Enough Draft? Wood quality vs draft?

oldspark · Jan 1, 2011

LLigetfa said:
oldspark said:

Some times I like to check the moisture content for the discussions on here.

Click to expand...

Ja, but then FUD sets in. Did he resplit and test on a fresh face? Did he stick the probes in far enough? With the grain or across the grain? On the end grain... that's not how to do it!

It gets to be like the conversations on cord measurement. Real cord, face cord, bush cord, stove cord? Then there's the conversation around thermometers... surface or probe? Where to place? Is mine accurate?

I'm just playing Devil's advocate. There is no perfect answer so we work with what we have/know/assume. Do we fly seat-of-the-pants or by the instruments?

That info all comes out, might have to ask a few questions but they get answered, I thought that was what forums are for.

guest5234 · Jan 1, 2011

branchburner said:
avc8130 said:

I THOUGHT 20-25% was ok.
ac

Click to expand...

I never measure, but it seems a 20% range is roughly the turning point for a good burn with some stoves. Going up to 25% may make a big diff.
Forgetting %, how long has your wood been cut/split/stacked, and what species?

I also find that burning a mix, small along with large, helps my stove run best. All big splits and sometimes it won't coal well for my burn chamber. I have some big oak about 24 mos cut and 12 mos split - decided I'm going to give it another year!

That should be ideal to burn now.

LLigetfa · Jan 1, 2011

oldspark said:
That info all comes out, might have to ask a few questions but they get answered, I thought that was what forums are for.

You're right, that is what forums are for. AC stated that he has a manometer and a wind meter and is looking for the magic numbers. I think however that the number is probably "that depends". Depends on the stove in question, the flue size, how one burns, what one burns. Sometimes raw data, particularly out of context, asks more questions than it answers.

When I had my HRV installed, the installer had a manometer to dial it in. After he left, I had to tweak it seat-of-the-pants style based on my observations (without instruments). My aircraft analogy doesn't really fit because aircraft have very sophisticated instruments and the science around their operation fairly precise. If planes were powered by firewood, it might be a little different.

oldspark · Jan 1, 2011

LLigetfa said:
oldspark said:

That info all comes out, might have to ask a few questions but they get answered, I thought that was what forums are for.

Click to expand...

You're right, that is what forums are for. AC stated that he has a manometer and a wind meter and is looking for the magic numbers. I think however that the number is probably "that depends". Depends on the stove in question, the flue size, how one burns, what one burns. Sometimes raw data, particularly out of context, asks more questions than it answers.

When I had my HRV installed, the installer had a manometer to dial it in. After he left, I had to tweak it seat-of-the-pants style based on my observations (without instruments). My aircraft analogy doesn't really fit because aircraft have very sophisticated instruments and the science around their operation fairly precise. If planes were powered by firewood, it might be a little different.

Very true but some people want to throw the baby out with the bath water.

Battenkiller · Jan 1, 2011

branchburner said:
I'll quote you: "Otherwise, the new charge will drop internal temps substantially, lowering flue temps, and reducing draft..."

Aren't you saying the draft strength is dependent upon the moisture content or size of the new charge of wood?

Sorry, I thought you meant split size, not the mass of the load itself.

No matter, if the coal bed is substantial you will get high flue temps even with marginal (i.e. between 25% and 30% water by weight) wood. Those extra few ounces of water in each split won't do much to immediately and directly rob the coal bed of the heat needed for combustion to begin, not compared to the effect of the actual total mass of the load (including all the wood fiber) itself. The only difference as far as an immediate cooling effect will be in the higher specific heat of water compared to that of wood fiber. And we're talking marginal wood here, not dumping massive rounds of green oak at 45% water by weight. Again, not that much extra water to account for the heat loss you are describing.

Even at 30% water, a 20 pound charge will only need 280 BTU to raise that extra water all the way to 212ÂºF. And that's assuming that all of that water will need to be raised to that temp before the wood will begin to burn. That is certainly not the case. Once the outside has charred and begins out-gassing, flame will be sustained while the inner water is still coming up to temp. Anyone who has ever dropped a large round of green wood directly on a massive bed of hot coals will see that it begins to burn almost immediately, and continues to burn, even while water is steaming out the ends.

As far as air vs. wood quality, I find it perplexing that folks will obsess over getting their saws at the ideal air:fuel ratio, but then assume that they don't need to do it in the stove. I have seen much more research that indicates that proper air is more crucial to a good burn than using perfect wood... however it is defined. And forget the old "you wouldn't want to use gas with water in your saw" argument. Firewood always contains a substantial amount of water, gasoline can't have any at all. Apples and watermelons comparison AFAIC.

In fact, I have seen some hard data that shows that oak in the 40% MC range burns substantially cleaner in a conventional airtight stove than oak at 20% MC. Yes, there is a small hit on overall efficiency Less than you imagine), but the measured emissions are actually significantly lower. This trend reverses itself with the modern designs, but since the new stoves burn cleaner by nature, the difference between burning seasoned and unseasoned is so low in total emissions that there is a greater difference between stove types/models. To wit, a cat stove will produce less PM and other nasties when burning marginal wood at lower temps that a non-cat will when burning bone-dry wood at the same temp.

You should remember that this isn't EPA/Hearth.gov. Just because many folks made the switch to an EPA stove doesn't mean everybody did. Many here still burn the older stoves to great success. They deserve to know the truth about their stoves' emissions under varying moisture contents.

avc8130 · Jan 1, 2011

Battenkiller said:
branchburner said:

I'll quote you: "Otherwise, the new charge will drop internal temps substantially, lowering flue temps, and reducing draft..."

Aren't you saying the draft strength is dependent upon the moisture content or size of the new charge of wood?

Click to expand...

Sorry, I thought you meant split size, not the mass of the load itself.

No matter, if the coal bed is substantial you will get high flue temps even with marginal (i.e. between 25% and 30% water by weight) wood. Those extra few ounces of water in each split won't do much to immediately and directly rob the coal bed of the heat needed for combustion to begin, not compared to the effect of the actual total mass of the load (including all the wood fiber) itself. The only difference as far as an immediate cooling effect will be in the higher specific heat of water compared to that of wood fiber. And we're talking marginal wood here, not dumping massive rounds of green oak at 45% water by weight. Again, not that much extra water to account for the heat loss you are describing.

Even at 30% water, a 20 pound charge will only need 280 BTU to raise that extra water all the way to 212ÂºF. And that's assuming that all of that water will need to be raised to that temp before the wood will begin to burn. That is certainly not the case. Once the outside has charred and begins out-gassing, flame will be sustained while the inner water is still coming up to temp. Anyone who has ever dropped a large round of green wood directly on a massive bed of hot coals will see that it begins to burn almost immediately, and continues to burn, even while water is steaming out the ends.

As far as air vs. wood quality, I find it perplexing that folks will obsess over getting their saws at the ideal air:fuel ratio, but then assume that they don't need to do it in the stove. I have seen much more research that indicates that proper air is more crucial to a good burn than using perfect wood... however it is defined. And forget the old "you wouldn't want to use gas with water in your saw" argument. Firewood always contains a substantial amount of water, gasoline can't have any at all. Apples and watermelons comparison AFAIC.

In fact, I have seen some hard data that shows that oak in the 40% MC range burns substantially cleaner in a conventional airtight stove than oak at 20% MC. Yes, there is a small hit on overall efficiency Less than you imagine), but the measured emissions are actually significantly lower. This trend reverses itself with the modern designs, but since the new stoves burn cleaner by nature, the difference between burning seasoned and unseasoned is so low in total emissions that there is a greater difference between stove types/models. To wit, a cat stove will produce less PM and other nasties when burning marginal wood at lower temps that a non-cat will when burning bone-dry wood at the same temp.

You should remember that this isn't EPA/Hearth.gov. Just because many folks made the switch to an EPA stove doesn't mean everybody did. Many here still burn the older stoves to great success. They deserve to know the truth about their stoves' emissions under varying moisture contents.

So with this said...is wood quality not that important in your opinion?

From reading on this forum it seems wood is the number one thing to pin blame on.

I have been fighting to find wood that I can burn this year from local suppliers. Luckily most have been willing to share "samples" with me to try.

I have used the moisture meter...my technique has been to cut a fresh split and then plunge the meter into the middle of the wood (both directions) until it gives a reading. Is this not the correct way to use it?

ac

Battenkiller · Jan 1, 2011

avc8130 said:
So with this said...is wood quality not that important in your opinion?

From reading on this forum it seems wood is the number one thing to pin blame on.

No, that isn't my position. Wood quality is extremely important, I just define "quality" over a broader band of moisture than others here may.

Once your wood is dry enough to burn well without fuss, it will be easy to get the flue temps up to the range where lots of adjustment is possible. I don't own an EPA stove, so I cannot speak from first hand experience, but I'm fairly certain based on all I do know about them that wood that is 20% water by weight is ideal in these stoves. You'll get a great, regulated burn with plenty of heat to drive the engine of your burner - the chimney. From that point, you will be able to easily adjust the air one way or another to get close to a perfect burn more often than not. If it's too dry it will be harder to control, if it's too wet it won't burn well at all. Like the Goldilocks story, I want my wood to be "just right".

Use that moisture meter, just understand what it means. I know I sound like a broken record here, but the reading you get on the meter is not the percentage of a given piece of wood that is water, it is the percentage of the pure wood fiber in that piece that is water. Failing to differentiate between these two ways to express moisture in wood is like failing to convert centimeters to inches. But forget the confusion about all that. You should be getting a reading of between 20 and 25% MC on the meter. That is between 16 and 20% water by weight - the perfect range. Or store it in single rows for 12-18 months in a dry, covered place that gets plenty of air movement and you should be close to perfect.

During a conversation with a tech from a well-respected stove maker, I was told that if they could just get folks to get their wood down to 30% on a meter they would have very few service related phone calls. Seems most folks still want to cut it today and burn it tomorrow... until they show their faces here on Hearth.com.

Big Al · Jan 1, 2011

avc8130 said:
Battenkiller said:

branchburner said:

I'll quote you: "Otherwise, the new charge will drop internal temps substantially, lowering flue temps, and reducing draft..."

Aren't you saying the draft strength is dependent upon the moisture content or size of the new charge of wood?

Click to expand...

Sorry, I thought you meant split size, not the mass of the load itself.

No matter, if the coal bed is substantial you will get high flue temps even with marginal (i.e. between 25% and 30% water by weight) wood. Those extra few ounces of water in each split won't do much to immediately and directly rob the coal bed of the heat needed for combustion to begin, not compared to the effect of the actual total mass of the load (including all the wood fiber) itself. The only difference as far as an immediate cooling effect will be in the higher specific heat of water compared to that of wood fiber. And we're talking marginal wood here, not dumping massive rounds of green oak at 45% water by weight. Again, not that much extra water to account for the heat loss you are describing.

Even at 30% water, a 20 pound charge will only need 280 BTU to raise that extra water all the way to 212ÂºF. And that's assuming that all of that water will need to be raised to that temp before the wood will begin to burn. That is certainly not the case. Once the outside has charred and begins out-gassing, flame will be sustained while the inner water is still coming up to temp. Anyone who has ever dropped a large round of green wood directly on a massive bed of hot coals will see that it begins to burn almost immediately, and continues to burn, even while water is steaming out the ends.

As far as air vs. wood quality, I find it perplexing that folks will obsess over getting their saws at the ideal air:fuel ratio, but then assume that they don't need to do it in the stove. I have seen much more research that indicates that proper air is more crucial to a good burn than using perfect wood... however it is defined. And forget the old "you wouldn't want to use gas with water in your saw" argument. Firewood always contains a substantial amount of water, gasoline can't have any at all. Apples and watermelons comparison AFAIC.

In fact, I have seen some hard data that shows that oak in the 40% MC range burns substantially cleaner in a conventional airtight stove than oak at 20% MC. Yes, there is a small hit on overall efficiency Less than you imagine), but the measured emissions are actually significantly lower. This trend reverses itself with the modern designs, but since the new stoves burn cleaner by nature, the difference between burning seasoned and unseasoned is so low in total emissions that there is a greater difference between stove types/models. To wit, a cat stove will produce less PM and other nasties when burning marginal wood at lower temps that a non-cat will when burning bone-dry wood at the same temp.

You should remember that this isn't EPA/Hearth.gov. Just because many folks made the switch to an EPA stove doesn't mean everybody did. Many here still burn the older stoves to great success. They deserve to know the truth about their stoves' emissions under varying moisture contents.

Click to expand...

So with this said...is wood quality not that important in your opinion?

From reading on this forum it seems wood is the number one thing to pin blame on.

I have been fighting to find wood that I can burn this year from local suppliers. Luckily most have been willing to share "samples" with me to try.

I have used the moisture meter...my technique has been to cut a fresh split and then plunge the meter into the middle of the wood (both directions) until it gives a reading. Is this not the correct way to use it?

ac

I've found it's all tied in somewhat. I've got some 14month seasoned oak now, not sure of the MC, but it still sizzles. When the temps are mild I can can only throttle the stove down so far or the fire smolders. I can get good secondary action....I'm watching an amazing light show right now and temps are still north of 40 outside...it's just at 15 to 20% greater primary ait setting then when it's cold out. When it gets below 30 or better, below 20, I can dial it down that extra 20% and get the same efficient burn. So I guess the colder it is, the stronger the draft. The stronger the draft the more air you can draw in to burn less than ideal wood more efficiently. Not very scientific, just my observations.

avc8130 · Jan 11, 2011

I guess the question it boils down to for me is this:
How do you KNOW you have adequate draft?
ac

Battenkiller · Jan 11, 2011

avc8130 said:
I guess the question it boils down to for me is this:
How do you KNOW you have adequate draft?
ac

It's like pornography. Hard to define precisely, but you know it when you see it.

oldspark · Jan 11, 2011

avc8130 said:
I guess the question it boils down to for me is this:
How do you KNOW you have adequate draft?
ac

The stove works!

Battenkiller · Jan 12, 2011

oldspark said:
avc8130 said:

I guess the question it boils down to for me is this:
How do you KNOW you have adequate draft?
ac

Click to expand...

The stove works!

If you're hinting around for us to chip and buy you three more feet of chimney, I think you may have finally worn us down. ;-P

oldspark · Jan 12, 2011

BK thank you for the laugh! :lol:

avc8130 · Jan 12, 2011

Battenkiller said:
oldspark said:

avc8130 said:

I guess the question it boils down to for me is this:
How do you KNOW you have adequate draft?
ac

Click to expand...

The stove works!

Click to expand...

If you're hinting around for us to chip and buy you three more feet of chimney, I think you may have finally worn us down. ;-P

Nah, not looking for that...just trying to figure out how one knows. My stove burns kiln dried wood like an animal, so I guess the common answer works.
ac

Renovation · Jan 12, 2011

avc8130 said:
How does one know if their setup has enough draft?

ac

1. Connect it to the tailpipe, and it will suck-start a Harley.

2. It will suck the chrome off a trailer hitch.

Renovation · Jan 12, 2011

avc8130 said:
I am not having any particular issues, I am mostly CURIOUS. Hence why I am looking for quantitative answer.
ac

Oh, so you are evil, like me?

You could check owners manuals. IIRC, Blaze King, or someone else specifies 0.02" of water of natural draft, measured by an instrument called something like a manometer.

So there's a bit of an idea.

Stay curious! :coolsmile:

Renovation · Jan 12, 2011

Battenkiller said:
Well, since you already have a manometer, might as well put it to good use.

For most wood stoves, you should have a measured draft of between 0.04" and 0.08" of water when the stove is at operating temps. Obviously, there is some room for variance, but not too many stoves can handle a draft as high as 0.12" without tending toward overfire, or below 0.02" without tending to smolder and burn inefficiently.*

* The above information regarding draft was taken from a recent phone conversation with an engineer at Pacific Energy.

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