How High is TOO High? (Chimney)

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hoverwheel

hoverwheel

Burning Hunk
Oct 18, 2013
184
USA
Academic question as the stove is installed and working pretty well.

Last fall I installed a 30NC using a 6" Class A (Selkirk) chimney. The chimney had to be on the rake end of the house and peaks at ~ 30'. That is, the chimney run is 30'. It exits the building envelope around 6' AGL.

The stove drafts well when it is very cold outside. In shoulder season it plods along and burns, but never develops the fierce fire I see when it's in the 20's.

As a first year burner, I have not yet developed the firewood inventory to be several years ahead. I'm positive at least some of the lackadaisical performance is due to wood that could be drier. It measures between 15% and 25%.

The question - at what point is a chimney too tall to be efficient and is poor shoulder season performance more likely a factor of the chimney or the quality of the wood?
 
I'd guess that poor wood is the cause of your problem but most of us find that weather conditions during shoulder season cause us headaches for burning. Going too high with a chimney could cause overdraft and a few of us have experienced overdraft. It's not fun watching your thermometer climb into the danger zone. A damper will correct that.

The cheapest answer is to save some of your drier splits for the shoulder season and that should help solve the problem.
 
With two changes in direction and most of the pipe outside, 30' is probably not too tall.
 
hoverwheel said:
The question - at what point is a chimney too tall to be efficient and is poor shoulder season performance more likely a factor of the chimney or the quality of the wood?
Click to expand...
What you are seeing is not untypical. The chimney is the engine, the wood is the fuel. Both will have an effect on burning, but they are separate entities. If your chimney was straight-up inside the house 30' could draft too strongly for some stoves and still be ok for others. All stoves do not breath equally due to the nature of their construction. If I had a 30' straight-up chimney I would probably add a key damper to the stove pipe to temper draft, especially in cold weather. But with two 90 deg. turns in your flue the effective height is more like 24' which should be good for draft especially in milder weather. However, if this is a basement install another factor could be a slight negative pressure in the basement which could be counteracting the strong draft.

Another factor that could affect the fire would be different loading in milder weather. How large is the shoulder season load of wood in the stove? Is it loaded E/W or N/S?
 
The stove is in the living room with a top exit, 90 deg pipe exiting the wall (not through the ceiling). From there it is straight up but with two 15 deg angles to miss a window.

I've been splitting fairly small with the idea being they'll dry faster.

Stove is normally loaded N/S and I've tried various loads, usually filling the box then adding a big honker near bedtime. In cold weather this seems to last through the night and have a nice bed to start again in the am. Shoulder, the fire is usually pretty well out in the am.
 
During shoulder season do you do a full load or partial load of wood?
 
I've experimented with both, generally a full load. The splits tend to be smaller and sometimes I'll add small unsplit pieces or "chunks" (knots, short ends, etc.) to fill it up.

I haven't done a "scientific" study, but lots of smaller wood seems to burn better than a few larger pieces.
 
Splitting some of the wood smaller will help it dry and combust quicker. They'll also burn up quicker. Are you burning the same species of wood all season? If the wood is the same, (which it isn't because it is still drying all winter long if top covered), then is sounds like draft is the variable. One thing that might affect shoulder season draft is if there is a upstairs window cracked open during milder weather.

Regardless, as your wood stash gets drier, your fires will improve. Less heat is being used to drive out the remaining moisture.
 
New burner, small inventory. I've been burning a wide variety of wood and sizes, mostly harvested from my property. I've not yet gotten to the point of sorting species.
 
That may also be a variable then. Different species wood burn at different rates. I often don't bother to try and burn 24/7 in mild weather. With a short hot fire the house and stove will stay warm long after the fire has died down. Typically I will burn less dense wood during these times and save the good stuff for the dead of winter when I want a long hot burn.
 
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