Re: At what rate does heated air rise?

Dear SB:

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?

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

Reply to
N:dlzc D:aol T:com (dlzc)
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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

Reply to
jim

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

*****
Reply to
Lance

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

Reply to
k wallace

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

Reply to
SB

Thus spake :

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,

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/PB

Reply to
PB

Dear SB:

I tried

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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

Reply to
N:dlzc D:aol T:com (dlzc)

Dear k wallace:

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

Reply to
N:dlzc D:aol T:com (dlzc)

Sounds unnecessarily complex. How about just a 2 speed fan and 2 thermostats one upstairs and one down. Then a logic scheme where if the temperature upstairs goes above some temperature the fan gets power (otherwise its off). If the temperature downstairs goes below some point then the fan kicks in to high (otherwise low). Probably would work best when the 2 setpoints would be quite close to the same temperature. After all the average house is quite comfortable in the winter with a scheme where the one speed blower (and furnace heat) comes on when the temperature drops below the thermostat set point and is otherwise off. So one thermostat and one fan (that kicks on when temp upstairs goes above some point) would probably be adequate. The thing to remember is as you blow air downstairs to where you want it, there needs to be air returning to the upstairs to replace it. Ideally this air that is drawn upstairs will come from the coldest area of the downstairs.

-jim

Reply to
jim

There are two possible reasons.

Since you're in the Northwest with hydro power coming out the kazoo, electricity is (or was) relatively cheap. Many many years ago, electricity was cheap all over and radiant ceiling heat was popular with builders.

If your neighbor uses natural gas, maybe there wasn't any nearby gas mains when the house was built.

Also see here:

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Lance

*****

k wallace thought carefully and wrote on 7/21/2005 2:00 AM:

Reply to
Lance

Hummm not bad Jim!..thats what I like about this group...smarts!!

Steve

Reply to
SB

Dear SB:

If you got a thermocouple driven temperature controller, put them

+- connected to -+ , then read this *difference* in temperature with the controller, you could set the controller for the temperature difference target. Over this difference, turn on, under this difference, turn off. The upstairs should be "polarized" to agree with the controller...the downstairs should be polarized opposite.

David A. Smith

Reply to
N:dlzc D:aol T:com (dlzc)

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