Support box length

Kincaid · Nov 30, 2017

I'm building a log home and need to do a roof penetration. The chimney will come out very near the ridge. It's cathedral ceiling with large log purlins every 5 foot on center and 8 in 12 pitch. I'm looking for a support box 4 feet long so that the single wall pipe below it will not be within 18" of the lower purlin.

Does anyone make a one piece 48" x 6" support box? Local supplier has "support box extensions" but I'm doubtful about how professional that will look, given the nature of sheet metal.

DAKSY · Nov 30, 2017

ICC EXCEL makes 24" support boxes with 24" extensions. Effective working length is probably 44 - 46"
Nobody will see the seam if you are mounting it near the center of a cathedral ceiling.

begreen · Nov 30, 2017

It's recommended to not have the connector pipe longer than 8ft. There is too much heat loss with greater lengths. This can contribute to creosote accumulation, especially with low burns. This is especially true for cat stoves. For a long stove pipe I would go with double-wall. Then you can use a 24" support box.

Kincaid · Nov 30, 2017

DAKSY said:
ICC EXCEL makes 24" support boxes with 24" extensions. Effective working length is probably 44 - 46"
Nobody will see the seam if you are mounting it near the center of a cathedral ceiling.

44" might be enough. The log staircase is right next to the flue so a seam would be visible. I could live with that if the seam is straight, tight and free of warpage, which is probably asking a lot.

begreen said:
It's recommended to not have the connector pipe longer than 8ft. There is too much heat loss with greater lengths. This can contribute to creosote accumulation, especially with low burns. This is especially true for cat stoves. For a long stove pipe I would go with double-wall. Then you can use a 24" support box.

My last home had a 13' black single wall pipe between the Pacific stove and the support box. Spent 6 winters there. It did accumulate creosote, but I don't know if it was more than it should accumulate since it was the only wood heated home I ever lived in. I cleaned the flue every year or two "whether it needed it or not". I don't think I'd like the aesthetics of visible double-wall pipe. With your advice I'll stay away from catalysts.
BTW, the flue will be straight vertical with no bends.

begreen · Nov 30, 2017

We have a PE with double-wall stove pipe. Our creosote accumulation is negligible, no more than a cup or so per year. There are different brands of double-wall stove pipe. Some look pretty much like single wall.

MagKarl · Nov 30, 2017

I like ICC Excel. They sell support box extensions, I used one on my house too. Get double wall pipe instead for 6" clearance.

Kincaid · Mar 19, 2018

Follow up. I bought an ICC Excell class A 9' chimney, 2 support boxes and a support box extension. I cut the bottom out of one support box and used it on top. The box now hangs about 5 feet below the rafters, giving me the 18" clearance from the lower log purlin. Looks OK from a distance. Even a 4' box would not have done it. I really think they should make longer support boxes so people wouldn't have to fabricate sheet metal abominations like this or run class A all the way to the stove. I wonder what people do in A frames...................................................................................................................................................................................

webby3650 · Mar 19, 2018

Most people would have went with double wall pipe, so a traditional ceiling box would work fine.

Highbeam · Mar 19, 2018

I think the op doesn’t realize that double wall interior black pipe is not the same as class a shiney stainless exterior pipe. This install should have been done with double wall interior pipe for sure.

Maybe I’m wrong but I’ve never met anybody so married to the single wall pipe that they would go so far as this.

Kincaid · Mar 19, 2018

Highbeam said:
I think the op doesn’t realize that double wall interior black pipe is not the same as class a shiney stainless exterior pipe. This install should have been done with double wall interior pipe for sure.

Maybe I’m wrong but I’ve never met anybody so married to the single wall pipe that they would go so far as this.

I wasn't familiar with the uninsulated double wall pipe when I started the thread, but my supplier enlightened me and tried to sell before I did the support box. I noticed that the inside diameter of the double wall was a very loose fit on the flue extension coming out of the support box so I declined. Someone really muddied the water by naming it double wall when class A already had that name.
I think it's the woodstove's job to get the fire as hot as possible and the job of the single wall pipe to dissipate that heat in my house. Maybe I'm wrong, but I think the single wall pipe puts out a lot of heat.
I suppose ICC excell is some of the best pipe around, but I'm not impressed with stainless on the exterior. Just makes it that much harder to get paint to adhere to it.
I'm not impressed with their installation manual either. On page 8 they say "make sure the chimney has 2" clearance at the top of the truss", but they don't specify whether it's 2" from the inside diameter or outside diameter. Then on page 11 they show a support box attached directly to two by sixes so one would assume they mean 2" to the inside diameter. My supplier told me otherwise so I used steel to attach the box to rafters at least an inch away from any wood. When I put up the T&G ceiling wood it will be less than an inch from the box, which is unavoidable.

webby3650 · Mar 19, 2018

At least it’s done now. Sounds like you needed to do a little more research to clarify clearances and components. This is a specialized field, there’s a lot to know about installation methods and proper use of components. I’m a little surprised your insurance company allowed you to do your install, particularly in a log home.

Kincaid · Mar 19, 2018

Since the home is under construction, I don't have insurance. That would have added 20 or 30 grand to the cost. As much as I despise insurance companies, I will try to get homeowners just to protect myself from lawsuits because this property includes significant acreage and trespassers wander through here frequently. They won't sell just liability though because they're control freaks, almost quasi-governmental agencies that want to micromanage every aspect of our lives because it's their "policy".
Getting homeowners for any log home is difficult as it is. Wood heat will make it more difficult. I do have gas heat and will call the woodstove "secondary heat".
I don't consider chimney installation to be very technical. I did mechanic work for 20 years and do all my own architectural, plumbing, electrical, HVAC, welding and solar power myself. I even built 3 woodstoves from scratch. The only thing complicated about it is understanding the specifications. If manufacturers used clear language in their instructions it would be much simpler.

bholler · Mar 19, 2018

Kincaid said:
View attachment 224614 Since the home is under construction, I don't have insurance. That would have added 20 or 30 grand to the cost. As much as I despise insurance companies, I will try to get homeowners just to protect myself from lawsuits because this property includes significant acreage and trespassers wander through here frequently. They won't sell just liability though because they're control freaks, almost quasi-governmental agencies that want to micromanage every aspect of our lives because it's their "policy".
Getting homeowners for any log home is difficult as it is. Wood heat will make it more difficult. I do have gas heat and will call the woodstove "secondary heat".
I don't consider chimney installation to be very technical. I did mechanic work for 20 years and do all my own architectural, plumbing, electrical, HVAC, welding and solar power myself. I even built 3 woodstoves from scratch. The only thing complicated about it is understanding the specifications. If manufacturers used clear language in their instructions it would be much simpler.

I find the instructions pretty clear. The clearances are always from the outside of the component to the closest combustible material. And no the pipes job is not to radiate heat at all it is to get the exhaust out as quick as possible while loosing as little heat as possible. You heat with the stove not the pipe.

begreen · Mar 19, 2018

Kincaid said:
I think it's the woodstove's job to get the fire as hot as possible and the job of the single wall pipe to dissipate that heat in my house.

That is wrong. It is the stove's job to heat the house. It's the flue's job to create the draft that powers the stove. You want it to stay hot enough to prevent creosote from condensing inside. Robbing heat for the room risks having a too cool flue at the top. This is why most stove mfgs and stove pipe companies say to not run single-wall stove pipe for more than 8'.

Kincaid · Mar 19, 2018

bholler said:
I find the instructions pretty clear.

The picture shows wood members attached directly to the support box. Seems to me there is a conflict when the drawing indicates something the instructions forbid.

bholler · Mar 19, 2018

Kincaid said:
The picture shows wood members attached directly to the support box. Seems to me there is a conflict when the drawing indicates something the instructions forbid.View attachment 224615

No that is how it should be installed. The support box gives you the required clearances.

Highbeam · Mar 19, 2018

I believe that the support box extensions shown in this thread were intended to maintain the 2” required clearance between the class a chimney pipe and insulation in the attic. Not to dangle the support box down into the room.

When you buy double wall interior connector pipe it is already black on the outside and stainless steel on the inside only. Should last many decades. Works better since the heat stays in the pipe. Most importantly for this job is it only requires 6” clearance to combustibles.

Homemade stove you say? Against federal law to install.

Beautiful location and logs.

Highbeam · Mar 19, 2018

bholler said:
No that is how it should be installed. The support box gives you the required clearances.

Correct. You’ll notice the support box is 2” bigger than the class a chimney to match the required 2” clearance to combustibles. You screw the support box straight to the supporting framing. In the attic (or elsewhere) you can have insulation right up against the outside of the support box. That ceiling material can also touch the box but there should be a trim piece that would hide any gap.

Kincaid · Mar 19, 2018

Highbeam said:
Correct. You’ll notice the support box is 2” bigger than the class a chimney to match the required 2” clearance to combustibles.

You must be referring to a 12" support box. They sold me a 10" support box to contain an 8" outside diameter class A pipe. One inch on each side does not add up to 2" clearance.
The other concern is that I will not have 2" clearance to the tongue and groove ceiling.

The homemade stoves I built were used in shops and a hunting camp. They are quite common in shops around here.

I haven't bought the single wall pipe yet. I'm just apprehensive about the double wall because it's such a loose fit. there is almost 1/8" of slop between it and the flue extension.

bholler · Mar 19, 2018

Kincaid said:
You must be referring to a 12" support box. They sold me a 10" support box to contain an 8" outside diameter class A pipe. One inch on each side does not add up to 2" clearance.
The other concern is that I will not have 2" clearance to the tongue and groove ceiling.

The homemade stoves I built were used in shops and a hunting camp. They are quite common in shops around here.

I haven't bought the single wall pipe yet. I'm just apprehensive about the double wall because it's such a loose fit. there is almost 1/8" of slop between it and the flue extension.

Did you read the instructions? I dont use excel so i didnt know the specifics. But in a couple mins i easily found that within the support box you only need 1". It is very clearly stated in the note on page 8.

As far as the double wall not fitting correctly you may not have the right components. What brand double wall was it?

webby3650 · Mar 19, 2018

The excel box only requires 1” clearance within the box, after it leaves the box, 2” is required to the chimney. That’s why the instructions mentioned making sure to maintain 2” clearance to the rafter or truss on the top side.

It really bothers me when people say there’s nothing to it, or anyone can do this stuff. So not true... all too often class A chimneys are installed wrong. I’m all for getting things done yourself, but respect should be given to the pros that know this stuff inside and out.

Kincaid · Mar 19, 2018

bholler said:
Did you read the instructions? I dont use excel so i didnt know the specifics. But in a couple mins i easily found that within the support box you only need 1". It is very clearly stated in the note on page 8.

As far as the double wall not fitting correctly you may not have the right components. What brand double wall was it?

In the instructions, the sentence that follows contradicts it, at least that was my interpretation. After reading it again, perhaps the distinction between "truss" and "cathedral" is important, but they don't mention cathedral ceilings until page 10, without mentioning clearances to anything other than flue extension to ceiling. Funny thing is the instructions don't mention the support box dimensions, whether it's 10" or 12". Presumably 10" is all Excell sells.
My supplier told me I needed 2" clearance in my cathedral ceiling and sold me the metal brackets to hang it. Everything I bought is ICC Excell. I'm not sure which brand of double wall they showed me. We held it up to their display support box and noticed the poor fit. I think it would draw room air up the chimney.
Another concern about double wall, as I have never used it... does it telescope and move sideways easily when cleaning the chimney like single wall does?
If I decide to go with double wall, is the support box Kosher the way it is?

webby3650 · Mar 19, 2018

Double wall has telescoping sections just like single wall only they work better.
The loose fit would only be pulling room air up through the outer cooling wall, not up the flue like you thought. It’s designed to let air flow throughout the outer wall.

bholler · Mar 19, 2018

Kincaid said:
In the instructions, the sentence that follows contradicts it, at least that was my interpretation. After reading it again, perhaps the distinction between "truss" and "cathedral" is important, but they don't mention cathedral ceilings until page 10, without mentioning clearances to anything other than flue extension to ceiling. Funny thing is the instructions don't mention the support box dimensions, whether it's 10" or 12". Presumably 10" is all Excell sells.
My supplier told me I needed 2" clearance in my cathedral ceiling and sold me the metal brackets to hang it. Everything I bought is ICC Excell. I'm not sure which brand of double wall they showed me. We held it up to their display support box and noticed the poor fit. I think it would draw room air up the chimney.
Another concern about double wall, as I have never used it... does it telescope and move sideways easily when cleaning the chimney like single wall does?
If I decide to go with double wall, is the support box Kosher the way it is?

Read the note on page 8 it is very clear. It doesnt matter if it is a catedral ceiling or truss or what you can have combustibles against the support box.

Yes double wall telwscoped but honestly in most cases you simply clean through the stove and never oull the pipe at all.

It really sounds like your supplier doesnt know their product well at all. Are you sure they were using the right double wall component? Was it rightside up?

Kincaid · Mar 19, 2018

bholler said:
Are you sure they were using the right double wall component? Was it rightside up?

Maybe the double wall component we were handling was a portion of telescopic pipe not designed for the flue extension. None of the sales crew are former installers and it seems every time I speak to an installer there it's a new young guy. Climbing steep metal roofs for a living pays better than sales associate, when they get tired of it they find another occupation with another company.
Maybe I need to drive further and do business with the other stove shop.
Will the insurance company balk at the long support box? The installer where I bought the components said it was Kosher.

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