How the wood stove works?

[DragonSmoke]

So, Ive been burning wood as a main heating source for 10 years (and through 2 different houses), and I'm STILL learning and adjusting the way I do things! lol For example this Year I've decided to split my would into bigger chunks instead of smaller ones or as I've tended to do, a mixture of small, medium, large, and rounds. (I still utilize rounds for good long overnight burns).
However, One thing that REALLY surprises (and confuses me) me is how the wood stove works! Its almost counter intuitive. Typically, One would think that a good rip-roaring fire is the best way to get the house nice and warm and get that stove nice and hot. BUT...in actuality, once you get that wood ignited well, you really wanna close that air supply down until that fire is just nicely, lazily rolling along and THEN that temperature sky rockets! Which is also really nice because you're practically sipping wood instead of flying through it.
My question is, why exactly is this? I know that its because the heat is staying in the stove instead of going up the chimney, but my question is WHY is it staying in the stove more that when the fire has more air to it? Is it simply because of a ratio? The fire with a little air has the air required for combustion but not an overload whereas a lot of air is more than combustion requires and therefor actually pushes the heat out of the stove so fast it doesn't have time to heat?? hmmm, I may have answered my own question but I'd still appreciate your feedback.

 

[bholler]

So, Ive been burning wood as a main heating source for 10 years (and through 2 different houses), and I'm STILL learning and adjusting the way I do things! lol For example this Year I've decided to split my would into bigger chunks instead of smaller ones or as I've tended to do, a mixture of small, medium, large, and rounds. (I still utilize rounds for good long overnight burns).
However, One thing that REALLY surprises (and confuses me) me is how the wood stove works! Its almost counter intuitive. Typically, One would think that a good rip-roaring fire is the best way to get the house nice and warm and get that stove nice and hot. BUT...in actuality, once you get that wood ignited well, you really wanna close that air supply down until that fire is just nicely, lazily rolling along and THEN that temperature sky rockets! Which is also really nice because you're practically sipping wood instead of flying through it.
My question is, why exactly is this? I know that its because the heat is staying in the stove instead of going up the chimney, but my question is WHY is it staying in the stove more that when the fire has more air to it? Is it simply because of a ratio? The fire with a little air has the air required for combustion but not an overload whereas a lot of is is more than combustion requires and therefor actually pushes the heat out of the stove so fast it doesn't have time to heat?? hmmm, I may have answered my own question but I'd still appreciate your feedback.

Less air moving through the stove allows for more heat transfer. That is the simplest answer. There is a whole lot more to it but that varies by stove.

 

[jetsam]

It sounds like you learned to burn on a pre-EPA stove and now you have an EPA stove.

With old stoves, more air was more heat. You were sending most of the VOCs (aka half of your fuel) up the flue on any setting, and the route from the firebox to the flue was straight up with no stops.

With new stoves, you also get to burn the wood gas and other volatiles. When you back off the air, you can burn mostly gasses for the first part of the burn.

More heat, less wood.

Also true what bholler said- it's less wasteful when you slow the speed that air moves through the stove. The stove only transfers heat out so fast, and if combustion air is flying through there very quickly, you are mostly heating the outside.

 

[begreen]

Closing down the airflow does a few things. It keeps the wood gases resident in the firebox longer for a more complete burn. The vacuum created by the draft increases and this pulls air through the secondary burn system (tubes or baffle box). This secondary air supply mixes with the hot gases just under the baffle and reignites them at high temperature. All of this combines for a more complete and hotter burn in the firebox and less heat and unburnt wood gases (smoke) up the flue.

 

[SteveKG]

Some good answers here. I'll add that I switched from an old barrel-shaped stove [Shenandoah, but there were many] I'd used to very comfortably heat our home for decades. Then I decided to get a catalytic stove and once I learned its differences to a non-cat [which took a few days], I heat the same area with about half the wood. Seriously. I had hoped to burn less wood, and I wanted to burn the gases I was letting up the chimney, but I was very surprised that the wood use went down so dramatically.

It did take me a few days, as I said, to get used to turning the air down so much.

 

[heavy hammer]

In my first house I had an old sears stove did an ok heating but burned a lot of wood for short periods. I upgraded to a jotul Oslo and what a difference more heat better burn times less wood consumed. Technology and improvements with designs have made vast improvements over the years.

 

[Ctwoodtick]

This video is very informative. The acting however...lets just say Mystery Science Theater 3k would have had a field day with it.

 

[DragonSmoke]

Actually both of my stoves are pre 90 pre epa. One of them is the Lopi 380 (The prototype predecessor to the lopi endeavor) circa 1988. That stove I bought while at my last house and removed and brought with me to this house! The other stove is an avalon 790.745 insert circa 1987? maybe? Can't remember, definitely 80's though.

 

[jetsam]

Actually both of my stoves are pre 90 pre epa. One of them is the Lopi 380 (The prototype predecessor to the lopi endeavor) circa 1988. That stove I bought while at my last house and removed and brought with me to this house! The other stove is an avalon 790.745 insert circa 1987? maybe? Can't remember, definitely 80's though.

Well, some stoves did have baffles and maybe even tubes before the EPA mandate forced manufacturers to make better stoves.

Hobbyists have been adding baffles to smoke dragons since.... well, franklin stoves had a vertical version in 1741, so I'd say the technology was around for a while before manufacturers got forced to use it.

No idea how long tubes have been around.

If you get more heat by turning down the air, you definitely have some sort of secondary combustion going on.

 

[begreen]

The knowledge of secondary combustion goes back aways. The Jotul F602 was designed in the 1940s I think and it had a baffle plus dual ports in the door to feed both the fire base and to introduce air as the smoke turned around the baffle. Lange and others had two port designs on some stoves too. The original VC stoves designed in the 70's had a secondary combustion chamber.

 

[DragonSmoke]

Well, some stoves did have baffles and maybe even tubes before the EPA mandate forced manufacturers to make better stoves.

True. I know the lopi 380 experimented with tubes before they turned it into the endeavour. The only difference is that the endeavor had 2 or 3 tubes while mine only has one at the front. Same with the avalon. (Same company as lopi so not a big surprise). Has one secondary burn tube at the front.

 

[blacktail]

More air going into the stove and up the pipe means more cold outside air being pulled into your house to replace it.

 

[xman23]

For more heat I need more wood and air. I judge the heat on stove top temperature. To slow the burn and reduce the stove temp I reduce the air. There must be fine line, where your sending to much heat up the chimney, thus pulling in an equal amount of cold air thru the house walls. But that would be hard to measure.

 

[Rhodie]

For more heat I need more wood and air. I judge the heat on stove top temperature. To slow the burn and reduce the stove temp I reduce the air. There must be fine line, where your sending to much heat up the chimney, thus pulling in an equal amount of cold air thru the house walls. But that would be hard to measure.

Wait.. I’m trying to square this with prior posts... I thought turning the air down increases the heat by encouraging the secondaries. Later in a burn I would think turning the air down slows things down and there are no secondaries left to burn.

 

[jetsam]

I don't think I exactly follow you. What is the relation between air entering the stove vs air entering the house?

The relation is 1:1- but not by volume because of the temperature difference.

1cf of air goes into the stove and up the flue. There is now a small negative pressure in the house, and some cold air comes in the house through wherever the gaps are.

The cold air expands a bit on its trip through the house and stove. If it's 0°F outside and your flue gas is leaving at 300°, you are pulling in 0.6cf of 0° air per 1cf of 300° exhaust. (This is actually the same amount of air coming in and going out.) If it's -30 out and your flue gasses are 400° at the top of the stack, you are taking in 0.51 cf for every cf of output.

Fluid dynamics is very complex, but this is a real simple way to think of it and have some idea what the system is up to.

 

[jetsam]

Wait.. I’m trying to square this with prior posts... I thought turning the air down increases the heat by encouraging the secondaries. Later in a burn I would think turning the air down slows things down and there are no secondaries left to burn.

It increases the heat because all the VOCs are burning instead of being whooshed up the flue. If you see pretty secondaries (or your cat gets hot, or both) you'll know this is going on.

After the wood is all coal, you may as well crank 'er up a bit because no secondary combustion is happening anyway.

 

[Rhodie]

It increases the heat because all the VOCs are burning instead of being whooshed up the flue.

Right, that’s what I understand. Note xman23 said turning down the air was one way he lowered the temp. People are not quantifying where in the burn cycle they are.

 

[begreen]

It's not only the time in the burn cycle but how much fuel is in the firebox and how strong the draft is. Our stove was running low and slow today because it was in the high 40s outside. The setting for that fire is quite different than for a fully loaded stove.