EPA Compliant Wood Stove + (Firebox > 3.5 cuft) = 8" Flu?

[Renovation]

BK prefers to highlight long burn times over high output. A 6" flue might be adequate for this, but they still have to allow for Homer jamming the thermostat open and burning 80 lbs of wood ASAP in an attempt to make it throw heat like his old smoke dragon.

Den, that is exactly my current thinking on the matter. I've considered the issue seven ways from Sunday, as some folks have noticed, and the only negative consequence I can come up with is reduced maximum burn. It seems to me that if a 6" chimney were otherwise optimal, its response and effects would be similar to an 8" chimney with a partially closed damper--good draft and minimal effect at low to moderate burn/flow rates, but the maximum flow/burn rate would be reduced. In other words, the stove would work fine and burn the same at low/moderate rates, but its maximum output would be reduced. That would be just fine with me, because I want the King over the similarly rated Princess for the longer burn times.

Thoughts? I've asked BK the same question, though I'm afraid they won't say much besides "don't do it" out of fear of liability exposure with no upside.

 

[ddddddden]

Similar thinking here, but a Homer situation could also happen accidentally. There were a few posts recently about new BK's with the thermostat installed backwards. . .dunno if that could cause overfire or not. You could always do the ol' doze off with the door open after loading manuever. . .and besides an overfire, you might have smoke spilling into the room because the 6" flue is maxed out. Doh! Might not happen like that, but maybe. Aside from stuff like that, the big negative is that no insurance is going to cover an installation that the mfr doesn't approve.

 

[branchburner]

It seems to me that if a 6" chimney were otherwise optimal, its response and effects would be similar to an 8" chimney with a partially closed damper--good draft and minimal effect at low to moderate burn/flow rates, but the maximum flow/burn rate would be reduced.

How would this smaller flue effect start-up fires, where you want good draft at maximum flow rate to establish the burn before shutting it back to your desired low to moderate burn/flow rates? And wouldn't stack height also be a factor in determining the viability of 6" vs. 8"?

 

[ddddddden]

Also, I have the impression that with the lower flue temp of a BK, getting a good draft can be an issue. (Apparently, they strongly recommend double-walled pipe for this reason.) You might have draft issues if you monkey with the recipe. . .always a possibility in any setup though. And yeah, PNW would be doug fir central, AFAIK. I was just reading in another thread that Mr. Green has several cords of it stacked.

 

[Renovation]

It seems to me that if a 6" chimney were otherwise optimal, its response and effects would be similar to an 8" chimney with a partially closed damper--good draft and minimal effect at low to moderate burn/flow rates, but the maximum flow/burn rate would be reduced.

How would this smaller flue effect start-up fires, where you want good draft at maximum flow rate to establish the burn before shutting it back to your desired low to moderate burn/flow rates? And wouldn't stack height also be a factor in determining the viability of 6" vs. 8"?

Actually, according to my calculations, and surprisingly, no.

This is one of the aspects that I explored in other threads, and here is the most relevant post:

https://www.hearth.com/econtent/index.php/forums/viewreply/711416/

And here's the most relevant part of that post:

Okay, I’ve been playing with 6” versus 8” chimney calculations, and have some results for comment, sanity checking, and general ridicule.

I Googled up this chimney calculator, which is pretty fun to fool around with:

http://www.engineeringtoolbox.com/natural-draught-ventilation-d_122.html?v=14&units=feet#

The part I played with is the “Natural Draft Air Flow and Velocity Calculator” about halfway down the page.

It’s in metric, so to use it you have to convert units using the widgets on the left side of the screen, and know that for circular pipes, the “duct hydraulic diameter (m)” is the chimney diameter.

Once I waded through that, the fun began.

I set the interior temp at 68*, outside temp at 20*, and varied chimney diameter and height, in the following format:

Each line shows the chimneyheight/houseTemperature/OutsideTemperature/ and the resulting DraftPressure/AirFlow:

height/dia/insidetemp,F/outsideTemp,F/DraftPressure,N/m^2/flow,ft^3/min

14/8”/68/20/5.2(0.02water”)/164
14/6”/68/20/5.2(0.02water”)/88
24/6”/68/20/7.3(?water”)/103

I found those results kind of interesting.

First, it seems the Draft pressure—the initial draw on the stove—depends on height and temperature only. But the resulting flow also depends on diameter, which determines the chimney’s resistance to flow. This seems to say that, for a given temperature difference and chimney height, a 6” pipe initially draws as hard as an 8” pipe, but has lower maximum flow, which all seems to make sense to me.

So yes, you want and need maximum draw at startup, and the 6" and 8" are virtually identical under those conditions--the flow of a fire is directly proportional to fuel burned,a startup's fires BTU output is nothing like a full on burn. This makes sense, since a startup fire in a small stove isn't much different than a big stove--it's only when the fire gets going that there is a difference.

As for stack height, those figures show that for a given height and temperature difference, 6" and 8" flues have identical initial draws and velocities. Again, it is only ultimate flow that is limited, as at high flow rates the 6's smaller area and higher velocities causes more drag and limits flow.

Since maximum flow at a given temperature is almost twice as much for the 8" than the 6", the 6" would probably have more smoking on reloads, already an issue for the King, according to tips on BK's website. So that's something else to consider.

Anyway, that's way I see it, and invite insights, corrections, comments, etc.

 

[Renovation]

Also, I have the impression that with the lower flue temp of a BK, getting a good draft can be an issue. (Apparently, they strongly recommend double-walled pipe for this reason.) You might have draft issues if you monkey with the recipe. . .always a possibility in any setup though. And yeah, PNW would be doug fir central, AFAIK. I was just reading in another thread that Mr. Green has several cords of it stacked.

Agreed, good sir. BK's site talks about smoke leakage on reload as being an issue, and ways to minimize it. Smoke leakage on reload, is another difference between 6" and 8" flues. om addition to maximum burn rate. For the firebox door open and a given height chimney and temperature differential, the 8" will have a greater flow greater by the proportion of its area almost twice as much. That means less smoke leakage on reload.

I have to weigh that against the advantages of my 24', straight, insulated chimney, and saving $1,200.

Of course, when all is said and done I may well just opt for the 8" flue. But I find trying to figure out why and how and different ways of doing things fun and useful. Even if I don't do what I'm pursuing and a particular idea is a dead end, as this may be, I find the process leaves me with a greater involvement, satisfaction, and understanding. It's sort of my carrot for learning. I hope it is helpful, entertaining, or useful for others too.

 

[ddddddden]

If you already have a 6" flue, hook it up to a BKK and see. No need to put an 8" flue in first. Of course, if it doesn't work for you, the $1200 for an 8" flue could make the BKK significantly more expensive than the Woodstock Anduril. But if you want those marathon burns, I don't think anything can touch the BK. Nothing else has jumb0 firebox + cat + thermostat.

 

[Renovation]

the Woodstock Anduril. .

I like it! The definition I saw was "flame sword of the West", if only it were East...

Thanks for your thoughts.