Why the 8" flue on big cat stoves?

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Highbeam

Highbeam

Minister of Fire
Dec 28, 2006
20,912
Mt. Rainier Foothills, WA
So I'm thinking about a stove for the new 30x60 shed out back with a 14' ceiling. My dream stove would be a BK King, more realistic is an englander 30, and another thought is a wood furnace, but the first hot box will most likely be the biggest Fisher freestander (grandpa or papa) since two of them available to me for free. Anyway, I need to plan for the flue and am deciding between 6 and 8" stuff.

The real big freestanders (king and fisher) have an 8" collar. But the real big furnaces and the big englander NC30 only need a 6".

6" is cheaper of course and seems that less heat would be wasted up the flue. More efficiency from an appliance means less need for a large flue to vent the heat.

The big BK cat stove. Does it really need an 8" flue or is it just because back in the olden days when they made the dies for this stove, that was a common flue size? Perhaps it is just a function of not smoking up the room when you open the loading door?

What would you do? The whole flue will be vertical and 22' from floor to cap.
 
I think I'd go with 8". Look at the true fire box sizes of the Englander compared to the BK. The NC 30 isn't 3.5 cu ft, more like 2.7 so 6" works fine. The BKK has a true 4 cu ft box which requires more exhaust volume running at higher burn rates to prevent back puffing. You may be able to get away with 6" for a BKK if you were running low and slow 24/7 but for cold starts and hot fires you may be better off with 8".
 
I think I'll go with 8". Codes usually allow the 6" stove to be used on an 8" flue but not the other way around. Usually, the stove will be run full throttle anyway to get lots of btus.

The roof jack for 8" is huge.
 
I have a High Valley and it has an 8" collar on it. I cant get an 8" liner down my flue tiles, only a 6". Some here have said that with my 25ft height i should be fine running it on a 6"er. I called The manufacturer and got a somewhat of a yes i can run it on 6" but was not totally satisfied with that answer? Still debating?
 
Todd said:
I think I'd go with 8". Look at the true fire box sizes of the Englander compared to the BK. The NC 30 isn't 3.5 cu ft, more like 2.7 so 6" works fine. The BKK has a true 4 cu ft box which requires more exhaust volume running at higher burn rates to prevent back puffing. You may be able to get away with 6" for a BKK if you were running low and slow 24/7 but for cold starts and hot fires you may be better off with 8".
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How do you know the NC30 is 2.7 cu ft? Did you measure it? I wish I knew what the real measurement was on the J.A. Roby Ultimate.
 
Dr. Richards said:
How do you know the NC30 is 2.7 cu ft? Did you measure it? I wish I knew what the real measurement was on the J.A. Roby Ultimate.
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Yep, some manufactures include the space above the baffle as fire box, so I'd advise everyone to measure themselves.
 
Todd said:
Yep, some manufactures include the space above the baffle as fire box, so I'd advise everyone to measure themselves.
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Wow, that's disappointing. 3.5 sounds like a monster, now it just sounds average. I'm surprised all manufactures don't just include the area above the baffle to inflate their number.
 
Dr. Richards said:
Wow, that's disappointing. 3.5 sounds like a monster, now it just sounds average. I'm surprised all manufactures don't just include the area above the baffle to inflate their number.
Click to expand...


The vast majority do.
 
Dr. Richards said:
I just asked stoveguy2esw (works tech support at Englander) if they include the space above the baffle in their 3.5 cubic feet and he said they do not.
Click to expand...

The "usable" space (and these are tight #'s) are 20" wide (E/W) X 20" long (N/S) X 12.5" tall. You do the math.
 
If you load up to the glass (N/S) you can get 22". But its still not 3.5.

Still bigger than a lot of stoves out there. Much bigger. Being able to load 19" split N/S or E/W is pretty nice ;)
 
Todd said:
The BKK has a true 4 cu ft box which requires more exhaust volume running at higher burn rates to prevent back puffing. You may be able to get away with 6" for a BKK if you were running low and slow 24/7 but for cold starts and hot fires you may be better off with 8".
Click to expand...

Interesting... the only time I get back puffing on my stove is when I'm running low and slow. I'm not running a BK King, but I am running a large cat stove.
 
Dr. Richards said:
I just asked stoveguy2esw (works tech support at Englander) if they include the space above the baffle in their 3.5 cubic feet and he said they do not.
Click to expand...


In the end, it doesn't matter to me. It's a big stove and the 30 will find a nice home in one of my three stove locations before fall arrives.
 
Joful said:
Interesting... the only time I get back puffing on my stove is when I'm running low and slow. I'm not running a BK King, but I am running a large cat stove.
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I think in your situation it may be more the design of the stove? BK has the cat on top of the stove and I think yours is in the lower rear like the VC stoves which also have been known for backpuffing. My theory on this and I may be wrong is the stoves with the cat on top have a more direct or easier path for the exhaust which keeps less gas buildup in the firebox that can cause backpuffing. Chimney and draft probably play a big roll as well.
 
Todd said:
I think in your situation it may be more the design of the stove? BK has the cat on top of the stove and I think yours is in the lower rear like the VC stoves which also have been known for backpuffing. My theory on this and I may be wrong is the stoves with the cat on top have a more direct or easier path for the exhaust which keeps less gas buildup in the firebox that can cause backpuffing. Chimney and draft probably play a big roll as well.
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All true, and the primary reason why I plan to install a 6" insulated liner in my 8" clay tile flue, on that stove.
 
So you plan on using a 6" liner for your stove with an 8" output?

I recontacted my stove maker and they sent an email saying my stove was aproved with the use of a 6" reducer.
 
Nope! My stove has a 6" output. I was surprised when I saw similarly sized BK's had 8" collars.
 
Highbeam,

What is this bad boy insulated with....?

How about a picture or 2?


I know that if/when I build a shop or a new house I'm gonna go with a furnace,

Like a BK Apex,These furnaces are made by VALLEY COMFORT MANUFACTURING and are available in the USA.

The APEX-CBT units use catalytic combustion and can be installed as a stand alone unit or added to another furnace system.
 
The floor slab is 5" on top of 2" of foam. Pex radiant tubing (1800LF) is in the slab for some sort of legal heating just to keep the barn in the 40+ range, though future owners could heat to 70 for all I care.

The walls are built with a true 8" cavity which will be filled with fiberglass batts of whatever thickness fits, probably some sort of ceiling insulation. R-20 at least. The ceiling will have blown in insulation. The three 12x12 doors are 2" thick insulated steel doors.

Right now the barn is functional and wired. Being used for storage.




Hiram Maxim said:
Highbeam,

What is this bad boy insulated with....?

How about a picture or 2?


I know that if/when I build a shop or a new house I'm gonna go with a furnace,

Like a BK Apex,These furnaces are made by VALLEY COMFORT MANUFACTURING and are available in the USA.

The APEX-CBT units use catalytic combustion and can be installed as a stand alone unit or added to another furnace system.
Click to expand...
 

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Todd said:
Yep, some manufactures include the space above the baffle as fire box, so I'd advise everyone to measure themselves.
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Just got back from Home Depot and did my own measurements and came up with 3.25 cubic feet. That's measuring 13" to bottom of tubes. The lip in the front on the bottom sticks up 2.5", so the bottom measurement is 20" x 20" x 2.5", then if you pack to the glass the rest of the space is 20" x 22" x 10.5". So it's not quite as far off as originally thought.
 
Highbeam,

You Sir....Hats off to You my Friend.....:eek:

That is one awesome "Shed" as You call it...I'm jealous ==c

Have You thought about using a wood fired boiler?

If You have radiant in floor heat, that may be the ticket?

The 2" foam from my understanding makes a huge difference?

Cheers Hiram
 
I can heat the floors with a very cheap wall hung boiler/water heater or a tank water heater. Well under 1000$. For me, the floor heat is to hold that base temp above freezing at all times to keep the liquids from freezing and condensation from occuring on the metal. The floor heat needs to be automatic and trouble free so propane is the most likely fuel. In the future, if I find myself spending more time in the shed or if the propane costs are huge then I may consider some other way to heat the water. The floor heat allows lots of flexibility in this way. Wood boilers are expensive and illegal in WA. All indoor solid fuel heaters are illegal as well. So if I get busted and need to remove it, the woodstove won't be a great loss.

I like to go to the shop for a few hours at a time. I need a rapid response wood heater to take the chill off. The fisher stoves will be good for this and I already have two of them but the NC30 might be ideal. I'll be installing 6" flue and using the fishers at first.

The shed has been a dream of mine for a long time. I was out there last night until 9:30 getting the Stihl ready for this weekend's adventure.
 

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One of my friends heated his shop that way. It is a newer pole building he built, and is decently insulated. He used a water heater for the first couple of years, and found it to be really expensive. The thing was burning constantly trying to keep up. He has since installed a proper LP boiler, and the cost is much more manageable. He also supplements with a pellet stove.
 
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