it come up out of the ceiling after that 45 elbow.

Jerry, that looks like an illusion created by the insulation on top of the flue pipe.

Nick,

That's what I thought ... at first, then I realized that bottom of the 45 elbow was at the level of the insulation (you can see the top of it) and that the B vent is going up at 45 degrees.

That means the B vent above the elbow is fully above the insulation ... unless I am looking at it wrong.

(That's one thing which bugs me with this new forum, there is no way to click on the photo thumbnails when in reply mode, I will have to go back and look at the photo again, then edit this post if needed.)

The insulation is all the way down at the bottom of the 45 elbow, then there is the strap which is above the insulation, then there is straight B vent, then it gets weird shaped at about the level with the lateral brace Raymond is standing on.

I don't know Jerry. I think it's just insulation. The truss to the right of the flue pipe seems to almost act as a form and is holding the insulation in place.

Who knows what you'll see if you pull back that insulation but I don't see any kinks or dents on the pipe suface. A different picture angle or two might make things more clear.

I guess this makes the point to check each shot and take tons. Digital film is cheap. Going back to re-inspect is not.

I blew up the photo and it appears to be some duct insulation below. However, all that insulation in direct contact with the vent is a fire hazard and the vent is improperly supported.

What you see that looks like a damaged area of vent is bath fan flex vent behind (not in contact with) the B-vent. The vent pipe is in perfect condition, the strange angle plays with the eyes. It was strapped again just out of frame and again about a foot from the roof penetration. I did recommend that the insulation be removed from contact with the vent.
WOW you guys can pick one apart... I just wanted to make sure that the hi / lows were ok in the attic...
Thanks for all of the feedback!

Originally Posted by Raymond Humphreys

It was strapped again just out of frame and again about a foot from the roof penetration.

I think what Bob meant by "and the vent is improperly supported" is that the vent needs to be supported in at least three axis: side-to-side, front-to-back, and up-and-down.

That one strap which is visible will allow the vent to swing side-to-side and maybe even front-to-back.

Originally Posted by Raymond Humphreys

I just wanted to make sure that the hi / lows were ok in the attic...

Code may say those pipes installed like that are combustion air ducts and alright but in reality they function as exhaust ducts for the space they are connected to.

Doesn't it seem odd that three pipes are run up vertical in that chase one is named a flue & the other two are combustion air pipes?

Air doesn't know to make this distinction in many cases.

That one strap which is visible will allow the vent to swing side-to-side and maybe even front-to-back.
__________________


Good point.

The one just out of frame is a collar / standoff that is mounted directly to the framing... She does not budge.

Originally Posted by DavidR

Code may say those pipes installed like that are combustion air ducts and alright but in reality they function as exhaust ducts for the space they are connected to.

Doesn't it seem odd that three pipes are run up vertical in that chase one is named a flue & the other two are combustion air pipes?

Air doesn't know to make this distinction in many cases.

NOW WAIT JUST A FRIGGIN' MINUTE!

I thought "combustion air" KNEW it was "combustion air" and that those pipes are one way only.

Not only the energy and air quality aspect, but what about protecting the roof framing from fire with at least gypsum board.

I know, I know, code does not require it, BUT, one must always remember 'code is minimum', nothing better, the 'crappiest way one is legally allowed to do something'.

Originally Posted by Jerry Peck

BUT, one must always remember 'code is minimum', nothing better, the 'crappiest way one is legally allowed to do something'.

Couldn't have said that any better Jerry.

I think those ducts are listed for use with Smart Air, which knows which way to go. Smart Air, when burned, becomes Smart Smoke, which knows to vent up and out instead of backdrafting.

just another reason to test so you know either things work or don't work.

Peoples Energy in Chicago has no problem with combustion air ducts in the attic., but only if the attic is properly vented. As reasonable view.

Has to make you ask the question "If the attic is vented how can it supply air to another part of a home?"

If the attic is venting, then path of least resistance means these ducts may feed the attic and depressurize the CAZ even worse.

Gee, should you tell David about the attic fans and turbines?

So when one finds a system where all the combustion air is coming from the living spaces, what is the best method to obtain combustion air? I'm asking about routing and materials.

Originally Posted by Thom Walker

So when one finds a system where all the combustion air is coming from the living spaces, what is the best method to obtain combustion air? I'm asking about routing and materials.

Fan powered combustion air is the only reliable & measurable way.

I'll ditto Davidr's response. Passive supplies are at the mercy of unpredictable forces. Only powered MUA is predictable. Still needs to be calculated, designed and tested that it does the job. Many methods from crude DIY cobbled parts to systems and hardware that has current sensors slaved to the largest exhaust fans in the home.

These systems need to be installed by someone with a grasp of building science and experience. IF you oversize the system and pressurize the home, you'll drive moisture into the interstitial wall spaces and grow goobers while losing R value. Best to maintain a neutral Delta P when possible.

Bob, Have you seen the ASHRAE study on passive combustion air versus fan powered combustion air?

Was conducted back in 2000 I believe.

Yes, there is that and a study by the Canada Mortgage & Housing Corporation on the topic. They measured actual airflows under various pressure gradients and the results are miserable. At 5 Pascals duct pressure a 4 inch duct will flow about 13 CFM. Again, at 5 Pa, an 8" duct will flow about 72CFM. The Canadian General Standards Board draft test std. CGSB 51.71 "The Spillage Test" is a chart showing the various depressurization limits for combustion systems. The open atmospherically vented appliances are at -5Pa, closed chimney vented heaters spill at -5 Pa. continuous/ 10 Pa intermittent and gas-tight sealed side wall vented heaters at 10/20 Pa.

Passive air intakes located in down wind/ leeward zones will often be under enough negative pressure to backdraft. Why else would the codes now require unlisted air kits to have a 1 inch clearance to combustibles for 5 feet within of the duct outlet (where it dumps into the fireplace)? Funny but the IRC does not come right out and require metallic ducts. Just the clearance, which means it must be non-combustible for the first 5 feet on masonry Fps. Factory built Fps require listed components only.

David, I find myself referring back to ASHRAE 62.2 most often for justification for MUA.

Do you have any other sources?

Originally Posted by Bob Harper

Do you have any other sources?

The ASHRAE study is all that I have in print, wouldn't happen to have a link to the CMHC study would you?

The more info I can gather the better.

https://www03.cmhc-schl.gc.ca/b2c/b2...00370000000051
This will give you a taste of what they're up to. On the last page David, you'll see contact info. Ask to speak to Don Fugler. He knows as much about this stuff as anyone on the continent. You can also get one of John Gulland's books. Either from his Wood Heat website or through the HPBA. From Woodheat.org, look for his The Fireplace in the House as a System, which contains many useful charts with house depressurization limits, airflows, and resources. Essentially the same book from the HPBA but in a 3 ring binder is the Venting Design Specialist Training Manual from the Hearth Education Foundation at HPBA: HPBA You can also email John Gulland and he'll be happy to chat about this stuff and give you more links. Another great Canadian out in B.C is Dave Hill from Eneready. I think Collin from the HVAC-Talk site knows him well. He has the gift of putting such matters into practical terms anyone can understand. His carboard dollhouse model is far more effective than any of the plexiglass models costing over $1K. Sorry, I don't have his contact info handy. Another industry great is Paul Stegmeir in Mn. He has written for yrs on this stuff, sits on many Standards cmtes. and is a private consultant/ investigator. I'm looking for his card as I used to carry it in my wallet. Anyway, just Google him and I'm sure "Steg" would be happy to help out as would Gary Nelson at The Energy Conservatory-- energy conservatory | home They make the Minn. Blower Door & Duct Blaster and can steer you to some links. Get their blower door manual for, I believe $10.00. Describes a Worst Case Depressurization Test and all. Lastly, go to INFILTEC Air Leakage Control - Radon Gas Mitigation & Blower Doors - test, testing and bookmark it. Dave Saum makes a blower door but has 10 pages of links you'll love! First link is Fred Lugano's Weatherization Pages then Joe Lstiburek's Building Science Corp. You'll love his Top Ten Dumb Things Builders Do for the North and the South. Great resource. Tons of links on duct sealing, building science, blower doors, CO, etc. at Infiltec.


Enjoy!
Bob

Thanks a million Bob!