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

Paul, nice explanation

Another thing to consider. By washing heat off a stove with air flow, it makes sense to me that radiant heat is being converted to conducted heat. Since the surface of the stove will be cooler there is less radiant heat. In exchange for radiant heat, there is a bigger volume of air that is warmer.

The additional heat in question by the original poster would be proven with lower stack temperature.

Sunny Boy wrote: By adding a fan you move a greater volume of cooler air over the heated surface area faster than natural convection can move it, thus being able to extract more heat in the same time period. Otherwise that extra extracted heat would have just gone up the chimney.

So far, I disagree. I don't think the fan is EXTRACTING more heat, it is simply dispersing it into a larger volume area and scattering it around the room faster.

Paul, with your statement above, you're force to agree that putting your identical heater setup at the north pole (where it's colder) would put out more heat because there's a "greater volume of cooler air over the heated surface area" ...right? That doesn't make sense to me. I don't think a 10,000 BTU electrical heater will get more heat by putting a fan on it. If it did, it wouldn't be a 10,000 BTU heater, it would be say 11,000. Then you put a fan on the fanned heater and then you got 12,000 BTU's until you have infinite heat, which we all know is false.

you still aren't counting the heat going up the chimney. That's where the gain is. Cooler stack temps equals more heat in the house. Take your 10000 BTU heater for example. 7500 is heating the house. 2500 BTU is going up the chimney. Now turn the blower on. We see a drop in flue gas temperature. Now 8000 BTU are heating the house and only 2000 BTU are lost up the chimney.

I'm running out of ways to tell the same story.

Here's how I see it right now (I could change my mind later): Metal has a certain property that transfers heat. Each metal allows heat to pass through it at a certain rate. Glass, for example, allows heat to pass through it much slower than metal, so every substance has a SET RATE of heat transference.

Does pointing a fan at a piece of metal that's being heated change that property of how much heat can transfer through that particular substance? I just don't see how a property of a metal can change (the heat transference property) on the fly, but I could be wrong.

Heat can only pass through that metal at a certain rate, there's nothing you can do to change that rate. You're not going to make heat pass through the metal faster or slower, the best you can do with a fan is what you do with the heat that has ALREADY passed through it.

On the other hand, the car radiator concept makes sense too. Blowing a fan over the coil DOES cool the liquid inside faster, which means more heat WAS extracted.

I may post this question in a physics forum and report back. And as last resort, I can call my wife's best friend's husband who is the chairman of the physics department at the local college. We'll all be eating crow then!

Come on Lee, you guys can complicate it all more betterer then you've done so far

Lee, there IS NO stack temp with an electric resistance coil heater at 10,000 BTU's in my example. It's a glorified toaster. My example shows that blowing air over the coil doesn't make more heat. That's basically what we're talking about.

tony17112acst wrote:Lee, there IS NO stack temp with an electric resistance coil heater at 10,000 BTU's in my example. It's a glorified toaster. My example shows that blowing air over the coil doesn't make more heat. That's basically what we're talking about.

Oh ok.. Then you are absolutely correct.. I thought we were discussing more heat from a coal stove. With the electric heater, radiant heat is converted to conducted heat with blowing air across it, there is no extra heat.

freetown fred wrote:Come on Lee, you guys can complicate it all more betterer then you've done so far

You see Fred Bro, its quite simple.. We can think of a BTU as being a marble, in that they are both tangible things. Matter and energy are the same (E=MC squared or Energy equals Mass times the speed of light times itself) As with BTUs, all the marbles must be accounted for.. None vanish, none magically appear.. Matter and Energy CANNOT be created or destroyed it can only be transferred from one form to another. So in the example of the electric heater with 10,000 marbles, scattering the marbles around won't produce any more marbles, or make any less marbles. In the more practical case, pertaining to using coal, we have some marbles going up the chimney.. With air washing across the stove, we keep more marbles in the house and less marbles going up the chimney.

So, now that I've lost my marbles, I'm gonna go looking for them..

freetown fred wrote:Come on Lee, you guys can complicate it all more betterer then you've done so far

Okay, put that 10,000 watt electric resistance heater inside Fred's stove and Fred just leaves the blower fan off, the MPD wide open and the ash pit door hanging open...Why is Fred cold? After all it's 10.000 watts.

freetown fred wrote:Come on Lee, you guys can complicate it all more betterer then you've done so far

Hey Fred....how's this for complicating it....
Seems to me that the electric baseboard example is a little too simplistic when compared to burning coal. The electric baseboard BTU output rate doesn't vary according to a draft or have its draft change according to it's temperature. It's either full on or off independent of its temperature or surroundings. So it doesn't matter that blowing a fan across it lowers its surface temp, the BTU's created by the 'fuel' are constant and never ending.

But a coal stove can have a variable BTU output rate from a lb of coal depending on the burn conditions that are affected by the stove temp, combustion air control settings (primary & secondary), draft (baro adjustment or MPD setting). You can get those BTU's out real quick and suffer short burn times since your fuel runs out or stretch them out longer.

The fan primarily affects the distribution of the BTU's created by getting them away from the immediate area of the stove more quickly. But secondarily, in doing that it affects the stove body temp which affects the burn rate of the fuel which affects the rate those BTU's come out of the fuel which can either be absorbed by the stove body via its size / design and transferred to the room or lost up the flue.

So my answer is 'It Depends' on what other adjustments are made to the burn conditions to account for the stove temps which would be cooler with a fan blowing across it affecting one of the variables that impacts the rate at which you get your BTU's out of that lb of coal.

A second level of 'It Depends' is the stove you are using since some automatically adjust combustion air, some have built in heat exchangers that promote natural air flow, some have larger heat exchanger areas.

And all this time I've wanted to warm up my house, by turning a fan on near my stove, I never knew it was getting warmer, "not" because more heated air was being blown into the room, but I was being pelted by marbles. Does this have anything to do with friction causing heat? Or that hot water freezes sooner than cold water. Or the molecules are further apart or together...or...something? (I'm so confused).

[quote="tony17112acst"]Here's how I see it right now (I could change my mind later): Metal has a certain property that transfers heat. Each metal allows heat to pass through it at a certain rate. Glass, for example, allows heat to pass through it much slower than metal, so every substance has a SET RATE of heat transference.

WRONG!!!!!!!!!!!

In the graphs above, the slope of the line represents the rate at which the temperature of each individual sample of water is changing. The temperature is changing because of the heat transfer from the hot to the cold water. The hot water is losing energy, so its slope is negative. The cold water is gaining energy, so its slope is positive. The rate at which temperature changes is proportional to the rate at which heat is transferred. The temperature of a sample changes more rapidly if heat is transferred at a high rate and less rapidly if heat is transferred at a low rate. When the two samples reach thermal equilibrium, there is no more heat transfer and the slope is zero. So we can think of the slopes as being a measure of the rate of heat transfer. Over the course of time, the rate of heat transfer is decreasing. Initially heat is being transferred at a high rate as reflected by the steeper slopes. And as time progresses, the slopes of the lines are becoming less steep and more gently sloped.

What variable contributes to this decrease in the heat transfer rate over the course of time? Answer: the difference in temperature between the two containers of water. Initially, when the rate of heat transfer is high, the hot water has a temperature of 70°C and the cold water has a temperature of 5°C. The two containers have a 65°C difference in temperature. As the hot water begins to cool and the cold water begins to warm, the difference in their temperatures decrease and the rate of heat transfer decreases. As thermal equilibrium is approached, their temperatures are approaching the same value. With the temperature difference approaching zero, the rate of heat transfer approaches zero. In conclusion, the rate of conductive heat transfer between two locations is affected by the temperature difference between the two locations.

joeq wrote:Does this have anything to do with friction causing heat?

Not in this case.. The shuttle re-entering the Earth's atmosphere then yes...

joeq wrote:Or that hot water freezes sooner than cold water.

That's a myth, Cold water freezes sooner than hot water..

joeq wrote:Or the molecules are further apart or together...or...something?

As molecules absorb heat, the molecules have more space between them as electrons move out further in their orbit of the nucleus of an atom.. With warmer air, the larger molecules occupy more volume which makes them lighter per volume unit.. So hot air is forced upward as cooler air comes in underneath it.. (Sunny Boy)

This thread =

So are you guys "bench racing" or what?

windyhill4.2 wrote:This thread =

Did you take my marbles??