Do you lose BTUs if you run your insert wide open?

[Toasty-Yote]

I actually did take some science classes in school so it seems like I should know this but I don't.

Do you lose some of the BTUs from a given amount of wood if you run your insert wide open? My preliminary thoughts are that more air equals more updraft and more heat leaving through the chimney. On the contrary, I would think that running it colder might make a dirtier fire with more unburnt fuel leaving in the way of smoke? Those are just my uninformed thoughts, I would appreciate if someone could tell me what is really happening.

 

[begreen]

Yes, definitely. In general terms, the longer the flame stays resident in the firebox the more heat the stove will radiate. The draft creates a vacuum in the firebox. With the air control wide-open, air is pulled in through the primary air port. If this is a secondary combustion stove, closing down the air will then cause the draft to pull air through and out the secondary air system. This enhances the combustion of the flue gases resulting in a lot less heat going up the flue and more heat emanating from the stove.

You can see robust secondary burn happening in this video.

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[stoveliker]

Another take on this is the following.

When burning low (I'm still assuming one burns clean in both cases here...), the hot/warm air flowing up the chimney lasts for far more hours than when burning high.
So a large number of hours with warm air up the flue (and a large number of hours with a low number of BTUs per hour into the room) versus a shorter number of hours with hot air (but also a lot more BTUs per hour into the room). What looses the most BTUs up the flue?

BKVP has mentioned (on this site) that the efficiency (as in the ratio of BTUs into the room versus the total BTUs put into the stove as wood) does *not* change significantly between burning low and high (for his brand stoves) - and that this was confirmed by independent testing.

The caveat is that this is for one (cat) brand only.

Nevertheless, this suggests the case is not that cut and dry.

 

[Eman85]

Just my findings burning my stove I can get my home warmer burning with more air and no catalyst but for shorter periods of time so obviously using more wood. With the air down and the cat on it's low and slow but definitely low on heat output. With the cat engaged I find it hard to get more heat output without overfiring the cat until hours into the burn. Then I can open the air and get the heat into the room.

 

[Garbanzo62]

Just my experience to draw on here. But when I close down the damper most of the way, I wake up in the AM and my stove fan is still blowing as the stove has enough heat in it. The few times I only closed it down part way, the stove had burned through the wood faster and the stove went cold by the morning. Now the room may have been warmer for those few hours, but I will take slow and steady heat over short high heat. The only time I really ever burn close to wide open is on cold starts where I want to establish a new coal bed.

 

[mellow]

If you have not insulated your fireplace and installed a blockoff then you will be losing BTU's no matter what, especially with an exterior fireplace.

I am guessing you are wondering where all that heat is going, take a look at the picture in this post:

Finally got around to insulating my fireplace

Mellow - How did you get the Roxul to stay in place on the top block plate? I have my 4" pipe coming down through what is left of my old damper with Roxul stuffed into the remaining cavity - nothing on the side walls.

www.hearth.com

 

[Toasty-Yote]

Thanks all! Very informative.

 

[joop]

from my seat of the pants take on this , it feels like more heat the more open the air.but does burn alot faster

 

[Garbanzo62]

The wood in the box has a total BTU content. The question is how much of that content can be transferred to the room. Burning with an open damper will burn hotter for a shorter period of time., but a lot of that heat is going to go up the flue. My suspicions are that more BTUs would be lost burning hotter as the draft would be greater and pulling more of that heat up the chimney. The big question is what is the percentage of difference of lost btu's between short hot burns and longer lower burns. Yeah there might be a difference, but it might be negligible.

 

[Highbeam]

I actually did take some science classes in school so it seems like I should know this but I don't.

Do you lose some of the BTUs from a given amount of wood if you run your insert wide open? My preliminary thoughts are that more air equals more updraft and more heat leaving through the chimney. On the contrary, I would think that running it colder might make a dirtier fire with more unburnt fuel leaving in the way of smoke? Those are just my uninformed thoughts, I would appreciate if someone could tell me what is really happening.

Since you’re asking about delivered btu per “amount” of wood what you’re really asking for is efficiency. Maximum efficiency for all woodstoves is at the lowest clean burning output level. For the reason you probably think, wasted heat up the stack at higher burn rates.

 

[stoveliker]

That is not what BKVP claimed (based on independent tests) on this site: efficiency is the same for burning low and high in BKs.

 

[Itslay90]

This is what my stove says

 

[stoveliker]

So it's different for different stoves, as expected.
Blanket statement one way or the other cannot be made.

 

[Highbeam]

That is not what BKVP claimed (based on independent tests) on this site: efficiency is the same for burning low and high in BKs.

I think you need to look closer to get this figured out. The newest manuals actually disclose emissions at different burn rates. It’s part of the new testing. I know that’s not the same as efficiency but you’ll see that Bk especially clean burning at low burn rates. Definitely not “the same” across all. I betcha bkvp could produce efficiency numbers at each burn rate too.

 

[stoveliker]

That is what was asked at that point, and to which he answered that the efficiency does not (appreciably) change between burn rates.

Emissions are measured in weight/hr, though, making it hard to compare emissions (in weight) per load when the burn rate is different.
Efficiency is the BTU load into the room as a fraction of BTU fuel load into the stove.

 

[bholler]

So it's different for different stoves, as expected.
Blanket statement way or the other cannot be made.

And a bk in reality can never actually be run wide open because the thermostat won't allow it. With a non thermostatic stove you will absolutely loose efficiency running wide open

 

[stoveliker]

That may be true.
Wide open on a BK is likely not as wide as non-Tstat stoves.
All I was saying is that the question received a blanket answer, and I brought up an example where that blanked answer does not apply (per independent data mentioned by the VP of the company...)

 

[bholler]

That may be true.
Wide open on a BK is likely not as wide as non-Tstat stoves.
All I was saying is that the question received a blanket answer, and I brought up an example where that blanked answer does not apply (per independent data mentioned by the VP of the company...)

Oh you are absolutely correct about the blanket answer being a problem

 

[Highbeam]

That may be true.
Wide open on a BK is likely not as wide as non-Tstat stoves.
All I was saying is that the question received a blanket answer, and I brought up an example where that blanked answer does not apply (per independent data mentioned by the VP of the company...)

Unless you get numbers it was a subjective claim. Hardly something you can base a decision on.

 

[stoveliker]

True.
But that depends on who you trust. I am not convinced BKVP is here telling things that are not true.

 

[BillBurns]

My take on this is it seems if its wide open, youre losing heat and burning more wood. I keep mine at a 45 degree angle for most of the fire, I figure its the happy medium.

 

[Highbeam]

True.
But that depends on who you trust. I am not convinced BKVP is here telling things that are not true.

Me either but you’re also splitting hairs in a bk type design that prevents super hot overfire waste. Thing is, bkvp never gave you numbers so you don’t really have anything conclusive from that data. I do agree with his opinion that efficiency should always pretty good on the bk. It’s definitely a wood sipper down low.

Everybody else with other stoves, including me, probably has witnessed the inefficiencies of very high burn rates.

 

[stoveliker]

Fair enough. I point again to my remark above about blanket answers and counter examples.