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View Full Version : Break Out Your Slide Rule ------ A Brain Teaser To Which I Have No Answer



SurferJoe46
05-05-2012, 04:44 PM
Just got back from putting the woodstove on 'full-damp', almost totally closing the air supply to it to keep it glowing hot with embers all night until morning --------- and I got to thinking.

Does extracting heat from the secondary jacket around the firebox cause the wood to be consumed faster that just letting the heat naturally flow from the stove walls into the room?

This particular stove has an outer jacket that has a blower forcing air from the room, through it to heat it and then discharges it out through louvers into the house.

If I don't turn the heat exchange blower ON - will the wood last longer?

https://lh3.googleusercontent.com/-wueL7YwImPY/T6Sf3vjMwBI/AAAAAAAAKEs/dygoKo4ANQs/s640/101_0049.JPG

Terry Porritt
05-05-2012, 05:04 PM
You have to ask yourself.."does the burning wood know what is happening outside of the combustion chamber ? Does the wood feel and have a brain ?" :)

SurferJoe46
05-05-2012, 05:10 PM
I'm thinking that sucking heat (energy) out of it would speed up the consumption. I would think a fire would have to maintain a certain level of heat to stay lit - right?

Maybe I need medication - but if you are more actively pulling heat from a firebox, it has to create some sort of shift in the Time-Space Continuum. The inner walls being washed with relatively cooler air has to account for something.

decibel
05-05-2012, 05:38 PM
This sounds something like what caused new rules over here about wetbacks (not YOUR sort of wetback).
If you cool the combustion chamber too much, the burning effiency goes down.

http://www.mfe.govt.nz/laws/standards/woodburners/woodburner-faqs.html#wetback

pctek
05-05-2012, 06:00 PM
Well more air makes a fire burn faster, so less air would cause it to die down.

SurferJoe46
05-05-2012, 06:26 PM
This sounds something like what caused new rules over here about wetbacks (not YOUR sort of wetback).
If you cool the combustion chamber too much, the burning effiency goes down.

http://www.mfe.govt.nz/laws/standards/woodburners/woodburner-faqs.html#wetback

OK --- now we're cooking! I got a feeling that I've discovered the 11th Law of Thermodynamics here. I don't know what the others are numbered, but I hope I left enough space for them by numbering mine #11.

SurferJoe46
05-05-2012, 06:29 PM
Well more air makes a fire burn faster, so less air would cause it to die down.

Uh huh - I buy that 100%. I wuz just wondering if taking heat away from the jacket surrounding the firebox speeds it all up.

BTW: the way we damp the fire here is not with a flapper in the exhaust - but we limit the air intake to the combustion area. I know that cokes up the flue faster, but a handful of tri-sodium phosphate (TSP) tossed into the flames cleans it well.

Paul.Cov
05-05-2012, 08:36 PM
Yeah, a cooler fire will burn less efficiently.

The other factor is the draw of air through the fire and up the chimney.

This relies on a temperature gradient. The hotter (lighter) air wanting to rise up the chimney, and consequently drawing more cool air into the fire, past the damper.

IF the fire gets too cold the draw may suffer, and the fire could stall and effectively die from a lack of oxygen.
However, a strong draw also draws away all that precious heat which goes blasting out the top of the chimney.
The slower the trip up the chimney, the more heat can be released to the house... there's a delicate balance (who knows where) where the combustion is effcient, but thr draw is slow enough to give the heat to the house, but not so slow that the fire dies.

Taking heat from the jacket will allow the fire to pass more heat to the jacket, rather than going up the flue, but runs the risk of sapping the draw and killing the fire.

If the jacket is left hot a greater proportion of the heat will go up the chimney instead.

Colder jacket = colder flue gases as well. Cloder flue=less draw. Less draw=slower fire, but also less heat vented to outside.

Sod all help probably... just more factors to confuse the issue.:stare:

R2x1
05-05-2012, 09:06 PM
Get a shot of Vegemite in you and it will all become clear in an instant. (You won't need the fire either, the Vegemite glow will warm all your family.) ;)

Greg
05-05-2012, 09:21 PM
Crumbs.... it's been years and years since I had an enclosed wood burner. Wish i could remember, sorry!

martynz
05-05-2012, 10:05 PM
Unless there is some sort of thermostatic control on your air intake linked to the outer jacket, the rate at which your fire burns is totally dependent on your manual setting of the damper.

SurferJoe46
06-05-2012, 01:58 AM
Yeah, a cooler fire will burn less efficiently.

The other factor is the draw of air through the fire and up the chimney.

This relies on a temperature gradient. The hotter (lighter) air wanting to rise up the chimney, and consequently drawing more cool air into the fire, past the damper.

IF the fire gets too cold the draw may suffer, and the fire could stall and effectively die from a lack of oxygen.
However, a strong draw also draws away all that precious heat which goes blasting out the top of the chimney.
The slower the trip up the chimney, the more heat can be released to the house... there's a delicate balance (who knows where) where the combustion is effcient, but thr draw is slow enough to give the heat to the house, but not so slow that the fire dies.

Taking heat from the jacket will allow the fire to pass more heat to the jacket, rather than going up the flue, but runs the risk of sapping the draw and killing the fire.

If the jacket is left hot a greater proportion of the heat will go up the chimney instead.

Colder jacket = colder flue gases as well. Cloder flue=less draw. Less draw=slower fire, but also less heat vented to outside.

Sod all help probably... just more factors to confuse the issue.:stare:

No - I followed all the logic - it makes sense to me. Maybe we can present each other a doctorate in Flueology.


Get a shot of Vegemite in you and it will all become clear in an instant. (You won't need the fire either, the Vegemite glow will warm all your family.) ;)

Eat it? Gack! I've been using it as a rapid fire starting paste! \0/


Crumbs.... it's been years and years since I had an enclosed wood burner. Wish i could remember, sorry!

That's OK, old feller! Hopefully when I get to your age, I'll not have these types of moments too --- at least that I can remember.


Unless there is some sort of thermostatic control on your air intake linked to the outer jacket, the rate at which your fire burns is totally dependent on your manual setting of the damper.

There is a defective sensing thermostat on the air circulation blower to keep it from grabbing outside (colder) air, and just work with the air in the house once the fire dies.

That way (if it worked as designed) it would switch to 100% recirc of the internal air and not bring cold air in from outside. This is needed to keep the house from combating with the draw and not allow CO into the house if the flow reverses.

When a house is sealed tightly like our is, it needs air flow from somewhere, and if it could not get it from outside then it might be dangerous. This is both for the fire air supply and the heat exchanger.