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At what rate does heated air rise?

At what rate does heated air rise? - Physics Forum

At what rate does heated air rise? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 07-20-2005, 12:33 PM
SB
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Default At what rate does heated air rise?



Hello all,
I have searched the web for information on this and found very little ...I
saw this news group and thought there may be someone here that could help.
I am looking for information on heated air.
I am looking to re-circulate my woodstove heat which rises from the 1st
floor to the 2nd floor - back to the 1st floor. I am wondering if the energy
used to run the blower (110v squirrel cage blower)would be more than the
energy benefited by the recycled heat.
My concern is that the blower would have to be sized (very large)to force
the rising hot air...back down .(.if in fact it could be)...approx 18
feet..how is the rate of rising heated air figured?
I welcome all thoughts and comments

Steve


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  #2  
Old 07-20-2005, 01:19 PM
N:dlzc D:aol T:com \(dlzc\)
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Default At what rate does heated air rise?

Dear SB:

"SB" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

Just to make sure, you are not trying to recycle flue gasses,
correct?

I can give you a handful of search terms that will get you close.
Also, cross posting to sci.engr.mech.

"natural convection"
"Grashof number"
"Rayleigh number"
"Nusselt number"

The first term is the name of the process that produces what you
are trying to recycle. The other three are dimensionless
groupings of dimensioned quantities important to natural
convection. There is in general NO closed form solution... only
the ability to scale an experimental model to real life via
"empirical relations".

Perhaps others can be more helpful. I know there are products
available that will blow air in a duct constructed *around* the
flue, to capture the extra heat, and blow it into a room.
Capturing hot air from a less well defined area will be more
difficult. And I'd start with 1000 scfm, and go from there.

David A. Smith


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  #3  
Old 07-21-2005, 12:31 AM
jim
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Default At what rate does heated air rise?



"N:dlzc D:aol T:com (dlzc)" wrote:

No, his upstairs gets warmer than his downstairs due to heat rising. He
wants to know if it would be efficient to blow the warm air back
downstairs or if it would be more efficient to just use the electrical
energy to heat the downstairs. This doesn't require complex calculations
a blower will move a lot more heat energy than the energy it consumes.

-jim

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  #4  
Old 07-21-2005, 01:42 AM
Lance
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Default At what rate does heated air rise?



The rate is barely measureable. Try googling "stack draft calculation"
(without quotes). 'Draft' is the pressure differnce created by a column
of hot air surrounded by cooler air. It's how your fireplace works, warm
air rises up the chimney and sucks in fresh air into the firebox. The
greater the temperature difference, the greater the draft.

In the case of your fireplace the temperature difference between fresh
air and combustion gases is 100's of degree's, for your house the
temperature difference from upstairs to downstairs is maybe only 10's of
degrees or much less. The draft your little fan has to push against is
very very small. Most, if not practically all, the "push" the fan has to
create to move air around is used up by friction between the air and the
ductwork.

The one more thing, *all* the energy the fan uses will eventually go to
heating up the air. So in a sense there's no wasted energy at all.

So the real questions become "Is it cheaper to use a 50W fan to move
upstairs air to downstairs or use an additional 50W heater downstairs?
Does this achieve the comfort levels upstairs and downstairs that I desire?"

Lance
*****
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  #5  
Old 07-21-2005, 09:00 AM
k wallace
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Default At what rate does heated air rise?(new tangent question)

N:dlzc D:aol T:com (dlzc) wrote:


This reminds me of a question I've had for quite a while. I live in a
15-year-old (approx) house. This house has ceiling heat. Why was this in
vogue for a while then, why was it considered a 'good idea', is there
any good reason to use it? My electric bills are quite a bit higher
than my next-door neighbor with forced-hot-air.
I am not (quite yet) an engineer- another 9 months till my BS in ME
degree- but having some elementary and introductory education in heat
transfer, ceiling heat just doesn't seem like the best idea. Are there
structural reasons it makes sense? Why not 'floor heat', if there is
such a thing?

-k wallace
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  #6  
Old 07-21-2005, 10:07 AM
SB
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Default At what rate does heated air rise?

Thanks for your replys
Ok -..so what you are saying is the the greater the heat faster the
rise...and the more pressure i will need to push it down stairs...I have a
thought..how about installing a variable speed fan using a"smart
thermostat" ( IF temp =x than rpm =x)...humm I wonder if there is a
thermostat that would measure the differance between up stairs and
downstairs and adjust the speed of the fan motor...sounds like a good
idea..any thoughts out there?
Steve

"Lance" <[Only registered users see links. ]> wrote in message news:dbmuin$rcs$[Only registered users see links. ].edu...


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  #7  
Old 07-21-2005, 10:42 AM
PB
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Default At what rate does heated air rise?(new tangent question)

Thus spake <k wallace>:


<SNIP>


Hi k wallace!

There is such a thing. In Sweden it has been quite popular
for some years to use floor heating as the sole heat source.
One supplier of floor heating systems is Devi, [Only registered users see links. ].

/PB

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  #8  
Old 07-21-2005, 01:01 PM
N:dlzc D:aol T:com \(dlzc\)
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Default At what rate does heated air rise?

Dear SB:

"SB" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

I tried [Only registered users see links. ], they didn't have such a thing. It does
sound like a good idea. Of course you'd need to duct the fan to
the "far corners" of the house, so that it didn't simply float
right back up.

David A. Smith


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  #9  
Old 07-21-2005, 01:06 PM
N:dlzc D:aol T:com \(dlzc\)
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Default At what rate does heated air rise?(new tangent question)

Dear k wallace:

"k wallace" <[Only registered users see links. ].edNOSPAMu> wrote in message
news:[Only registered users see links. ]...
....

Our heat comes from the Sun, when it is up. It does absolutely
*require* good attic insulation. One good thing is the radiating
surfaces are always visible from anywhere in the room. Another
is that all places in the room should be close to the same
temperature. Unfortunately, radiant-only heating isn't terribly
efficient.

David A. Smith


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  #10  
Old 07-21-2005, 09:21 PM
tadchem
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Default At what rate does heated air rise?


"SB" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]...

A ceiling fan located in the stairwell should do the trick. It did for the
place I lived in Colorado.


Tom Davidson
Richmond, VA


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