Chimney for boiler shed

[joecool85]

Ok, planning on putting my Memco MW100 into a 10' x 6' metal shed. What do I need for chimney? I was hoping I could just use single wall stove pipe and do it on the cheap...now I'm not so sure. I keep reading about needing a Class A insulated pipe etc. Is that necessary for a metal shed install?

Also, any ideas how tall my chimney should be? I'm going to be contact the local code enforcement office before the install, but right now I'm more curious as to how tall the chimney needs to be for the boiler to run properly. It will be 25-30' from my house which is a two story 1,200 sq ft home. I'd like to put it closer, but I think codes require a certain distance from the house.

 

[Gooserider]

Ok, planning on putting my Memco MW100 into a 10' x 6' metal shed. What do I need for chimney? I was hoping I could just use single wall stove pipe and do it on the cheap...now I'm not so sure. I keep reading about needing a Class A insulated pipe etc. Is that necessary for a metal shed install?

Also, any ideas how tall my chimney should be? I'm going to be contact the local code enforcement office before the install, but right now I'm more curious as to how tall the chimney needs to be for the boiler to run properly. It will be 25-30' from my house which is a two story 1,200 sq ft home. I'd like to put it closer, but I think codes require a certain distance from the house.

You definitely need a Class A - it isn't legal to run single wall past a wall / ceiling no matter what it's made from, and code says that you can't switch out of class A once you go into it. The only thing the metal shed gets you is lower clearance to combustible requirements, but since you really ought to insulate the shed even that is iffy... For height, check the manual for the unit, you should only need the lower of whatever the 10-3-2 rule gives you, or the minimum called for by the manual, unless local codes require more. (Ditto on the distance - see what the rules are)

It is possible with the house that close, you might need to go up higher than the minimum, but that depends on local conditions and is mostly one of those "experimental" things - try it with the minimum, if you don't get enough draft add another length, etc... If you are able to get closer, then the odds of needing more stack go up - essentially it is a bit of an extension of the idea behind the 10-3-2 rule...

Gooserider

 

[joecool85]

How's that work for an OWB? Do they use Class A? Just curious.

Also, isn't Class A chimney like...$100 for 3ft? Ouch.

 

[stee6043]

Anything "outside" you see will be stainless class A if it isn't masonry. Even on OWB's.

You can get Supervent class A at some big box stores in the $90 per three foot range for 8", $70 for 6". It's not cheap, especially if you're reaching 20+' into the air with it...

 

[joecool85]

Good to know. Looks like I'll be spending a few bucks there. 18' would be $540 plus tax...here's hoping local codes don't require that kind of height.

 

[joecool85]

Starting to think this is going to get expensive quick. Might be a lot cheaper to just do a small add-on to the side of the house and put it in there. Current regs in Maine require the stack to be 2' taller than the building the boiler servers (for OWB, including those in a shed/garage etc). If I built it onto the house, I would only need the stack a minimum of 3' from where it exits the roof, and 2' taller than anything within 10' of it. Depending on where I put the addition etc, this could be ALOT cheaper on chimney. It would also eliminate dealing with underground pex and wiring and eliminate freeze issues as well. I would need to contact my insurance company though and see if it would bother them.

The codes enforcer is in the office tomorrow and I think I'm going to set up an appointment and run a few things by him, see whats what around here. At this point I'm so fed up I almost feel like swapping the oil boiler for the wood one and being done with the damn thing. Maybe use a propane boiler as backup with a direct vent situation. Alas, that would be to costly - but so sweet.

 

[joecool85]

Talked to our code enforcement guy today, turns out I'm good to go. He said there is no minimum setback from the house, but he did recommend a minimum of 10' to keep the insurance co happy. He also said that we could just put the chimney a minimum of 2' from the top of the shed roof, no worries about calculating in for the house's roof height. Because of where we're going to put it and the direction of the wind around here, we shouldn't have any draft issues, and if we do we can just raise the chimney at that point in time.

So, 10' from the house and a 3' section of class a pipe = $140 in insulated pex and $100 in chimney. I can deal with that.

 

[stee6043]

Don't forget the cost of the adapter to go from single wall to Class A in your cost. There may also be some brackets required depending on your setup. Personally, I wouldn't go shorter than 10/12' to start. You'll thank yourself later by starting a little taller...

 

[Gooserider]

I agree - 10' from the house isn't much, you could easily get draft issues unless you get the top of the pipe up to at least flush, maybe a foot or two higher than the roof of the house... No matter which way the wind is blowing, you will get a lot of turbulence in the air right around a house, and you want to get up high enough to avoid that...

Gooserider

 

[joecool85]

Alright, well, since I haven't poured anything yet (and won't be till summer), what would you guys recommend for a good distance from the house? I was thinking the closer the better effectively, to limit line loss through the underground pex etc and also to limit cost due to the pex.

Originally I was thinking 25-30' from the house. Would I be able to run a 3' stack on peak of the roof of the 7' tall shed at that distance do you think?

 

[Gooserider]

Alright, well, since I haven't poured anything yet (and won't be till summer), what would you guys recommend for a good distance from the house? I was thinking the closer the better effectively, to limit line loss through the underground pex etc and also to limit cost due to the pex.

Originally I was thinking 25-30' from the house. Would I be able to run a 3' stack on peak of the roof of the 7' tall shed at that distance do you think?

You don't say what the peak height on the house is, or how it would be oriented to the shed, which makes it a bit difficult to answer. However since you said two stories, I will guess at 25', for 2 floors @ 10' each, and 5' for an attic...

There was a link mentioned just a little while ago in another thread that led to an Astroturf site run by Central Boiler that presented their "Best Procedure" recommendations - They seemed to say 2' above the peak of any house within 300' of the boiler... If you look at the cost of doing proper underground lines, I think even a short run would be more than what it costs for a taller stack, and it isn't at all clear to me that a short run would get rid of the tall stack.

My choice would be keep it close to the house (maybe even up against the house wall, with a fire barrier wall construction, then running the stack up the side of the house?) and put up the tall stack.

Gooserider

 

[joecool85]

Since I (legally) only need a 2' chimney, I'm not worried about the laws anymore.

For insurance reasons I don't want the shed closer than 10' from the house. 25' sounds about right for the height of my house. So from your calcs I would want 27' of chimney height. The shed is 7' tall at the peak (which is where the chimney will exit. So I would need 20' of Class A pipe. If I went with 3' sections that would give me 7 sections (21') and at $100 a piece, that's $700 in chimney cost.

In doing some reading, it is important to have at least 15' of vertical from the bottom of the wood burning appliance to the top of the chimney. Since my shed is 7' tall, that would mean I would need 8' of pipe above the roofline of the shed. I'd probably do 9' just to make it easy, 3 x 3' sections at $300. This is also the max height you can do before you start needing all kinds of bracing on the chimney.

What would the worst case scenario be if my chimney was too short? Just hard to keep a fire going? Also, is it important to have a barometric damper on the flue?

I'm real tempted to run a 3' section of chimney for $100, then add 3ft at a time until I am happy if it isn't great.

 

[Gooserider]

I would do some serious research into the insurance question - it has come up in many contexts, and from what I've seen in the responses, if a unit is installed per codes and / or manufacturer instructions, some companies may be bothered by it, but others aren't at all... If your present insurance co. wants to give you a hard time, be willing to switch, and make sure your agent knows it...

In terms of what happens if the chimney is short -

First off, you might have draft problems with the appliance. Since most gasifiers have some sort of combustion fan to force air through the unit, this is probably not going to be a big issue.

The BIG problem however is what the exhaust gasses do once they exit the chimney... Obviously you don't want them hanging around where you will be breathing them, and a short chimney may not get them high enough to dissipate properly. Worst case is they collect in the low pressure area between the boiler and the house (and remember I'm just GUESSING at the height there, you need to actually measure it...) and possibly cause problems with smoke odors and fumes inside - with potentially toxic effects. You might also get staining from combustion byproducts blowing against the house, etc...

Some ways the OWB smoke dragon is better in this regard - it stinks so the problem is obvious, a gasser doesn't have as many visible issues with it's output, but you still want to get it away from where people will be breathing...

Gooserider

 

[joecool85]

Yeah, this isn't a gasifier, just an old 1978 Memco MW100 indoor wood boiler.

It has a 7" flue, should I try to find 7" chimney or run 6" or 8"?

 

[Gooserider]

Yeah, this isn't a gasifier, just an old 1978 Memco MW100 indoor wood boiler.

It has a 7" flue, should I try to find 7" chimney or run 6" or 8"?

Tough call... Usual standard is to match the flue diameter, but this would be difficult as 7" is pretty rare. 6" is much cheaper, but a lot of gasifiers like 8", and it's likely to be a safer bet overall... Might be worth checking manual to be sure, but I'd probably go w/ 8"

Gooserider

 

[joecool85]

Yeah, this isn't a gasifier, just an old 1978 Memco MW100 indoor wood boiler.

It has a 7" flue, should I try to find 7" chimney or run 6" or 8"?

Tough call... Usual standard is to match the flue diameter, but this would be difficult as 7" is pretty rare. 6" is much cheaper, but a lot of gasifiers like 8", and it's likely to be a safer bet overall... Might be worth checking manual to be sure, but I'd probably go w/ 8"

Gooserider

Again, this is not a gasifier. Pretty sure those didn't exist in 1978 when mine was built. Regardless, I am going to get the manual from Memco before I do the install, maybe it will say in that.

Also, this is a natural draft boiler, no fans or anything.

 

[Gooserider]

I realize it's a conventional boiler, (which BTW does suggest a stronger need to have a tall stack) but I'm working under the assumption that at some point you may need / want to upgrade to a gasser - In which case it would be nice if you had minimal amounts of things that you had to change to drop in the new boiler...

(BTW, It would be nice if you could edit your signature (go to the user control panel at the top of the screen) to include a capsule description of what you are trying to set up... It really helps those of us that follow a lot of threads to have a reminder of what a particular user is doing...)

Gooserider

 

[joecool85]

I realize it's a conventional boiler, (which BTW does suggest a stronger need to have a tall stack) but I'm working under the assumption that at some point you may need / want to upgrade to a gasser - In which case it would be nice if you had minimal amounts of things that you had to change to drop in the new boiler...

(BTW, It would be nice if you could edit your signature (go to the user control panel at the top of the screen) to include a capsule description of what you are trying to set up... It really helps those of us that follow a lot of threads to have a reminder of what a particular user is doing...)

Gooserider

Good idea about the sig. And also good point about upgrading later, which is a definite possibility later down the road. I think a 15' tall chimney should give me a good starting point. The closest point of the shed will be 10' from the house, and the shed is 10' x 6', I will make sure the chimney goes on the far side of the shed, giving me about 20' from the chimney to the side of the house.

I may have to consider getting my land surveyed, because if I put it on the other side of the house (which would be close to the property line), the land itself is up higher than the house by 4-5 feet, which would allow me a shorter chimney I would think.

 

[mrhaney]

put that baby 50 to 100 feet away keep your mess away from the house no sute or smoke doing damage to house insurance people will leave you alone wife wont make you paint or clean the side of house and stack 2-3 foot above ridge of shed all its going to cost you is the underground pipe thats cheap compared to all thse other things i mentioned especialy the wife!

 

[joecool85]

If I put it 50' away, the short stack (of only several feet) would still leave a lot of smoke if its burning smokey. I've seen OWB with 3' stacks that mess up the air 500' feet away. Also, even at 50' away that would cost me $700 in pex, AND I'd have to dig a 4' deep trench 50' instead of only 10'. AND I'd have to go 50' from my house to load it in the winter, which means I'm shoveling or snow blowing a path all the way out to the thing every storm we get (which is normally a lot here in Maine).

There is no way it's going further than 30' from the house, and at this point I am pretty settled on keeping in 10-15' from the house. Now it's more of deciding how tall my stack needs to be. It appears from all my research so far that a 15' chimney height would be the minimum to have effective draft. This is the required minimum on a "regular" indoor woodstove. The Tarm AD-24 (very similar in size and build, including a 7" flue, to my Memco) requires a 18' of chimney above the boiler itself. That would mean for me a chimney height of 22'.

For now I will tentatively plan on doing a 9' tall chimney on top of my 7' roof giving me 16' to the top. This will be at the end of my shed, so depending on where I put the shed the chimney will be anywhere from 20-30' from the house. I will be getting the manual from Memco before the installation, and if it indicates a taller chimney I will do whatever is necessary.