Burn time on a wood stove--how could this be a function of the stove rather then the wood?

[jbeall]

Hi All,

I've been looking into wood stoves a bit, and one thing that has me a bit confused are the claimed burn times. For instance, consider this Osburn stove:

http://www.osburn-mfg.com/product.aspx?CategoId=1&Id=512&Page=spec

They say that it will burn up to 10 hours in "slow combustion mode." What in the blazes (no pun intended) is "slow combustion mode?" How could a wood stove have multiple "combustion modes?" It would seem to me that any variance in combustion (and consequently, burn time) would be due to the fuel/air mixture, not the stove having different "modes"...???

They have other stoves that have slower advertised "burn times," so I'm asking myself--why is one stove going to burn the same fuel longer than another stove?

-Josh

[rocklock]

http://www.woodheat.org/planning/buyright.htm
"Burn time
How long will a given stove burn on a single load of wood? The only reasonable answer is: It depends. Burn time depends on wood species and moisture content, and on how much heat is needed during the burn. My experience is that a medium or large stove sized correctly based on all the issues discussed here will give a reliable overnight burn with enough coals remaining to kindle a fire in the morning. Stoves in the small category may or may not give an overnight burn, but they tend not to be used for whole-house primary heating."

Now my answer... The really easy answer is burning is the oxidation of the carbon (or what ever else can be oxidized) in the fuel. If you decrease the flow of o2, then the oxidation decreases which lengthens the time necessary to burn up the load of fuel... Primarily this can happen in two ways, pre-combustion chamber or post-combustion.
Hope this helps

[panderson03]

I looked into this too when we were researching wood burning stoves. found that with the blazeking stoves you can specify burn time too and that burn time is controlled by amount of air allowed into the burn chamber.

[rreidnauer]

Yes, dampering down and limiting oxygen does extend burn time, but more or less obvious, reduces BTU/hr and generates less complete combustion. This in turn reduces flue temperatures, and increases creosote deposits, which means cleaning the flue more often or risk a chimney fire. There is a limit to how far you can safely damper down a fire for regular woodstoves. However, catalytic converter equipped stoves, like the Blaze King, King and Princess models can be dampered way down, since the catalytic converter will reburn the incomplete combustion gasses. This reburning nearly eliminates creosote and efficiency goes way up as well, since most of the energy (82.5% as advertised by Blaze King) is released at the stove, rather than up the flue. (I believe flue gas temperatures are around 200F for them) This is how they get their long burn times. For example, the King model, advertised as 90,000 BTU, only generates about half that when on it's "low" setting, but burns for 47.5 hours on just 54 pounds of Tamarack.

[surinate]

I live in the Yukon and burned about 10 cords of wood a winter in My Blaze King Princess. Since there isn't large stands of hardwood to be found anywhere close I used white spruce or pine like everyone else around these parts. The longest burns I could get were around 20 hours give or take a few hours but the stove was putting out way less then half the btu's of hot burn. The thing it with the catalitic combustor is that it needs to be warm before you can flip the bypass off so it is about 15-25 minutes of babying time for a cold start. I would use the long burn times to save enough coals and keep the catalytic combustor warm for when I got home and could throw on more wood. Most of the time in the dead of the Yukon winter I wouldn't have to start a new fire for 3-4 months since there was always a bed of coals left to just throw more wood on.

The thing is that you need to burn it hot for the first hour atleast if you are going for a long burn so you don't end up with creasote problems, after that you can set the burn rate at whatever you want it.

[clairenj]

About burning soft woods successfully. I have a defiant knockoff from the late 70's and it's in great shape. 8" pipe. The chimney will go straight up through the roof. We have plenty of 60 yr old, harder "soft" red pine from the old log house. We were hoping to use it up in our woodstove next season. Most Everyone we have talked with around here has said NO because of creosote. One person said as long as we mix in the pine with our hard woods it would be OK . Whaddya think?

[surinate]

The way to avoid creosote buildup is to burn hot fires. If you only need a little heat just build a little hot fire. As long as the wood is well seasoned and you burn it hot for the first hour atleast you shouldn't be building almost any creosote. When I was burning 10 cords of softwood a year the only times I would have to sweep the chimmney was in the shoulder season of October and March-April. The rest of the winter there was nothing to clean even though my stove was going 24 7. One note though I did have a high efficency stove with a catalytic combustor in it.

If your worried about a chimmney fire just keep sweeping it often for the first while to figure out how your stove and wood combinations work together.

[Timberwolf]

Surinate, I agree 100%. As long as the wood is seasoned (the drier the better) and you are getting complete combustion (hot fire, and/or catalytic combustion), creosote should rarely be a problem.