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

Sunny Boy wrote:

kstills wrote:
What is the difference in btuh output between the starting temp of the stove and when it drops 25 Deg?

I get about 324 btuh less, while the stack is putting out about 20btuh less to the atmosphere.

The stove ? I have no idea. I didn't check the entire stove's surface area, or it's internal temps. However, the surface temp of the two plates directly in front of that small fan dropped, as I said, about 25 degrees.

Within fifteen minutes, the room was measurable warmer with just a small fan blowing across just that part of the top of the stove.

And, within a short time, the test points returned to the same non-fan temps when I shut the fan off to cook my lunch.

Now, . . . where's your test results ?

Paul

Well, it was a trick question.

You replaced radiant energy with conductive energy, and managed to convect it around the room in a different pattern than you had before.

The gain appears to be dropping the stack temp 3 degrees, which nets about 20btuh.

How can you put a defined number on gained heat from a drop in stack temp?

Did you account for the actual drop in flue gas temperature and count the volume that is leaving the chimney?

Lightning wrote:How can you put a defined number on gained heat from a drop in stack temp?

I'm calculating the radiation that is lost from the stack. I'm sure air velocity plays a part in that also, however unless the combustion rate or the air going into the stove has changed that should be constant, no?

As for my tests, last year I thought I would be energy efficient, so I went out and bought a programmable thermostat for my hw boiler.

I would set it for 60 while I was out of the house, and 68 for when I was heading home. Weekends were turned down, and I would cut back on the heat at night.

That's when I learned a hard lesson about equilibrium, and the amount of time it takes to reach it......

Now, I set the tstat to 71, with a 0.5Deg delta.

With 6 in the house now (2 last year), dhw demand out the wazoo and Nov, Dec and Jan being much colder than last year I'm on pace to use just a little bit more coal (relatively speaking) than I did last year.

All without a single fan blowing on any of the radiators.......

There is more involved than a small drop in radiant heat off the pipe. You gotta count the actual drop in temperature of the flue gases times the volume rate they are leaving the chimney. A few degree drop on the pipe is only evidence that flue gas temperature has fallen.

And labeling the drop in temperature on the flue pipe with a specific quantity of gained Btus isn't gonna work.

kstills wrote:

Lightning wrote:How can you put a defined number on gained heat from a drop in stack temp?

I'm calculating the radiation that is lost from the stack. I'm sure air velocity plays a part in that also, however unless the combustion rate or the air going into the stove has changed that should be constant, no?

So, without running any tests yourself, your still basing your theories on guessing part of it. By any chance have you ever worked on climate science ?

I'll stick with the simpler approach to the results I got - the one that skin temp only cares about. The fan went on, the room got warmer, my skin got warmer, the stove top got cooler. Fan off , room and skin got colder. Stove kept running either way.

Paul

Sunny Boy wrote:

kstills wrote:
I'm calculating the radiation that is lost from the stack. I'm sure air velocity plays a part in that also, however unless the combustion rate or the air going into the stove has changed that should be constant, no?

So, without running any tests yourself, your still basing your theories on guessing part of it. By any chance have you ever worked on climate science ?

I'll stick with the simpler approach to the results I got - the one that skin temp only cares about. The fan went on, the room got warmer, my skin got warmer, the stove top got cooler. Fan off , room and skin got colder. Stove kept running either way.

Paul

I wonder what happened to the stored energy in the stove?

Lightning wrote:There is more involved than a small drop in radiant heat off the pipe. You gotta count the actual drop in temperature of the flue gases times the volume rate they are leaving the chimney. A few degree drop on the pipe is only evidence that flue gas temperature has fallen.

And labeling the drop in temperature on the flue pipe with a specific quantity of gained Btus isn't gonna work.

Probably.

We don't have those numbers, however, all we have is a reading off the stack. And everything else being equal, that indicates a fairly low amount of heat.

kstills wrote:We don't have those numbers, however, all we have is a reading off the stack. And everything else being equal, that indicates a fairly low amount of heat.

I agree it could be a low amount of heat and good luck strapping a number to it ... Point is, there is a difference, and those Btus are staying in the house ultimately because of the added heat transfer the fan is providing.

By removing heat from the outer surface, you have increase the Delta T across the surface, which induces more rapid transfer of heat from the inner surface to the outer surface. This way you get more heat transfer. However, if you remove heat from all the surfaces by convection, then the lower temperature of the outer surface means LESS heat is conducted by radiance.
As someone noted, the only way to tell if you are getting more heat into the room is, with the stove at steady state conditions, measure the stack temperature before and after turning on the fans. Change nothing! If the stack temperature goes down, then energy into the room has gone up. Should be a simple enough experiment to conduct.

I did something similar on my Quadafire pellet stoves. I installed springs and turbulators in the heat exchanger tubes and saw the hot air temps rise as the stack temp decreased.

I AM a physicist. I stated at a Holiday Inn too.

I'm the OP. *bows*

My assumption in my question is that the same amount of energy is used inside the stove with and without the fan.

Please consider the original question with say, an electric resistance coil heater set up inside of the stove. This heater can only put out a certain number of BTU's ...say 50,000 BTU's ...and no more!

So I ask you recent posters: Will more heat transfer to the room by pointing a fan on the stove that contains this 50,000 BTU electric heater? You cannot claim that more heat will come out of the electric heater inside the stove (because it's impossible). The clear answer is "yes." It is "yes" because a fan will cause a lower temp on the outside of the stove and more heat will transfer to the room and not out the pipe/chimney because of the principle that exists that heat transfers through a medium more quickly as the temperature difference is greater on either side of it.

tjnamtiw wrote: If the stack temperature goes down, then energy into the room has gone up. Should be a simple enough experiment to conduct.

But we don't need an experiment because everyone here agrees that pointing a fan on a car's radiator transfers heat more quickly from the medium with heat inside (radiator coil with hot liquid). Are all of the car manufacturers wrong then? I think it's possible, but very improbable.

W-O-W

Are radiators placed on cars to increase engine efficiency, or to increase engine longevity? The analogy seems to be a grand stretch with regard to application regarding coal buring appliances.

Just remember that the various fundamental laws of thermodynamics can not and will not be violated. That is why they are labeled as fundamental laws. And remember that all of the heat energy available to the system is within the coal (unless the fan itself is generating a good load of measurable heat, but even so it is at great electrical expense). The laws of thermodynamics state in the most rudimentary terms that no matter what your whims, wishes, or desires or group theropy sessions (like this forum) may be or decide, you simply can't have your cake and eat it too.

Science by consensus is a dangerous practice. The analogy someone made earlier to global warming science is spot on.

Now riddle me this: Why is it that across the board, no matter what fuel is involved, the efficiencies of boilers seem to lag a bit with regard to the efficiencies achieved via furnaces or mere stoves. If radiators increase efficiency, wouldn't boilers have the highest efficiencies of the bunch?

I would say no because much of the time a boiler is idling or it is running with water at near maximum temp on the other side of the skin. Lower the water temp and get greater transfer, increase efficiency. A hot air furnace is heating a thin skin which puts it in closer proximity to "cold" return air coming along and scrubbing heat off the surface. I think it has already been mentioned that the sheet metal barrels of some of the antique baseburners throw much more heat. Makes sense, you are closer to the flame with sheet metal and there is less slowing it down on its way to the room.

Kevin