Does a Fan Produce More Heat or Just Blow the Air Around?

 
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windyhill4.2
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Post by windyhill4.2 » Sat. Jan. 11, 2014 5:45 pm

Why have windows gone to muti-pane ?? heat/cold transfer, or the radiator on a car?? get rid of the fan on your car radiator,see what happens,fan blowing air on surface = greater heat transfer,plain & simple,transferring engine heat to the atmosphere is more effective WITH the fan running,turn fan off,let radiant heat do it's job,in the summer time the car engine will overheat,just my .0000000000000000001


 
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Post by tony17112acst » Sat. Jan. 11, 2014 5:47 pm

waldo lemieux wrote:I seem to remember something from physics class. That " the greater the differential in temp of water or air on each side of a surface the greater the rate of heat transfer across that surface.I believe that it also flows faster from high density to low. :
It just doesn't seem intuitive to me that heat transfer depends on ambient temperature. So you're saying if I have a constant BTU output say at 20,000 BTU, I'll get MORE BTU's if it's colder in the room? I don't see that, but I could be wrong.
-Tony

 
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Post by Lightning » Sat. Jan. 11, 2014 5:50 pm

This...
waldo lemieux wrote:I seem to remember something from physics class. That " the greater the differential in temp of water or air on each side of a surface the greater the rate of heat transfer across that surface.
and to prove it, this....
lsayre wrote:You would have to watch the stack temperature fall
8-)

 
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Post by Rigar » Sat. Jan. 11, 2014 5:50 pm

... I got to chime in

. if the fan was capable of actually decreasing the temperature of the side of the stove it's blowing on... then technically you are decreasing the rate of heat transfer of that metal..
.. because air is a poor conductor of heat... the fan helps distribute the heat better

 
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Post by lsayre » Sat. Jan. 11, 2014 5:51 pm

waldo lemieux wrote:I seem to remember something from physics class. That " the greater the differential in temp of water or air on each side of a surface the greater the rate of heat transfer across that surface.I believe that it also flows faster from high density to low. :gee: Its all kinda cloudy back there :oops:
Air has mass and moving air will assuredly take heat away from the surface of the stove, but if you don't subsequently burn more coal, all you will end up with in a hurry is a colder stove surface. Once the equilibrium of the wind blown system is restored, unless you are burning more coal or seeing a measurably lower flue pipe (stack) temp, the energy per unit of time evolving form the air blown stove surface (that is being cooled by the blowing air) is ultimately the same as the energy evolution without any air blowing across it.

 
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Post by jpete » Sat. Jan. 11, 2014 5:51 pm

Other than the heat produced through the use of electricity, a fan doesn't "MAKE" heat.

It moves the heat produced by the stove around more efficiently.

Heat moves from an area of high concentration to lower concentration and the greater the difference, the faster the rate of transfer.

So imagine your stove with no fan. It heats up the actual stove, then the area immediately around it, then the area around that and so on as it radiates out into the room. At some point, it heats up the outside walls which very quickly transfer the heat out of your house.

Maybe it makes it to the other side of your house, maybe it doesn't.

Think of the fan as a "conveyor belt" for heat, moving it out faster and further than it would otherwise naturally move. This distributes the heat to where you want it in the shortest time necessary.

Any hot air heating system is all about air management. What you'd want ideally is a high volume, low velocity fan because the faster you move the air, the faster it gives up its heat.

Do you blow on your soup vigorously when you want to cool it down or do you barely whisper at it?

HVAC systems use low speed for heat but high speed for cooling for this reason.

The ultimate answer is turn your fan off for a week and see if your house is still comfortable.

 
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Post by tony17112acst » Sat. Jan. 11, 2014 5:54 pm

windyhill4.2 wrote:Why have windows gone to muti-pane ?? heat/cold transfer, or the radiator on a car?? get rid of the fan on your car radiator,see what happens,fan blowing air on surface = greater heat transfer,plain & simple,transferring engine heat to the atmosphere is more effective WITH the fan running,turn fan off,let radiant heat do it's job,in the summer time the car engine will overheat,just my .0000000000000000001
I ague that the fan simply sends the heat off and away from the object, not that you get more of it. If you had no fan blowing air for heat in your car, yes, it would be cold for a while, but it would eventually get hot ...it would just take longer ...the heat was simply NOT MOVED from it's spot; it's hotter than hell where the heater coil would be.
Last edited by tony17112acst on Sat. Jan. 11, 2014 7:39 pm, edited 1 time in total.


 
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Post by Lightning » Sat. Jan. 11, 2014 5:55 pm

Ultimately, If blowing air across the stove results in lower flue gas temperature then where is that heat going?? Its going in your house instead of out the chimney.. That's the bottom line... :lol:

 
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Post by Lightning » Sat. Jan. 11, 2014 5:57 pm

Lightning wrote:Ultimately, If blowing air across the stove results in lower flue gas temperature then where is that heat going?? Its going in your house instead of out the chimney.. That's the bottom line... :lol:
I can vouch for this since between blower cycles I see my flue pipe temperature rise and fall a couple degrees..

 
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Post by windyhill4.2 » Sat. Jan. 11, 2014 5:59 pm

I wasn't referring to heater core,i was referring to the engine radiator removing heat from the engine,by itself low heat removal,with fan running,high heat removal,simple to me ,but then I qualify as a simple minded person.

 
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Post by Rigar » Sat. Jan. 11, 2014 6:00 pm

Lightning wrote:Ultimately, If blowing air across the stove results in lower flue gas temperature then where is that heat going?? Its going in your house instead of out the chimney.. That's the bottom line... :lol:
... if it were even possible to lower the temperature of the outside of the stove with a fan... technically the flu temps would increase... not decrease
... this is probably not even something we could measure without sophisticated equipment... but if you decrease the heat transfer rate on the outside... then technically the flue gas temp will increase

 
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Post by Lightning » Sat. Jan. 11, 2014 6:04 pm

Rigar wrote:... if it were even possible to lower the temperature of the outside of the stove with a fan... technically the flu temps would increase... not decrease
The fan would definitely lower the stove temp some.. And why would the flue gas temp increase?? This doesn't make sense to me at all Bro... :D

 
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Post by franco b » Sat. Jan. 11, 2014 6:16 pm

Blowing air across the heat exchange surface increases the efficiency of that surface and flue gas temp. will go down just as lightning has observed.

 
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Post by oliver power » Sat. Jan. 11, 2014 6:23 pm

oliver power wrote:
waldo lemieux wrote:OH,OHHH>>>> I got this one. It Depends :rofl:
I agree Waldo. If the stove has fixed air control, pretty much you're just moving heat around. I'm sure taking heat off the surface would allow a faster rate of transfer. Not sure how much faster, if any. However, if the stove has a bi-metallic primary air damper, blowing heat off the stove would cool the stove mass. This would cause the flapper to open, and allow more air, which would create more heat.
OK, I finished off my sentence by adding "if any". I was going to say NO to getting more heat when fan blows across stove. Then, I too remember something about density, and heat transfer. Yes, I do believe you'll get more heat transfer if blowing fan across metal. But, not enough to make a difference that amounts to anything. I then went on to say if the stove has a bimetallic damper, it would open. Then you WILL get more heat off the stove.

 
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Post by lsayre » Sat. Jan. 11, 2014 6:27 pm

oliver power wrote:I then went on to say if the stove has a bimetallic damper, it would open. Then you WILL get more heat off the stove.
Agreed, through burning more coal.


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