Forced convection for a wood burner
Forced convection for a wood burner
(OP)
Its my first post...
Its been over 20 years since I completed my Mechanical engineering degree..and I've forgotten many of the heat transfer calcs.
I am pondering a challenge. I have a 45ft lounge and a woodburner at one end in an alcove. It heats up part of the room, but the other end is cold. I bought a 15" fan and pointed this at the stove and it heated the room up considerably. The problem is that it is incredibly noisey.
I would be grateful if you could help me:
1. How do I work out the difference in stove efficiency, i.e. heat transfer into the room rather than combustion, for forced vs natural convection. I've googled but it looks more complicated than it probably needs to be and I don't have some of the source information for the parameters. Can anyone help or point me in the direction of existing analysis.
2. Advise on whether I would be better off with a film of air over the stove top through a long slot or just blast air everywhere via a fan (i.e. trying to make efficient use of the airflow).
3. I've looked at the eco fan type arrangement, but air volumes are very low. How much heat can I extract from the stove before I affect the combustion efficiency or condensation of flue gases = chimney fires, and how would I calculate the required airflow?
A huge thanks in advance
Its been over 20 years since I completed my Mechanical engineering degree..and I've forgotten many of the heat transfer calcs.
I am pondering a challenge. I have a 45ft lounge and a woodburner at one end in an alcove. It heats up part of the room, but the other end is cold. I bought a 15" fan and pointed this at the stove and it heated the room up considerably. The problem is that it is incredibly noisey.
I would be grateful if you could help me:
1. How do I work out the difference in stove efficiency, i.e. heat transfer into the room rather than combustion, for forced vs natural convection. I've googled but it looks more complicated than it probably needs to be and I don't have some of the source information for the parameters. Can anyone help or point me in the direction of existing analysis.
2. Advise on whether I would be better off with a film of air over the stove top through a long slot or just blast air everywhere via a fan (i.e. trying to make efficient use of the airflow).
3. I've looked at the eco fan type arrangement, but air volumes are very low. How much heat can I extract from the stove before I affect the combustion efficiency or condensation of flue gases = chimney fires, and how would I calculate the required airflow?
A huge thanks in advance





RE: Forced convection for a wood burner
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RE: Forced convection for a wood burner
I assume that it is something to do with the temperature differential between the stove and local ambient: natural convection has a smaller temperature differential (the air immediately around the stove is warm) than forcing cooler air onto the stove, therefore the heat extracted with forced convection is greater.
If I can explain it and refine it, I see an opportunity to extract more efficiency from a wood burning stove. But I would like to understand the science, and understand whether its a volume of air issue, or whether its just as efficient to push a film of air over the surface.
Any help gratefully received.
RE: Forced convection for a wood burner
However, the short answer lies in the fact that many HT texts include "Mass Transfer" in their titles. That's because the movement of the air molecules, laden with heat energy is what convection is all about.
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RE: Forced convection for a wood burner
rmw
RE: Forced convection for a wood burner
Note also that as the room gets warmer, the draft up the chimney will increase, which will increase the rate of combustion. The room may be warmer with the fan running, but you will also have to feed more wood into the stove...hence, BTU for BTU, "efficiency" likely hasn't changed much.
You probably don't need as large and loud a fan as you are using. I would start with the outside combustion air RMW suggests because drawing make-up combustion air into the room via cracks in windows, etc. is a sure way to cool off a room (modern woodstoves feature an outside air connection). For example, some open fireplaces draw so much make up air from the room that they actually have a net cooling effect on rooms that are also heated by central heat (I.e. the heating load on the central heat system goes up when the fireplace is used, as the heat required to warm the make-up air is greater then the heat provided by the fireplace).
Another thing to consider is an air duct running from one end of the room to the other. An small fan installed within will probably help keep the air well-mixed much better than a big fan randomly blowing air all over the place. The fact that the woodstove is in an alcove doesn't help much either. A longer stovepipe (which would allow placement out of the alcove) might also do the trick.
Good luck,
Dave
RE: Forced convection for a wood burner
I'm still pondering how much additional energy can be extracted from a stove through forced convection rather than natural convection. I assume I have to extract a huge amount of heat before I start to affect condensates. I've been reading up on heat sink design, which is a similar challenge. Forced convection makes a massive difference to heat extraction because of the increased molecular transfer of energy, but I can't find any like for like comparisons with natural convection. I also concluded that the greatest effect is by disturbing the boundary layer, where the temperature gradient is the greatest. Turbulent flow appears to improve performance. I assume a curtain of air blowing across the surface is more efficient than just blowing air at the face of the stove. But how do I work out the volumetric flow rate?
RE: Forced convection for a wood burner
I suggest you find a quieter fan. I have a 1940's Emerson 15" fan next to my wood stove. They really just do not make fans of such high quality anymore.
RE: Forced convection for a wood burner
> Right now, your convection heat output is basically localized near the stove, and along the surface of the ceiling, neither of which helps you to feel warm. Counting on diffusion or convection is slow. A fan will mix the air more thoroughly, thereby significantly speeding up the warming of the nether regions of the room.
> Thermal transfer is directly proportional to temperature difference. With natural convection, the outer surfaces of the stove at kept at a relatively higher temperature, so heat output is limited. By forcing cooler air to be nearer the stove, there should be some increase in heat output into the room.
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RE: Forced convection for a wood burner
Buy two of the 20inch square box fans and arrange them to cycle the air around the entire room to keep the temperature everywhere equal.
RE: Forced convection for a wood burner
RE: Forced convection for a wood burner
Unless you are located in deep South America or Southern Africa (or Oz) this seems a strange topic for the weather outside my home right now. Last time I looked earlier this afternoon, the outside temperature was equal to average normal body temperature; 98.6F.
rmw
RE: Forced convection for a wood burner
I would have to dig up the concrete and oak floor to change the went for the fire. Stoves are rated at 85% efficiency but I'm not convinced that all of this heat is transferred into the room. The challenge remains, how do I calculate the volumetric flow rate assuming I blow air over the surface of the stove? If there are no reliable calcs, I can knock something up and see what works.
I know that inset stoves have forced convection but don't understand why freestanding stoves don't. If it increases efficiency in our green age, why not fit them? The ecofan circulates heat around the room, but it doesn't really extract more heat from the stove - as far as I can make out.
RE: Forced convection for a wood burner
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RE: Forced convection for a wood burner
Modern freestanding stoves are available with circulation fans. They don't actually increase the combustion efficiency of the stove--they just increase the convection coefficient from the stove to the air and help circulate the air around the room a little...reducing the temperature variation from one side of the room to the other. You might not see them in use very often as people are apt to trip over the cord! An internet search for "wood stove blower" will return hundred of hits.
Good luck,
Dave
RE: Forced convection for a wood burner
RE: Forced convection for a wood burner
Tobalcane
"If you avoid failure, you also avoid success."
RE: Forced convection for a wood burner
rmw
RE: Forced convection for a wood burner
Having said that, this doesn't seem to be a work-related project, so I'm not sure if it meets the posting policies for the forum.
Patricia Lougheed
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