Myth or science? Ceiling fan in winter mode reduces heat bill

Gsmith

I think this whole concept of reversing the ceiling fan rotation was developed to minimize stratification in tall spaces. In my experience, but not documented with energy use comparisons, it does work to minimize the energy use in a two story high living room in a rental house, BUT only if the fan is used at a very low speed to just GENTLY mix the air. This allows the thermostat to sense the overall air temperature rather than the hotter temp on the upper level or the lower temperature on the lower level. But, if the fan is run too fast the cooling effect mentioned by @paul formisano above will make the residents feel the cool breeze and lose the benefit of preventing stratification.

ChrisJ

Paul Formisano said:

Increased air velocity increases the convection coefficient and energy transfer from air to surfaces increases.  That means more rapid energy loss from the heated space to the wall will result from using a fan.  

However, the air will mix and become less stratified with the fan on, which will make the dry bulb temperature at lower elevations in the room increase when the fan is on, and also make the dry bulb temperature decrease at higher elevations as the colder lower air gets mixed with it.  Depending on the vertical location of the thermostat, the new room temperature could increase or decrease at its location.

Meanwhile, the wet bulb temperature (we humans are like wet bulbs because we perspire) will decrease when air velocity increases, due to the latent heat of evaporation becoming sensible on the skin surface. So it will feel colder in moving air compared to still air of the same temperature if the air temperature is lower than body surface temperature.

So, with a fan on, you’re more likely to use more energy to heat your home, but depending on the geometry of your room and your thermostat location, and your location relative to moving air, it could make you feel warmer or colder.

I’m a Professional Mechanical Engineer with a lot of thermal design experience.

For all intents and purposes of heating a building in the winter I don't think wet bulb temperature is very meaningful. No one should be perspiring.

ethicalpaul

We may not be actively perspiring like in a workout, but our skin has moisture to varying degrees. Some people have clammy skin all year long regardless of temperature or activity level.

And even with no evaporation, the increased air flow will remove the cushion of warm air that is adjacent to all of our bodies, causing a cool sensation.

Voyager

hot_rod said:

hot goes to cold always. The rate of heat exchange is based on the temperature difference. With forced air the warmer the stratified air at the ceiling the greater the loss through the insulation.

That was always the "selling feature" of radiant floor heat, little to know stratifications, lowers the losses up top.

I guess the same applies for lowering your indoor temperature, less loss through the building shell, lower energy required.

I suspect everyone is different, but I tend to be more comfortable in the winter if my feet are warm. However, I own a home with forced air and a nearby workshop with in-slab hydronic so I get to compare them on a daily basis. The difference is not even close to what the graph shows. And the beauty of graphs like that is that they have no reference source so I can’t look up the data behind them. I have not seen good data on temp stratification in a residence, but I am sure it must exist somewhere. If someone knows where, please share as I would love to see it.

I am equally comfortable in my home and my shop, but the one difference is that I probably could go barefoot in my shop and still be comfortable and I wear slippers in my house to keep my feet comfortable. I always have at least sneakers on in the shop so I can’t say for sure, but I believe that would be the key difference. I don’t think the temp stratification is even close to what this graph shows based on simple thermometer checks I have done in my home, but I’d love to see some real research quality data on this.

Voyager

Jamie Hall said:

Too many variables. What they do is reduce stratification in the space. Now -- is that necessary or even desirable? Well... In some applications overhead fans are unquestionably useful. High bay work areas and big barns are some examples (big barns particularly) -- but those fans are huge (ten to 20 foot or more diameter) and the ceiling (often the underside of the roof) is a long way up -- and is a major heat loss.

In a residence... I'll have to contemplate this. I think the key variables will turn out to be the type of emitter (e.g. forced air vs. radiators vs baseboards vs. radiant floor) and the geometry of the space (including where the heat sources are, how effective they are in reducing stratification, and so on).

Let me think about it.

I agree it is not as simple as just reducing stratification. The body loses heat through many mechanisms, and evaporation and conduction are two significant ones. And airflow over the body increases loss via both of these mechanisms. I have not seen any research that shows how much one factor (warmer) air offsets the other (heat loss due to convention and evaporation increase). I suspect, but can’t prove with data, that the increased airflow causes enough additional conductive and evaporative heat loss that the one or two degree increase in temp of the air is completely washed out. It would neat to see some research on this though.

Voyager

JakeCK said:

Talks cheap. Time to post up the evidence.

Clearly it made a difference.

I don’t doubt it makes a difference in temperature. The question is: does it make a different in comfort?
I suspect it doesn’t as the additional airflow likely provides as much additional cooling to the human body as does the additional 0.5 to maybe 1 degree increase in air temp. I have not yet come across a study on this, but it would be interesting to see.

My church has a 14’ ceiling. We have two ceiling fans in the sanctuary and in the winter everyone complains they are colder with them on so we leave them off. I am not sure how much they mess up the rising heated air from the baseboard heaters as they try to “wash” warm air down the outside walls in counter flow.

Voyager

ethicalpaul said:

JakeCK said:

I never claimed it affected fuel usage much. Overall average  temperature of the building shouldn't change much. Boiler run time averaged out might change a few seconds.

What I did say quite forcefully is that it does change the comfort of this house. It is old, it is drafty. There is always a slug of cool air rolling around the main floor until it finds it's way to the basement. That cool air gives me cold feet. And I find that particularly uncomfortable. The fan eliminates that. Arguing otherwise... Well if one believe that massive fan doesn't move around the warm air by the ceiling may I suggest you remove the fan from the heat sink of your PC. If you are using a PC. You obviously do not need it since fans apparently don't move warm air. I might add or rather warn this debate will be over quickly after that fan is removed.

Edit: I also want to add that those couple of hours were just the last few hours. I can turn that fan on and off all day and watch that same pattern occur in the graphed data. After doing it a dozen times and seeing it, being told it isn't real is akin to walking into a room and flicking the light switch to only be told it isn't the switch. Could it be coincidence? Sure anything is possible. But absent convincing evidence to the contrary I'm going to go with the simplest answer. Turning the fan on is moving the air and causing the sensors half way down the wall to report a warmer temperature. And I don't walk on the ceiling(I'm not spider pig), I walk on the floor. I'd rather feel the warm air down there.

Sorry I'm not trying to fight but the title of the thread is asking if it affects the fuel bill. I said in my reply that it might make people feel warmer or cooler, depending, but that the fuel bill will be higher. I will respectfully stick with that.

The problem is that the heat bill depends also on the thermostat setting. If the people feel cooler with the fan running, even though they might save on heating bills in that condition, most won’t tolerate feeling colder and they turn up the thermostat a degree or two to regain comfort. I am not at all sure that there is a net energy saving when this happens. Almost all energy saving studies I have read ignore that every residence has a control system in the room that will mess up your nice neat science data every time.

ChrisJ

ethicalpaul said:

We may not be actively perspiring like in a workout, but our skin has moisture to varying degrees. Some people have clammy skin all year long regardless of temperature or activity level.

And even with no evaporation, the increased air flow will remove the cushion of warm air that is adjacent to all of our bodies, causing a cool sensation.

SteamingatMohawk

Differences Between Centripetal And Centrifugal Force

Methinks ye art referring to centrifugal force.

Centripetal Force

Centripetal force is the component of force acting on an object in curvilinear motion which is directed towards the axis of rotation or centre of curvature.

It is observed from an inertial frame of reference.

If a car is travelling through a curve on a circular horizontal road, the centripetal force provided by the force of friction between the tyres of the vehicle and the road surface allows the car to negotiate the turn.

Centrifugal Force

Centrifugal force is a pseudo force in a circular motion which acts along the radius and is directed away from the centre of the circle.

It is observed from a non-inertial frame of reference.

When a car in motion takes a sudden turn towards the left, passengers in a car experience an outward push. This is due to the centrifugal force acting on passengers.

The link: https://byjus.com/physics/centripetal-and-centrifugal-force/

SteamingatMohawk

Add to this conversation that allegedly, humidity in a room affects comfort. Apparently somewhat higher humidity makes lower temperature feel more comfortable.

Another link to drive us wild:

https://todayshomeowner.com/video/how-to-make-your-home-feel-warmer-in-winter/

Lance

There are only 4 factors to consider in heating or cooling a creature's body for comfort in a room, or even outdoors. 1) Your body which by itself has many factors. 2) The heat loss or gain of sensible temperature to the body from air or surfaces it touches. 3) The humidity or latent heat in the air surrounding our body which is affected by; 4) the velocity of air movement over the body. 90% of any two humans in the same environment will always disagree on comfort especially if humidity levels are at the extremes. Extreme is RH% lower than 45% and higher than 55%. To prove my point how tough it is to find comfort we can agree on, the variables between bodies are multiplied by too numerous ways we differ, not to mention nude optional. Ceiling fans can only vary direction and CFM. They are the least of the variables. Frankly if I want cooling, I want dry air movement to evaporate my sweat. For warmth I want humid and still air and maybe some clothing. As for the building it could care less, as for our budget, fans use energy when they run. and air movement can remove a layer of still air on surfaces which has an insulation factor. Unless than fan compensates a gross construction design flaw, I see it as not worth it. My opinion excludes the radiant effect of sunlight. And for the scientists, the greater the temperatures and humidity differ, the greater the change the affect has.

ChrisJ

SteamingatMohawk said:

Add to this conversation that allegedly, humidity in a room affects comfort. Apparently somewhat higher humidity makes lower temperature feel more comfortable.

Another link to drive us wild:

https://todayshomeowner.com/video/how-to-make-your-home-feel-warmer-in-winter/

I think it can.
But I also think humidity can also make it feel colder, if it's cold out. I guess it amplifies how the ambient temperature feels.

johnkern9068

vhauk said:

heat rises. The theory is that a gentle push of the warmer air near the ceiling towards the floor would more equally distribute the available heat energy. But with radiators convection currents are important, maybe more important than moving already heated air towards the floor. Something one of my hvac instructors said has always stuck with me: “you cannot destroy heat, you can only move it.”  

Please don’t say heat rises. It hurts my head.  

wam525

Thank, Jake. Nothing like some good data to provide proof. No one can deny that the temperature at the thermometer goes up with the fan on. a few hours data is more than enough. Could have proved it with a few minutes of data.

ChrisJ

wam525 said:

Thank, Jake. Nothing like some good data to provide proof. No one can deny that the temperature at the thermometer goes up with the fan on. a few hours data is more than enough. Could have proved it with a few minutes of data.

The temperature at that thermometer in that room went up.

But a few minutes wouldn't tell anything.

scott w.

I hate to feel air movement by a fan in the winter unless its sitting on a hot radiator or the room is heated above 78 degrees. Fans are the worst with forced air heat. All fans make some kind of draft and makes the room feel chilly. INMO fans are only good for summer use.

JakeCK

SteamingatMohawk said:

Differences Between Centripetal And Centrifugal Force Methinks ye art referring to centrifugal force. Centripetal Force Centripetal force is the component of force acting on an object in curvilinear motion which is directed towards the axis of rotation or centre of curvature. It is observed from an inertial frame of reference. If a car is travelling through a curve on a circular horizontal road, the centripetal force provided by the force of friction between the tyres of the vehicle and the road surface allows the car to negotiate the turn. Centrifugal Force Centrifugal force is a pseudo force in a circular motion which acts along the radius and is directed away from the centre of the circle. It is observed from a non-inertial frame of reference. When a car in motion takes a sudden turn towards the left, passengers in a car experience an outward push. This is due to the centrifugal force acting on passengers. The link: https://byjus.com/physics/centripetal-and-centrifugal-force/

Right you are. Wrong choice of word.

Voyager

ChrisJ said:

ethicalpaul said:

We may not be actively perspiring like in a workout, but our skin has moisture to varying degrees. Some people have clammy skin all year long regardless of temperature or activity level.

And even with no evaporation, the increased air flow will remove the cushion of warm air that is adjacent to all of our bodies, causing a cool sensation.

Actually, ethicalpaul is absolutely correct and this is very relevant and probably the dominant factor in regard to ceiling fans.

ChrisJ

Voyager said:

We may not be actively perspiring like in a workout, but our skin has moisture to varying degrees. Some people have clammy skin all year long regardless of temperature or activity level. And even with no evaporation, the increased air flow will remove the cushion of warm air that is adjacent to all of our bodies, causing a cool sensation.

Actually, ethicalpaul is absolutely correct and this is very relevant and probably the dominant factor in regard to ceiling fans.

I'm just giving him a hard time.

mvickers

In air, heat > cold works in the horizontal plane, but not so much in the vertical.  My ex house had 22ft vaulted ceilings, wood heat in forced or convection, and without closing the 2nd story master bdrm door, sleeping wasn't possible, and without the down flow in winter ceiling fans, the loft was also unbearable.  Also, the 1st floor living area, office/bathroom/dining rm/kitchen wasn't as evenly heated.
Did a staple-up, series/parallel, single zone radiant, with a small propane Burnham but didn't like it per propane cost & overall comfort

EdTheHeaterMan

JUGHNE said:

A hardware store owner told me that he had a lady complain that the ceiling fan he sold here did not make the room warmer, she ran it for an hour and the blades never even got warm to help heat the room.

That reminds me of a Paul Harvey radio show when he would tell "the rest of the story"
One of Paul's sponser's was True Value Hardware. He told of a women who was in the store one Friday afternoon and ask the True Value associate if this Electronic Stud Finder really worked. Sounded like a regular commercial. The employee say YES they work amazing well, So the woman purchased one. Paul went on with the radio broadcast of the day.

Near the end of the program, Paul told "The rest of the story" about the woman with the stud finder who came to the store the following Monday. She thanked the associate for the great recommendation on the stud finder. You see, she was going out with her girlfriends that weekend and wanted a way to find "studs" at the night club. It really works she said!

And now you know THE REST OF THE STORY

EdTheHeaterMan

ChrisJ said:

Voyager said:

ChrisJ said:

ethicalpaul said:

We may not be actively perspiring like in a workout, but our skin has moisture to varying degrees. Some people have clammy skin all year long regardless of temperature or activity level.

And even with no evaporation, the increased air flow will remove the cushion of warm air that is adjacent to all of our bodies, causing a cool sensation.

Actually, ethicalpaul is absolutely correct and this is very relevant and probably the dominant factor in regard to ceiling fans.

I'm just giving him a hard time.




As it should be!

JHamburger

A great topic for discussion. Good information and entertainment, too.
I leased a building with 16 foot ceilings, I do not have any proof, however, it felt warmer in the winter when the ceiling fans ran and I hoped I wasn't supplying all the heat for the upstairs renters.