How bad are bends in flues?

[Hawkridge]

We are planning to install a woodburning cast iron stove which heats the house and is the oven and water heating, in a new house. We do not know which waty to go with our flue, so would appreciate advice on this.

We have limited locations for the stove, and can either have:

• a completely straight flue, and no seating space in the main bedroom, and have a wall that appears to bulge out randomly into the bedroom, and more space in a hallway where it is less use, - or
• have two bends in the flue, the first , 1.3 m above the top of the stove (but not visible), would be 35º from the vertical, the flue would run at that angle for 750 mm, then a bend return it back to vertical, (the bend changes where the flue passes through the next floor's structure), and have a what looks like more logical wall layout in the bedroom, and a more useful bedroom space.

I have heard there are influencing factors from firewood, flue height climate, height, etc so here are some details on our situation.
The flue internal diameter is 150 mm.
For each option, the flue height will be over the supplier's minimum recommended height of 4.2 m. It will be at least 6.7 m to reach appropriate clearance from the roof.
We are growing our own firewood, which is nearing its first harvest, and plan to dry it very well before use. It is mainly eucalypt and acacia, but we have some pine, some of which is already harvested and dry. A softwood like pine is recommended by the stove supplier when lighting the fire, but a hardwood for general running.
Our climate is maritime temperate with a range of -3ºC to 35ºC at the most extreme. The woodburning would be unlikely on those warmest days!

Thanks for any help on this.

 

[Rob_Red]

My flue has a 90* bend in my stove connector pipe and then an 18 inch wall pass through going to a clean out T (essentially an even sharper 90* bend) in the chimney itself. Overall height is around 25 feet tall and it has a great draft. In fact My draft is so strong and I should dampen it down with an additional damper.

I live on the coast about 100 feet above sea level.

You should be fine with either setup

 

[brenndatomu]

have two bends in the flue, the first , 1.3 m above the top of the stove (but not visible), would be 35º from the vertical, the flue would run at that angle for 750 mm, then a bend return it back to vertical, (the bend changes where the flue passes through the next floor's structure),

Is this stove pipe, or chimney pipe? If its insulated chimney pipe, are you sure about that angle? I've never seen a 35* elbow in chimney pipe.

 

[begreen]

Straight up is definitely preferable, even if it means relocating the stove sometimes. That said, if the offset is in the connector (stove) pipe that is within the stove room, then straight up, that can work ok too as long as the overall flue system height meets or exceeds the stove's requirements for sufficient draft.

 

[Hawkridge]

Thanks for the replies.

We can't really relocate the stove, because of other conflicts . We have adjusted as much as we can within the small house footprint of about 70 m2 and it leaves us with these options. Anything else would require a complete new house design.

I think we have different terms for things. I have never heard of stove pipe or chimney pipe here, so I do not know the difference between them .
What we refer to here as the flue is a steel tube, and runs from just above the woodburner to the very top. It sleeves at its base over the woodburner flue socket, which projects about 100-200 mm above the firebox. The flue will have casings around it as heat shields everywhere it gets near anything combustible. usually a couple of layers of larger steel tubes with steel spacers, ventilated at top and bottom . Each is generally 50 mm diameter larger than the one inside it.
Flues and casings are often made to suit the installation. so the 35 degree angle would be made to suit the job.

To clarify, our non straight flue option would run in the room the stove is in (stove room?) straight up for 1.3 m, then inclined by 35 degrees for 750 mm as it passes through the floor structure above, then vertical for another 4+ m through the next floor & above the roof structure. The flue would be cased from 1.2 m above the stove to about 300 mm above the roof, including the inclined portion.

 

[Rob_Red]

I think the blaze king requirements are very sensical for any stove. I based my instal on BK specs. All of the manufacturers should be including this info.

My chimney is a great performer even in shoulder seasons. When on the coldest winter days with the stove stuffedI need to watch my temps, so it would be nice to be able to regulate the draft with a stove pipe damper in that circumstance.

 

[Hawkridge]

Thanks. That is not a brand we have here. I just had a search and read through some specs.

 

[begreen]

Within the room envelope, stovepipe is permitted. This can be single-wall or double-wall stovepipe. Sometimes single wall requires shielding attached to it to meet clearance requirements. At the wall or ceiling, the stovepipe must transition to insulated chimney pipe. It sounds like in NZ they may use a double-walled air casing pipe to shield the flue pipe. That is not permitted here.

A consultation with a certified Solid Fuel Appliance Installation Technician (SFAIT) would help. The products used in NZ will most likely be different brands. Here is a link to NZHHA that will help locate a technician:
(broken link removed)

 

[Hawkridge]

Thanks. What is the difference between stove and chimney pipe?

 

[SteveKG]

I have two stoves with 45 degree bends, one just above the stove top and one to re-establish vertical at the chimney pipe. I also have a stove with no bends, straight up all the way. All three stoves perform flawlessly, so I would expect your slightly smaller angle will cause no issue as long as the total height of the pipe run is adequate to make good draft in your firebox.

 

[begreen]

Thanks. What is the difference between stove and chimney pipe?

Stovepipe is uninsulated. Chimney pipe is insulated and certified to HT-2100 standards here. Again, NZ appears to do things differently. I looked at an NZ stove manual and saw what I think you were describing. It had the flue pipe as a continuous run, but was jacketed with a double-walled casing starting at the ceiling.

 

[Waingro]

This is my flue pipe, newly installed last month (the last one was 25 years old!):
Inside
Outside
It is all single-wall (0.8mm?) type. Here in Japan, double-wall pipe is insanely priced.
What you see in the above two photos cost the equivalent of US$2700 (including labor for installation)!
As you can see, I have 90 degree bends both inside and out.
Annual outside temperatures are in the range of about -10c to +40c in the run of a year.
I clean it once a year. I don't get much creosote build-up as I burn well-seasoned wood.
The draft is good!
I also use this stuff regularly:

 

[begreen]

That is a completely illegal and dangerous installation in the US and Canada. I can understand the concerns about creosote. The temperature loss due to the thin, uninsulated pipe guarantees the flue gases to be condensing in the pipe outside.

 

[Waingro]

That is a completely illegal and dangerous installation in the US and Canada. I can understand the concerns about creosote. The temperature loss due to the thin, uninsulated pipe guarantees the flue gases to be condensing in the pipe outside.

I thought it would probably be illegal in North America.
Never had any issues though. (Japanese building standards are very poor)
No idea why you think it's dangerous. The clearances are all fine. The ceramic insert in the wall works perfectly well, too.
As I said above, I don't get much creosote build-up.

 

[Poindexter]

@Hawkridge , the thing about bends or elbows in the pipe coming off the stove is they make it harder to clean.

I routinely see -40C (I am at 64 degrees north latitude), I burn about 8 cords (about 16 cubic meters) per year, and need to brush out my pipe twice annually. Once in the warm months is no problem, but in the depths of winter having a straight pipe with no elbows makes it easy for me as the homeowner to brush out my own pipe.

Getting a chimney brush or a pipe brush around those two elbows at 35 degrees each is going to require a tool other than commonly and cheaply available rigid chimney cleaning rod.

Once you are well established, if you are burning four cords or less per year, about 8 cubic meters of wood, you can _probably_ get away with brushing out your pipe once annually, but the first couple years I would brush or sweep more often to prove it to myself before I relaxed about it. You will need a moisture meter to satisfy yourself that your fuel really is dry enough to burn safely without risking your loved ones dying or being permanently maimed by a tragic fire.

A chimney or flue system with two elbows in it can work very well. They are more difficult to keep clean. I would suggest burning only fuel at 20% moisture content or less.

After the first cord (2 cubic meters burnt) have the chimney pipe professionally cleaned - and make sure that shyster brushes out the section between the two elbows. If the sweep is kinda apologetic about charging you so much for recovering such a small amount of crud that is a good thing, long term. Then burn two more cords, about 4 cubic meters, and have it pro brushed again, including the middle section between the elbows or angles.

If the report is good and the recovered sweepings are a healthy relaxing color you can most likely have the chimney brushed every four cords (about 8 cubic meters) or most likely once annually given your relatively mild winter time temperatures.

Keep an eye on the gear your sweep is using. Sweeping your own chimney is easier than making a good whiskey sour - if you are willing to invest in the correct gear to get around the angles.

 

[Waingro]

@Hawkridge , the thing about bends or elbows in the pipe coming off the stove is they make it harder to clean.

I routinely see -40C (I am at 64 degrees north latitude), I burn about 8 cords (about 16 cubic meters) per year, and need to brush out my pipe twice annually. Once in the warm months is no problem, but in the depths of winter having a straight pipe with no elbows makes it easy for me as the homeowner to brush out my own pipe.

Getting a chimney brush or a pipe brush around those two elbows at 35 degrees each is going to require a tool other than commonly and cheaply available rigid chimney cleaning rod.

Once you are well established, if you are burning four cords or less per year, about 8 cubic meters of wood, you can _probably_ get away with brushing out your pipe once annually, but the first couple years I would brush or sweep more often to prove it to myself before I relaxed about it. You will need a moisture meter to satisfy yourself that your fuel really is dry enough to burn safely without risking your loved ones dying or being permanently maimed by a tragic fire.

A chimney or flue system with two elbows in it can work very well. They are more difficult to keep clean. I would suggest burning only fuel at 20% moisture content or less.

After the first cord (2 cubic meters burnt) have the chimney pipe professionally cleaned - and make sure that shyster brushes out the section between the two elbows. If the sweep is kinda apologetic about charging you so much for recovering such a small amount of crud that is a good thing, long term. Then burn two more cords, about 4 cubic meters, and have it pro brushed again, including the middle section between the elbows or angles.

If the report is good and the recovered sweepings are a healthy relaxing color you can most likely have the chimney brushed every four cords (about 8 cubic meters) or most likely once annually given your relatively mild winter time temperatures.

Keep an eye on the gear your sweep is using. Sweeping your own chimney is easier than making a good whiskey sour - if you are willing to invest in the correct gear to get around the angles.

I agree, sweeping out is an annoyance with bends, but not particularly difficult.
Best to take it all apart and clean it in sections to be safe.

 

[begreen]

I thought it would probably be illegal in North America.
Never had any issues though. (Japanese building standards are very poor)
No idea why you think it's dangerous. The clearances are all fine. The ceramic insert in the wall works perfectly well, too.
As I said above, I don't get much creosote build-up.

The clearance to single-wall stove pipe is 18" for a reason. It radiates a lot of heat. The stove pipe appears to be about 6" away from the soffit. Also, single-wall corrodes quickly outdoors with the combination of heat and rain. And lastly, the single-wall pipe should not pass through a wall, particularly a combustible wall.

What are the flue temperatures on the stove pipe?

 

[bholler]

I thought it would probably be illegal in North America.
Never had any issues though. (Japanese building standards are very poor)
No idea why you think it's dangerous. The clearances are all fine. The ceramic insert in the wall works perfectly well, too.
As I said above, I don't get much creosote build-up.

The ceramic insert (if it's anything like what we use here) offers very little insulating value and the heat transferred can easily ignite combustibles. Here when doing a masonry passthrough you need 12" of solid masonry on all sides of that crock. That is the spot most structure fires start from chimneys

 

[begreen]

Yes, this setup has the earmarks of a pyrolysis bomb in the making.

 

[Waingro]

The clearance to single-wall stove pipe is 18" for a reason. It radiates a lot of heat. The stove pipe appears to be about 6" away from the soffit. Also, single-wall corrodes quickly outdoors with the combination of heat and rain. And lastly, the single-wall pipe should not pass through a wall, particularly a combustible wall.

What are the flue temperatures on the stove pipe?

The clearance to single-wall stove pipe is 18" for a reason. It radiates a lot of heat. The stove pipe appears to be about 6" away from the soffit. Also, single-wall corrodes quickly outdoors with the combination of heat and rain. And lastly, the single-wall pipe should not pass through a wall, particularly a combustible wall.

What are the flue temperatures on the stove pipe?

Not sure of the temperatures on the flue.
All I can say is : "Evidently not hot enough to cause a fire in the past 25 years".
When I had the new flue installed last month, I had a good look at the ceramic insert; no scorch marks on it at all.
The previous flue was in bad shape, hence replacing it. The previous owners of the house did little in the way of maintenance.
Still, it lasted 25 years, which is pretty amazing given all the rain we have here in southern Japan.
I can live with that flue lifespan.

 

[Waingro]

Yes, this setup has the earmarks of a pyrolysis bomb in the making.

That must be why I've had so many house fires in the past 25 years.😂

 

[Waingro]

The ceramic insert (if it's anything like what we use here) offers very little insulating value and the heat transferred can easily ignite combustibles. Here when doing a masonry passthrough you need 12" of solid masonry on all sides of that crock. That is the spot most structure fires start from chimneys

There weren't any burn marks on the ceramic insert when I had the flue replaced last month.
The set up you see is how it has been for 25 years. No issues whatsoever.

 

[stoveliker]

You know, it's always like that with any disaster based on behavior: I always did it like this, and it went well for xyz years.

That's true. Because that disaster is the first and last time it'll happen to you (unless one has their head in the sand completely and can't learn from a disaster).

 

[bholler]

There weren't any burn marks on the ceramic insert when I had the flue replaced last month.
The set up you see is how it has been for 25 years. No issues whatsoever.

Of course there won't be burn marks on the ceramic. That isn't the concern. It is pyrolysis of the combustible material on the other side. It slowly lowers the ignition point over years of exposure until it gets low enough to ignite

 

[Waingro]

Of course there won't be burn marks on the ceramic. That isn't the concern. It is pyrolysis of the combustible material on the other side. It slowly lowers the ignition point over years of exposure until it gets low enough to ignite

Ah, I see!
Maybe it will happen in the next 25 years.....