Article claims reversing ceiling fans in winter is dumb...

[offingmoot]

he makes a good point but i have had more success so far with the reversal and the fan on low

http://www.bigassfans.com/pdf/research/Why Reversing Ceiling Fans In Winter Is A Dumb Idea.pdf


Why Reversing Ceiling Fans In Winter Is A Dumb Idea
Dr. Richard Aynsley, Ph.D. - Director Research & Development - Big Ass Fan Company
As an architect and engineer, I can understand why the architects for some projects seek trully reversible air
movement from ceiling fans. This simplistic notion of ceiling fan use has been around for decades. Let me explain
as an architect with a Masters degree in Engineering, why this is not a sound idea.
Use of ceiling fans for air movement to energy-efficient summer comfort is straight forward. In summer, providing
air movement of 160 fpm in the occupied zone will allow the thermostat to be raised 4.7°F while maintaining
the same thermal comfort. Raising the thermostat 1°F typically saves between 3% and 4% of cooling energy
cost*. Raising the thermostat 5°F will typically save between 15% and 20% of cooling energy costs. In summer,
providing air movement of 230 fpm (2.6 mph) in the occupied zone of an air conditioned space will allow the
thermostat to be raised 10°F while maintaining the same thermal comfort. This thermostat increase would save
30% to 40% of air conditioning cooling costs.
Use of ceiling fans at low speed with air flow downward for energy efficiency by destratification in winter is
well established. Circulation of indoor air at 3 to 4 times per hour, particularly in spaces with high ceilings, can
allow the thermostat to be set 10°F to 15°F lower while maintaining the same thermal comfort. For each 1°F the
thermostat is lowered in winter the typical heating energy cost saving is 1% per 8 hour period, or 3% per day.
Lowering the thermostat in winter by 5°F from 85°F to 80°F will typically save approximately 15% of heating
energy cost per day.** Field data from clients using Big Ass ceiling fans indicates monthly winter gas consumption
can be cut by up to 30%.
Here is the catch. If the airflow from the ceiling fan is reversed with airflow upward, even at low speed, the
velocity of air across the ceiling above the fan is high, typically around 400 fpm. At this speed, the heat transfer
coefficient at the ceiling is around 1.87 Btu/h.ft2.°F. However if the ceiling fan is running at low speed with airflow
downward the airflow across the ceiling is low, typically 80 fpm. At this speed, the heat transfer coefficient
at the ceiling is around 0.051 Btu/h.ft2.°F.
In short, reversing the air flow direction from ceiling fans in winter the heat loss through the ceiling is increased
by a factor of around 3.7 times due the increase in surface conductance.
*Exeloncorp, formerly Consolidated Edison,
(broken link removed)
** US Dept of Energy, (broken link removed)
Richard Aynsley, Ph.D., M.ASHRAE,
B.Arch (Hons I), MS(Arch.Eng), Registered Architect, QLD.
Director Research & Development
Big Ass Fan Company
Lexington, KY, USA

 

[DiggerJim]

he makes a good point but i have had more success so far with the reversal and the fan on low

Neither link in his article is live anymore so it's hard to tell what's up with that research, but he's got a quirky little company (bigassfans.com) Personally, due to the way our house is constructed, heat rising pays off by allowing me to turn down the zone upstairs (although net I have to run the downstairs zone more - just don't have to power a fan to pull the heat back off the ceiling). I've got a center chimney full dormer cape with a family room in the center on the second floor overlooking the living room downstairs which has a cathedral ceiling. The center chimney is flanked by two half walls so the family room is actually very open to the warm air rising from the living room (which is where I have my pellet stove). The two rooms that flank the family room need a bit of assisted circulation but otherwise it's ideal for heating with pellets.

 

[cac4]

sounds like a completely "egg-headed" analysis, which works out well on paper, but not quite the same in practice.

first of all, who heats their house to 85 degrees F???

as for the rest, it seems to me to be dependent upon 2 things:
a: what is on the other side of that ceiling? if its the roof, well, ya, I guess you're going to be loosing. But if its my second-floor bedroom that I'd actually like some heat to get transferred to, then...not so much.

b: what is the proximity of my comfy chair to the ceiling fan in question?

if its really close, (and it is), you don't want that thing blowing "down", because that higher velocity air blowing across you will "feel" much colder than it is. a labratory thermometer will say that the air is warmer. but that dry winter air will cause evaporative cooling to things that are alive, so you'll ("I'll") feel colder, which totally defeats the purpose, which is to save $$ by making it "feel" warmer, rather than generate more heat from my heating system.
With the fan blowing in reverse, the air is still circulated, which helps distribute the heat, but it doesn't "feel" like its being circulated.

Have to also keep in mind that he's talking mostly about "cathedral ceiling" type situations, and "destratification", which (I think) is the vertical movement of heat, and here in pellet-stove world, we're talking more about moving air around laterally, in an effort to turn a space heater into a central heater...again, theory and practice conflicting. these things aren't supposed to heat a whole house, but in practice, they can put out enough btu's to do it, but they can't distribute the heat evenly...so we help them along with fans and such.

anyway, the point is that running a fan in reverse isn't only to save $ on fuel. If that is your only purpose, then perhaps it is "dumb" to run a fan in reverse. But there are other reasons to do it than just btu savings.

 

[dave1966]

i find that running the ceiling fan in reverse made a big difference in my home,if i started the pellet stove and forgot to turn on the ceiling fan it seemed to never warm up but within minutes of turning on the ceiling fan it would get nice and warm made a big differance

 

[GVA]

The article is meant more for industrial buildings/ warehouses, where they would have 20'+ ceilings. where the heat rises and keeps the roofline 85* and the floor at 65*.
Destratification is refering to in laymans term - Mixing the air
He is correct that destratifying the air is the answer forward or reverse doesn't really matter (depends where you are seated in the room in question)..

the major key is the speed.......................... put it on the slowest speed.... you want to move the air but not feel a breeze....
my .02

 

[webbie]

This is one of those topics where the real answer is "it depends".....

on many things....from ceiling height, to the insulation of the roof above, to the climate, etc. etc. etc.

I'm usually in the camp of keeping it pushing down slowly, because throwing a bunch of air up against a cold ceiling (even insulated) would seem to create heat loss....or at least more than air moving slower over that same surface.

 

[freebird77]

I noticed no difference when trying to circulate the heat from my stove last winter. I put a thermometer under the fan and in other locations and got zero difference as opposed to not using it. I have a one story double wide

 

[moralleper]

A little off topic but I will in Ikea the other day and the warehouse part of the store was windy, when I looked up there was the ceiling fan. Not just a ceiling fan but the mother of all ceiling fans. this thing has to be 40+ feet in diameter, the name on the fan, Big Ass Fan Co. I tried to take a picture but it did not turn out well.

 

[offingmoot]

when i 1st posted the article i didnt realize that he was talking about warehouses and the connection to the big ass fan co.

 

[mccabedoug]

I've a newly installed Harman P61A on the opposite end of my house from my great (family) room that has a cathedral ceiling and ceiling fan. Obviously that room is cooler than the other end of the house which contains my pellet stove. What direction should the air from the ceiling be flowing in order to warm up that room? Up or down?

 

[offingmoot]

to me it just makes sense to have the fan blowing down on low just to get the heat off the ceiling
if you imagine your fan in "winter" mode it actually pushes the heat up onto the ceiling where it would probably float the outside walls and then maybe down
my experience over the past 2 months of burning is like i said blowing down on low when in the room cause it will create a slight breeze and on medium/high when not in the room
i also use a box fan on the floor to push the cold air towards my insert to pull to heat to that area

 

[Firenutz]

I expeimented with this while at my parents house. Their woodstove is on the lower level in a small florida room and they keep it uncomfortably hot in there. In the meantime, the upper level living room stayed quite cool. It was 71 in the living room when I turned on the living room fan on reverse. Within about 15-20 minutes, it was 76 in the upper level. I think a lot of it depends on where the fan is in relation to the stove. During structure fires, smoke and hot gases head to the ceiling and roll down at the walls. I would think that stove heat would behave the same. I'm gonna use some of my wind current checker that I use for hunting and experiment in my home this winter. It's a fine white powder that stays airborne for quite some time. Or we could always call up Mythbusters!

 

[mccabedoug]

Well I can confirm that in my cathedral ceiling family room (with the pellet stove on the opposite end of the house, same floor) with the ceiling fan slowly blowing the air down (same as I do in the summer), the temp in the family room has dropped 3oF. You can feel that it is not warming the room (drawing air from the warm end of the house) at all. So, that did not work. I'll try reversing it so that the air is being drawn toward the ceiling.

 

[kenzmadie]

I have cathedral ceilings over my living room and kitchen. The wall between them only goes up about 10 feet and it is open above it. I have my ceiling fan in the living room. This summer I had it in the summer position. This winter I changed it over and it blew all the dust off of the top of my kitchen cabinets. So, there is a difference in how it moves the air around and I no longer have to vacuum the top of my cabinets. I can stand in my kitchen and feel the heat being blown off of the ceiling. So, I think it's working........but I lost my PhD in a card game.

 

[hossthehermit]

This is one of those topics where the real answer is "it depends".....

on many things....from ceiling height, to the insulation of the roof above, to the climate, etc. etc. etc.

I'm usually in the camp of keeping it pushing down slowly, because throwing a bunch of air up against a cold ceiling (even insulated) would seem to create heat loss....or at least more than air moving slower over that same surface.

Gotta say, you reach an age where "Depends" is the answer to a lot of questions

 

[JimmyMood]

I think Mr. Webmaster got closest to the point. I take from the article, that circulation of the room air 3-4 times an hour will destratify the air, independent of whether the fan is blowing up or down. HOWEVER, heat loss is increased when the fan is blowing up because that creates a higher air velocity against the cold ceiling, which allows that surface to extract heat faster due to convection. If the fan is blowing down, the air velocity moving over the ceiling is lower which creates a lower heat loss on that surface. The argument that a fan blowing against the ceiling increases the room heat loss to the attic or outdoors makes sense. If you have a heated room above that ceiling, then the point is invalid.

This is the same reason standing under a fan blowing down makes you feel colder than standing under a fan blowing up. Now if the fan blowing down on you makes you feel colder, reducing the heat loss by blowing the fan down might be a moot point....

 

[jams002]

thanks for the idea and info that you share to this forum,,wonderful interesting to read thanks a lot
Ceiling Fan Reviews

 

[ablejoy]

I've a newly installed Harman P61A on the opposite end of my house from my great (family) room that has a cathedral ceiling and ceiling fan. Obviously that room is cooler than the other end of the house which contains my pellet stove. What direction should the air from the ceiling be flowing in order to warm up that room? Up or down?

I have the cathedral ceilings in the living room kitchen and dinning room. I have an open wall between living room where the stove is and the kitchen.I reverse both fans in living room and dinning room room temp stay about the same pulling heat off ceilings. Temp out side 35 inside its 72 in living room 70 in kitchen. Sure beats firing the propane heater at $2.83 a gallon.

 

[richkorn]

I've been trying the ceiling fan in the Living room where the stove is both up and down. I'm not sure i see a difference in temp in the house, but i find with it blowing down, that it is much more comfortable sitting in that room because it can get quite warm in there as compared to the rest of the house.

 

[billb3]

It would seem to me that calling a potential customer dumb isn't too bright.

 

[kofkorn]

I think Mr. Webmaster got closest to the point. I take from the article, that circulation of the room air 3-4 times an hour will destratify the air, independent of whether the fan is blowing up or down. HOWEVER, heat loss is increased when the fan is blowing up because that creates a higher air velocity against the cold ceiling, which allows that surface to extract heat faster due to convection. If the fan is blowing down, the air velocity moving over the ceiling is lower which creates a lower heat loss on that surface. The argument that a fan blowing against the ceiling increases the room heat loss to the attic or outdoors makes sense. If you have a heated room above that ceiling, then the point is invalid.

This is the same reason standing under a fan blowing down makes you feel colder than standing under a fan blowing up. Now if the fan blowing down on you makes you feel colder, reducing the heat loss by blowing the fan down might be a moot point....

Actually the logic behind the article is a bit flawed. Heat transfer occurs between two objects that have different temperatures. The greater the temperature difference, the greater the heat transfer. When you introduce a fluid movement across a surface, the heat transfer is increased, essentially wind chill (the article says 3.2x, sounds reasonable). However, most of the people on this forum are heating an INSULATED home. Therefore, the ceiling surface is insulated against the outside air, and nearly the same temperature as the inside of the room. Over a period of time, in a well insulated home (I have over 18" of blown-in insulation above my great room), for all purposes the temperature of the ceiling is about the same as the temperature of the air around it (Steady State). The additional heat loss due to the air movement (3.2 x 0 temp difference) is negligible.

Now, for all of you sorry souls that have no insulation, your screwed. The temperature of the ceiling is likely to be quite a bit lower than the temperature of the air. When you turn that fan on, you will definitely see an increase in your heat loss.

I myself, prefer the fan pulling up towards the ceiling in the winter, it still destratifies the air, but it also does not give a breeze concentrated in any one area.

Now the bad part about this is that your body is nearly 99 degrees in a 70 degree room. The air movement DOES increase the heat transfer of your body relative to the room air. So while the temperature at the floor may increase by a couple of degrees, it will probably actually feel colder.

I guess it's a no-win!

Later,
Ray

 

[hossthehermit]

Different situations require different solutions

 

[itworks]

Different situations require different solutions

AMEN. It's taken me a while to "tweak" my stove's heat distribution to my home, but it kind of defies intrinsic rationale. We are more comfortable not using an OAK, and moving the cold air towards the stove. A tea kettle of water on top of the stove adds enough humidity to add to our comfort. I'm far from an expert, but I believe you must experiment to find what works best for your situation.

 

[SmokeyTheBear]

I have three multi speed low wattage large bladed fans that I use in the lowest speed setting to hall air up and out of the stairwell. It does what I need it to do which is get the heat out of the lower level of my house where my home office and the stove is and up to the main living level.

The good thing is I don't feel the draft that I would if those fans were blowing down. I like not having to wear another shirt than my t shirt when I down a brew and watch a movie.

That's my story and I'm sticking to it.