[Greenbuilding] Ceiling Fans at Night?

[nick pine]

Sacie Lambertson <sacie.lambertson at gmail.com> writes:

> I rarely leave our fans on at night because no one is there
> to enjoy the warmth they create as they bring the warm air down.

Why is there warm air near the ceiling in wintertime? Lights,
or warm air supply ducts near the ceiling? A woodstove that
radiates more heat up than down? A dark curtain over a south
window, or a passive solar air heater on a south wall?

Ceiling and floor temps would be the same in a room with infinite
insulation, given the lowish (~R1) radiation resistance between
floor and ceiling, no? With less insulation, and heat sources near
the floor, the ceiling temp would be lower, no? The room might
require less heating energy with the fans off... 

> ... fans turned on immediately below normal ceiling level ie 8
> or 9 feet, might make the air feel cool to the occupants below.

The ceiling air would have to be warmer to make up for the cooling
effect of higher velocity air near the person.

> There must be a right ratio between the volume of hot air above
> before a fan can be efficiently employed to warm the air below. 

A volume ratio? Hmmm.

Corwyn <corwyn at midcoast.com> writes:

> On 1/6/2011 11:35 AM, Ron Cascio wrote:

>> Ceiling fans work the same on the human body in both summer and winter;
>> convective cooling. It is my belief that a ceiling fan is useful in
>> winter only when there is a large degree of temperature stratification
>> in high ceiling areas and the fan is able to push that hot air (which
>> wants to stay up there) down to a lower level at a very low velocity,
>> low enough as to not cause convective cooling to the occupants of that area.

That seems doable. If the upper and lower halves of an 8' R16 cube each
have a 6x8^2/R16/2 = 12 Btu/h-F conductance and the upper half temp is
80 F and the lower temp is 70, the cube needs a total (80-30)12+(70-30)12 
= 1080 Btu/h on a 30 F day. Picture 600 Btu/h of lights above and a 480 Btu/h
heater with a 70 F thermostat below.

If a 12 cfm ceiling fan circulates air between the halves, making the upper
and lower temps 75 and 70 F, the lower half receives (75-70)12 = 60 Btu from
the upper half, and the lower heater only has to supply 480-60 = 420 Btu/h,
and the cooler upper half only loses (75-30)12 = 540 Btu/h to the outdoors.

If the ceiling fan raises the air velocity from 0 to 12cfm/32ft^2 = 0.375
lfm (0.0019 m/s), we need to raise the 70 F still air temp to... 70 F :-)
for equivalent comfort. The air velocity would have to rise to 28.5 lfm
(still short of a paper-rustling 130 lfm) to require increasing the air
temp to 71 F.         

> Many ceiling fans come with a reversing switch.  And everybody seems to 
> get it wrong.  In the summer, the fan should blow DOWN to provide 
> evaporative cooling.  In the winter, the fan should blow UP to mix the 
> air and reduce stratification, but keep the evaporative cooling to a 
> minimum.

It seems to me the cooling effect would be the same in either direction,
but why fight natural room air convection currents? Up in winter and down
in summer fits better with natural airflow near walls.

"JOHN SALMEN" <terrain at shaw.ca> writes:

> Paddle type fans are not that efficient or comfortable in heating mode.

In "heating mode"? :-) Grainger's 4C721 ceiling fan seems efficient, at
46K cfm with 0.96A at 120 V, ie 400 cfm per watt, at full speed, with
higher efficiency at lower speed, eg 0.96x120(12/46K)^3 = 2 nanowatts
at 12 cfm, theoretically-speaking.  

> One simple design I worked with for passive assist with no additional fan
> power consumption was to create a dropped plenum in a cathedral peak and
> utilized the ceiling gypsum which can work effectively to store heat (simply
> feel the ceiling over your woodstove)

Or above your air heater. A shiny surface under hot ceiling mass with a slow
ceiling fan controlled by a room air thermostat can storelots of heat while
giving good room air temp control and avoiding overheating by radiation.

"Corwyn" <corwyn at midcoast.com> writes:

>> On 1/5/2011 3:10 PM, Frank Cetera wrote:

>>> Even if a programmable is used and the thermostat is reduced to 55 at
>>> night, would it still be valuable to keep the ceiling fans running at
>>> night to move the warm air from the ceiling to the living/thermostat
>>> layer, and thus prevent the furnace from turning on during the night?

Maybe.

>>> Or should turning the thermostat down to 55 pervent it from coming on at
>>> all so the fans question is mute?

Maybe.

>> I would say that ceiling fans are only useful when you are in the room.

Not so, in the example above.

>> Proper insulation levels would remove both the need for a fan, and the
>> need for a set-back thermostat.

No. Set back thermostats can save energy, even with "proper insulation."

> Frank Cetera wrote:
>> 
>> "We do have a programmable thermostat and have it set for 66 during the
>> day; at 5pm it goes to 60 and at 7pm it goes to 55 (on the weekends it
>> may even be lower). Brent's (the building manager) theory is that even
>> at 55, keeping the fans on all night (and weekends) will keep the space
>> slightly warmer and therefore the furnace would need to kick in slightly
>> less often. Our theory is that the energy expended keeping the fans
>> going when the thermostat is set for the lower temperature is more than
>> that saved by the furnace kicking in a little less often. Two more
>> details - the heat registers are at the ceiling level (go figure); the
>> thermostat is on a post about 5 feet up, near an edge of the room. Any
>> idea who is correct? 

I'd vote for Brent, given the high heat registers.

> How many degrees of stratification are you seeing on a night when the 
> inside temperature as at 55?

The ASHRAE 55-2004 comfort standard requires less than 5 F.

> 1) Turn them off.  Then turn the thermostat down to 52.  I guarantee 
> that is better than either of your options.

Got numbers?

20 CLO = 1'clothing insulation (clo)
30 MET=1.1'metabolic rate (met)
40 WME=0'external work (met)
50 RH=50'relative humidity (%)
60 DATA 70,0
70 DATA 70,0.375
80 DATA 71,28.504
90 FOR CASE = 1 TO 3
100 READ TF,V
110 TA=(TF-32)/1.8'air temp (C)
120 TR=TA'mean radiant temp (C)
130 VEL=V/196.9'air velocity (m/s)
140 DEF FNPS(T)=EXP(16.6536-4030.183/(TA+235))'sat vapor pressure, kPa
150 PA=RH*10*FNPS(TA)'water vapor pressure, Pa
160 ICL=.155*CLO'clothing resistance (m^2K/W)
170 M=MET*58.15'metabolic rate (W/m^2)
180 W=WME*58.15'external work in (W/m^2)
190 MW=M-W'internal heat production
200 IF ICL<.078 THEN FCL=1+1.29*ICL ELSE FCL=1.05+.645*ICL'clothing factor
210 HCF=12.1*SQR(VEL)'forced convection conductance
220 TAA=TA+273'air temp (K)
230 TRA=TR+273'mean radiant temp (K)
240 TCLA=TAA+(35.5-TA)/(3.5*(6.45*ICL+.1))'est clothing temp
250 P1=ICL*FCL:P2=P1*3.96:P3=P1*100:P4=P1*TAA'intermediate values
260 P5=308.7-.028*MW+P2*(TRA/100)^4
270 XN=TCLA/100
280 XF=XN
290 N=0'number of iterations
300 EPS=.00015'stop iteration when met
310 XF=(XF+XN)/2'natural convection conductance
320 HCN=2.38*ABS(100*XF-TAA)^.25
330 IF HCF>HCN THEN HC=HCF ELSE HC=HCN
340 XN=(P5+P4*HC-P2*XF^4)/(100+P3*HC)
350 N=N+1
360 IF N>150 GOTO 480
370 IF ABS(XN-XF)>EPS GOTO 310
380 TCL=100*XN-273'clothing surface temp (C)
390 HL1=.00305*(5733-6.99*MW-PA)'heat loss diff through skin
400 IF MW>58.15 THEN HL2=.42*(MW-58.15) ELSE HL2=0'heat loss by sweating
410 HL3=.000017*M*(5867-PA)'latent respiration heat loss
420 HL4=.0014*M*(34-TA)'dry respiration heat loss
430 HL5=3.96*FCL*(XN^4-(TRA/100)^4)'heat loss by radiation
440 HL6=FCL*HC*(TCL-TA)'heat loss by convection
450 TS=.303*EXP(-.036*M)+.028'thermal sensation transfer coefficient
460 PMV=TS*(MW-HL1-HL2-HL3-HL4-HL5-HL6)'predicted mean vote
470 GOTO 490
480 PMV=99999!:PPD=100
490 PRINT TF,V,PMV
500 NEXT CASE
510 'Innova AirTech Instruments has an excellent comfort web site...
520 'http://www.impind.de.unifi.it/Impind/didattica/materiale/
530 'microclima/innova/thermal.htm
540 LIST 60-80

70            0            -.2857543
70            .375         -.2857543
71            28.504       -.2857581

Nick
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