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

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:01 pm

I probably should ask an actual physicist this, but here it goes:

Does blowing air onto a hot object (like a coal stove) give you MORE HEAT output from the object, or does the fan just spread that heat around the room?

I have been using a fan, but I hate to use more electricity if it doesn't produce MORE heat; I really don't need to spread it around the room in my specific situation.

I am leaning towards that the heat output is the same whether there's a fan blowing on the metal or not. I'd think how much heat that transfers through metal is a function of the properties of that metal, if everything else is the same. I'm thinking that the fan cannot make more heat transfer through the metal frame of the stove or exhaust pipes. But I'm hoping it would!

Thanks!
-Tony

KLook · Post by **KLook** » Sat. Jan. 11, 2014 5:05 pm

I think it moves it around and maybe makes the room more comfortable. And I think it removes more heat from the box. The cooler the outer surface of the box, the more heat is transferred.

Kevin

waldo lemieux · Post by **waldo lemieux** » Sat. Jan. 11, 2014 5:06 pm

OH,OHHH>>>> I got this one. It Depends

KLook · Post by **KLook** » Sat. Jan. 11, 2014 5:14 pm

OH,OHHH>>>> I got this one. It Depends

Wow, everyone is channeling sting! LOL

KLook · Post by **KLook** » Sat. Jan. 11, 2014 5:16 pm

Lets put it this way. I have a metal fireplace "box". Yes POS. But people that have them with fans in them like them. Mine does not have a fan. I hate it. My propane fireplace in Maine worked without a fan....turn on the fan it did have and it was great.
YMMV I guess. I think fans work great.

Kevin

oliver power · Post by **oliver power** » Sat. Jan. 11, 2014 5:21 pm

waldo lemieux wrote:OH,OHHH>>>> I got this one. It Depends

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.

Post by **Lightning** » Sat. Jan. 11, 2014 5:24 pm

KLook wrote:The cooler the outer surface of the box, the more heat is transferred.

I agree with this logic.. Conducting heat from the metal by passing cool air across it should also mean you are conducting more heat from the contents burning inside the stove.. It also makes for better distribution of that heat. BUT, by cooling the metal, the radiant heat may sag a little. Overall I think the fan is good

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:31 pm

But I do not agree with this logic...

"The cooler the outer surface of the box, the more heat is transferred" ...It could just mean that the air was distributed from that spot and sent elsewhere ...not that more heat was produced.

-Tony

P.S. Again, we all know a fan distributes heat in a room better, but that's not my question. It's whether more heat is extracted form the object.

lsayre · Post by **lsayre** » Sat. Jan. 11, 2014 5:34 pm

The heat comes from the coal. You would have to watch the stack temperature fall (and/or watch the coal disappearing faster than normal) in order to conclude that more heat is going into the home while air is blowing across the stove.

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:37 pm

When you turn on your heater in the car as it is warming up for the first time, do you ever wonder: If I put the fan on "high," am I getting more heat, or is it just blowing the existing heat out faster, making it feel cooler, because the heat extracted is now in a larger volume of area. So if you leave the fan on "low" it feels warmer/hotter, because the available heat is concentrated in the smaller volume/area. The available heat didn't seem to change, just that the heat was spread out into a larger/smaller area ...making the small spot you were sampling with your hand/fingers "feel" like there was a difference in temperature.

McGiever · Post by **McGiever** » Sat. Jan. 11, 2014 5:38 pm

Simply put, an air stream washes heat off much the same as water will.

Signed,
A. COAL PHYSICIST, PHD

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:39 pm

McGiever wrote:Simply put, an air stream washes heat off much the same as water will.

...but that doesn't answer the question. That's the SAME question.

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:39 pm

lsayre wrote:The heat comes form the coal. You would have to watch the stack temperature fall (and/or watch the coal disappearing faster than normal) in order to conclude that more heat is going into the home while air is blowing across the stove.

...I think that would be a good test to answer the question.

tony17112acst · Post by **tony17112acst** » Sat. Jan. 11, 2014 5:42 pm

oliver power wrote: I'm sure taking heat off the surface would allow a faster rate of transfer. Not sure how much faster.

...I don't agree (yet). The rate of transfer of heat doesn't seem like it would go faster just because the surface was cooler. I could be wrong, but I don't think heat acts that way ...it's radiated through a medium as a certain rate, depending on the properties of that medium is how fast it transfers.

If what you say is true, then we'd get more heat if we put the stove in -200 degrees in space or something, and I'm not convinced that you'd get more heat just because the surface was cooler.
-Tony

waldo lemieux · Post by **waldo lemieux** » Sat. Jan. 11, 2014 5:44 pm

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.

Its all kinda cloudy back there