wood burning stove

[SNUNEZ]

What is the trick to build a stove that burns wood and 3 or 4 logs will last 8 to 10 hours

[shovelon]

What is the trick to build a stove that burns wood and 3 or 4 logs will last 8 to 10 hours

You talking about your logs or your stove lasting 8 to 10 hours?

[Country Metals]

Air control... The more air, the faster it burns. I am actually designing one that I can fold down one side and load logs inside of and have a thermostat control the air. This plus a pipe welded inside the stove that will go to the duct work will give cheap heat to the entire shop.

[Meltedmetal]

Air control... The more air, the faster it burns. I am actually designing one that I can fold down one side and load logs inside of and have a thermostat control the air. This plus a pipe welded inside the stove that will go to the duct work will give cheap heat to the entire shop.

Country metals pretty well nailed it. Tight draft control will extend your burn time. Just remember to maintain sufficient minimum draft to keep your flue temperature high enough to preclude creosote formation or you will end up with a spectacular chimney fire or worse. Burn time is also somewhat dependent on what you are burning.
Meltedmetal

[Old Skool]

Burn green wet wood, it'll smolder for a week.

[Doughboyracer]

Burn green wet wood, it'll smolder for a week.

Pete moss works good for that too.

[Dobermann]

But seriously folks......
Look at some good woodstoves and see what features they have. In most cases, they seem to be designed to keep a lot of heat in the firebox to keep combustion temperatures up. They also seem to have some air passage arrangements that inject air into the burn process fairly high up in the fire box.

My Nestor-Martin stove has a ceramic brick lining on the sides and top of the burn chamber. There are two sites where air in injected through small (less than 1/8" dia) holes in some sort of manifold that runs the length of the firebox. It can burn fairly slowly with the air shut down quite a bit and still hold a high enough temp to keep from forming creosote in the flue.

The Nestor-Martin also has some way of managing the burn gases so that really hot gas passes down over the glass door to keep carbon burned off the glass so you can see the fire. Although I've watched it burn lots of times, I have no idea how they've managed to accomplish this.

Older stoves such as my Jotul don't have any ceramic in them but they do have a fairly long burning path for the burning gases to travel. They also have cast iron shields along the sides of the firebox to help keep heat in the burning chamber.

I don't see any reason that a person couldn't weld one of these things up out of steel plate but there might be some "cut and try" so far as getting the thing configured so it will do what you want.

Go look at some stoves and kick some tires....er....cast iron to see how they're put together.

[kevin]

I gave the " building the stove for the shop" alot of thought, how to do it, what thickness steel ect. Then it popped into my head, this could be a problem if there was ever a fire, a home built stove is not ul approved, if you ever had a fire, this really opens the door for an insurance adjuster to find a way as to not pay a claim. There are tons of used stoves out there, better yet, call your insurance people and run it by them

[walker]

A masonry fireplace is also not UL approved, but there are a few out there.

[Helios]

What is the trick to build a stove that burns wood and 3 or 4 logs will last 8 to 10 hours

Make the stove burn reeeeeealllllllyyyyyyy ssssssllllllloooooooowwwwww (think: SMOLDER).

Why would you want to do that? All it will do is creosote up your chimney, and it won't put out enough heat to melt an ice cube.

If you want heat, you need to be able to burn X pounds of wood in Y hours ... and all woods, if dry, have about the same amount of energy per unit of weight (about 7,000 BTUs per pound).

That means that if you want your woodstove to put out as much heat per hour as a typical kerosene heater (which puts out 23,000 BTU per hour) then it will need to burn AT LEAST (23,000 ÷ 7,000 = ) 3.28 pounds of wood per hour.

But wait: A kerosene heater has a combustion efficiency of 100% but a wood stove has a combustion efficiency (and an overall efficiency) that is far less than 100%.

Why? Well, for one thing, with a wood stove you have products of combustion that you need to get out of the house (carbon monoxide, fly ash, soot, etc.), and to get them out of the house, you need to have a draft up your chimney, which requires heat to draw.

And it can't just be "warm" ... it needs to be hot. In fact, you need to maintain a minimum stack temperature of about 400°F just to prevent creosote condensation (think of this as "overhead" or "the energy cost of running a woodstove"), so a lot of your heat goes up the chimney. Also, you lose fuel in the form of smoke – especially in a fire that smolders.

The only way to get "3 or 4 logs to last 8 to 10 hours" in the stove is to make a stove that puts out almost no heat whatsoever. Why? Do the math.

Assuming those 4 logs weigh 7 pounds each, that's 28 pounds of wood.
If the wood is dry, that 28# of wood represents 196,000 BTUs.
If your combustion efficiency is 50% (and this is optimistic) then you would get 98,000 BTUs.
Factor in how much heat goes up the chimney and let's call it 50,000 BTUs.
Spread that out over 10 hours and you get 5000 BTUs per hour.
That's about one fifth as much heat as a common convection-style kerosene heater.
(And that's assuming you could even get it to "burn" for 10 hours without smoldering. If it's smoldering, your combustion efficiency probably drops to less than 5%.)

There ain't no free lunch, despite what our fearless leader might want you to think. I wouldn't even attempt to make a stove that only burns 3 or 4 logs in 8 or 10 hours. To me there's no point. If all you want it for is the ambiance or spectacle, buy a fake gas or electric "fireplace."