Re: At what rate does heated air rise?

Question

Dear SB: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 "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

jim · Accepted Answer

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

Lance · Answer

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

*****

k wallace · Answer

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

SB · Answer

Thanks for your replysOk -..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

PB · Answer

Thus spake :Hi k wallace!There is such a thing. In Sweden it has been quite popularfor some years to use floor heating as the sole heat source.One supplier of floor heating systems is Devi, /PB

N:dlzc D:aol T:com (dlzc) · Answer

Dear SB:I tried  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

N:dlzc D:aol T:com (dlzc) · Answer

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

jim · Answer

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

Lance · Answer

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

SB · Answer

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

N:dlzc D:aol T:com (dlzc) · Answer

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

Re: At what rate does heated air rise?

Join the Discussion

Didn't find your answer?