Ok, I'll buy that. Here, on my ranch-style house, my roof overhang is about
2.5 feet which is great in the summer. The sun is high enough in the sky
that the roof blocks the sun from hitting the windows. In the winter when
the sun is much lower on the horizon, the sun does hit the windows, but who
cares in the winter.
I would put up trees if I could - that would be the best solution - but my
front yard which faces south is only 10 feet deep.
Open the DOWNSTAIRS windows as well. The hot air needs to be replaced with
Most thermostats that I've seen have a switch for "Auto" and "On". Normally
it's in the auto position to let the furnace control the fan. In the on
position, the fan runs all the time. Might help out as well.
I don't have that. I would have to run an extra pair of wires down to
the furnace, and figger out where to hook them up; the thermostat is
on/off, heat only. I assume that this is something that can be done
easily as I have seen thermostats as you describe (and indeed if I ever
add A/C I will need to wire up this functionality.)
replace "roosters" with "cox" to reply.
I live in south Texas.. Been running the A/C since February.
I like the ceiling fan idea the best, but I'm also in the security
business and leaving windows open makes me cringe. In northern Texas
and Louisiana I've seen big attic fans to suck air up from ground
level and exhaust to the outside. I guess if humidity is low outside
a breeze may make the space livable, but that is not an option on the
Window A/C units are pretty cheap now. Maybe get one or two for
comfort and heat waves that are inevitably coming to your area.
so if you don't like leaving windows open, that pretty much rules out
anything but a swamp cooler or real A/C. Of course, I supposeI
*could* rig a swamp cooler to use cistern water... is that acceptable
How do PIRs deal with ceiling fans, anyway? I agree, if I'm leaving
windows open, the only real security would be a system using PIRs.
You have to move the hot air out and the cooler air in. A good powerful
window fan can do wonders. You have the fan blowing out of the bedroom and
only open the windows in the rooms to be used. You can suck air a long
distance through the house that way and cool everything along the way.
Ceiling fans just circulate the hot air already in the room, floor fans
don't do much better. You have to blow hot air out and draw cooler air in.
You may even want to consider a whole house fan that blows up through the
attic and out the eaves. That also gets rid of the blanket of heat above
OTOH, I bought my first AC for our bedroom back about 1968 and have not been
without at least a cool bedroom since. You can get them for $99 these days
and on a hot humid night, nothing beat real AC.
And make sure the cool air scrubs the heat out of the room surfaces, vs
just passing through rooms with still air near the surfaces (including
people surfaces, which feel cooler in moving air.) Ceiling fans can help
whole house fans. Picture surface thermal mass in series with an airfilm
conductance Ga that increases with air velocity (Ga = 2+V/2 Btu/h-F-ft^2,
with V in mph, approximately) in series with a conductance to outdoor air
(cfm Btu/h-F, approximately), like this, viewed in a fixed font:
Tout ---www------www----------- Tsurf
--- Csurf = 0.5A Btu/F/ft^2
| for A ft^2 of 1/2" drywall.
A 10'x20' room with 880 ft^2 of drywall might have Csurf = 440 Btu/F.
With a 1000 cfm window fan and a ceiling fan that raises the airspeed
near the surface to V = 2 mph, it might have a natural time constant
RC = Csurf(1/cfm+1/(A(2+V/2)) = 440(1/1000+1/(880(2+2/2)) = 0.6 hours.
In 2 hours with Tout = 70 F, a Tsurf = 80 F room would would cool to
70+(80-70)e^-(2/0.6) = 70.4 F. With a 500 cfm window fan and no ceiling
fan, RC = 1.1 hours, so it might only cool to 70+(80-70)e^-(2/1.1)
= 71.7, or more, if the bulk of the air near the surface stays warm,
hardly moving at all.
With R20 walls, after 8 hours with Tout = 90 F with the window fan off
and no internal heat gains and RC = 10 hours, the 70.4 F drywall temp
would climb to 90+(70.4-90)e^-(8/10) = 81.2, or less, with 2 layers of
drywall with RC = 20 hours and 90+(70.4-90)e^-(8/20) = 76.9. We might
have a lot more mass and a lower temp if we cooled a basement at night
and circulated house air through the basement during the day.
Very nice, but ...
For those of us who's most recent engineering, physics, thermo, fluids,
etc courses were a LONG time ago (early 60's for me), please expand
your very-interesting text with perhaps a few definitions, therefores,
etc, so we can better *understand* your surely-excellent points.
Still air is a poor heat conductor, about R5 per inch. A square foot of
still air 1" thick is like a 5 ohm resistor, for heatflow through the 1"
thickness. DEFINITION: Ohm's law for heatflow is just like Ohm's law for
electricity with different units, thermal resistances vs ohms, Btu/h vs
amps, and Fahrenheit temperature differences instead of voltage diffs.
For example, 4 Btu/h flows in this circuit, viewed in a fixed font:
| -----> |
| 12 4 |
THEREFORE, if a person generates 300 Btu/h at 100 F internally with a 91 F
skin temp (measured with a $50 Raytek IR thermometer), we have something
like this, with an internal resistance Ri and a skin surface resistance Ra
in 70 F slow-moving air:
Ri | Ra
| --------------------> |
| I = 300 Btu/h |
| 100 F | 70 F
300Ri = (100-91) makes Ri = 0.03 and 300Ra = (91-70) makes Ra = 0.07, no?
If the slow moving air has a film conductance Ua = 2 Btu/h-F-ft^2 and
Ra = 1/(UaAs) = 0.07, the person might have As = 1/(RaUa) = 7 ft^2 of
exposed skin, out of 20 ft^2 of total average Dubois skin surface.
Now if the room temp rises to 85 F and everything else is the same, the skin
temp rises to 85+300Ra = 106 F, very uncomfortable. What to do? Rest vs work,
eg a siesta to lower heat generation, which can vary by 10:1, depending on
activity, or lose weight, since heat output increases with body mass, or
decrease Ri (some people in Arizona adapt to heat with more blood flow near
the skin and higher skin temps), or evaporate sweat, or stand in a bucket
of water, or increase the airspeed near the skin, and/or remove clothing.
To be equally comfortable with a 91 F skin temp, we might reduce Ra until
300Ra = (91-85), ie Ra = 0.02 or Ga = 1/Ra = 50 = AsUa. Increasing airspeed
to 6 mph (enough to blow papers off desks) makes Ua = 2+6/2 = 5 Btu/h-F-ft^2
and As = 10 ft^2, with more exposed skin, and so on.
DEFINITION: fans are thermal conductors. A 1000 cfm airstream with a dT (F)
temperature difference moves about 1000dT Btu/h, ie it has a conductance of
about 1000 Btu/h-F.
And 1 ft^2 of 1/2" drywall is like a 0.5 farad capacitor...
DEFINITION: an RC time constant is a measure of how fast a circuit
reacts to change.
The wall starts at Tsurf = Ti and becomes T(t) = Tout + (Ti-Tout)e^(-t/RC)
t hours after the fan starts. Initially, T(t) = Tout + (Ti-Tout)e^(-0/RC)
= Ti. After a very long time, T(t) = Tout + (Ti-Tout)e^(-oo/RC) = Tout.
Nice, huh? Most of this happens in 3 or 4 time constants. In between,
the exponential function e^(-t/RC) gradually squashes the initial temp diff
(Ti-Tout) to 0 over time.
Adding resistors in series, RC = (1/500+1/(2x880))440 = 1.13 hours.
RC = 20ft^2-F-h/Btux0.5Btu/F-ft^2 = 10 hours.
... as before, with a final temp and a negative initial temp diff that
gets stomped down to 0 over time by the cruel but patient exponential.
Same old stuff, with slower stomping.
Basements have lots of thermal mass. If we don't live in them, we can cool
them below the comfort temp, but we have to be careful to avoid blowing
moist outdoor air through a basement, with condensation.
I live in Baltimore. Flash your lights a few times so I can see where
My house has central AC, but I only use it 2 or 3 weeks a year.
Be sure you have enough insulation between your attic and top fllor,
if the attic isn't living space.
I have almost enough.
I have a townhouse that was built with fullwidth soffit screens front
and back and full width ridge vent. Not nearly enough in my opinion.
I put in a roof fan the first summer and I love it. Before the roof
fan, it was so hot when I got home at 5 or 6 that I couldn't go
upstairs at all. I relaxed and slept in the basement, and went
upstairs in the morning to shower and get fresh clothes. AFter roof
fan, I started sleeping upstairs. At least 10 degrees cooler and 20
to 30 in the attic. I have had to replace the motor 2 or 3 times in
the last 24 years. Some last a lot longer than others, I don't know
But I use fans too, a lot.
I don't like window fans because it would take an awful lot of fanning
to really put a breeze through my house, if nature hasn't provided
one. And they obstruct the window, and I don't have many windows.
I only use table fans, and I have one wherever i spend any time, which
means above my bed, facing my chair when I'm sitting at my desk,
facing my chair when I'm sitting at the kitchen table, and facing the
sofa when I'm in the living room. And one at work on my desk, facing
me. Even though work has AC, there are times when it is not enough.
So the fan is always blowing straight at me.
I don't use the oscillation when the fan has it, but I live alone. I
think if other people lived here, and we didn't sit right next to each
other, I'd buy more fans. (rummage sales and thrift stores mostly.) I
would definitely have a second fan for the bed, although it would be
hard to find another one small enough to fit on the sill.
The below-grade basement is never hot enough to need a fan.
I don't like the noise of fans, so every fan has a speed control. I
plug the control into the wall and plug the fan into it. 2 or 3
controls I bought as table-top lamp dimmers (the brown box with the
slide control), and the others were home made: one I bought a ceiling
fan speed control and mounted it my own plastic box, with a cord to
the wall, and a receptacle I can plug the fan into. And one was a
dimmer that went in the wall under a standard wall plate, that I ended
up mounting in a fairly big metal box (big enough to use in a wall for
a switch or receptacle)
Several people here don't like the idea of using lamp dimmers as motor
speed controls, but I've been doing this for 24 years with no
problems. A couple motors wouldn't work with a lamp dimmer, and it's
for those fans I made up the table-top fan dimmer. I use that in the
kitchen, with the fan on top of the tv. It was a new fan about 10 or
15 years ago, with a plastic housing.
I think I can use the one that is actually a fan control to control
the speed of electric drills etc. maybe, but I have variable speed
drills and so far I haven't had occasion to try this.
I almost always set the speed of the fan to just below the speed at
which I hear the fan. So I only get the breeze. One fan, the one at
my desk, has to go a bit higher for some reason, but it's not as loud
as even the slow speed out of 3.
I never sat the dimmer so slow that the fan doesn't spin at all. It
needs to turn or it will overheat.
Also I never leave the fan alone on the dimmer until I've run it say 5
or 10 hours in a row without overheating problems. Just feel the fan.
It should be warm, but if it is hot, maybe it's being used with the
wrong dimmer, although I've never had that except the one or two that
would barely spin on the light dimmer. (There are differnt kinds of
I have my bed just below the bedroom window, and there I use a fan
that I think was once riveted to some big machine at a factory, to
blow on the operato. It might be 50 or 70 years old. It only has two
blades, that is, one blade with two ends. And the protective screen
is barely any protection at all. But the motor is pretty weak and
when the blade wasn't firmly attached to the shaft, or even now, I
could just put my finger in with no problem. When started, it would
take 15 seconds for the blade to catch up to the shaft. But it got to
where it would never catch up, so I glued the blade on.
It has a tiny flat cast metal base, 4"x4" and would always be knocked
over or pulled over if I didn't nail one corner to the window sill.
This old fan doesn't have self-lubricating bearings and I have to oil
it 2 to 5 times during the summer. Sometimes it will last 2 months on
one oiling and sometimes only a few days. I guess it depends on how
well I do it. Sometimes when I didn't keep the oil right by the fan,
I would let it slow down some, and then in the middle of the night it
would slow down to a stop. But it had already done this a few times
in the middle of the day too. Motor was quite hot, but after oiling
it was always good as new.
For a while, I thought it would make me too cold if the outside temp
went down during the night, so I took a thermostatic contro from
another fan, mounted in the plastic cap from some aerosol can, and
wired that into the power cord. I don't use it much anymore but it's
adjustable and sometimes I set it just warmer than the current temp,
so the fan has just gone on as I turn the knob. Then it will turn off
if it gets any colder inside.
OH YES VERY IMPORTANT. I learned to sleep with no blanket or sheet
covering me. It was hard to do. I used to use a blanket or
comforter no matter how hot it was. It took a couple weeks to get
used to no covers. That was worth 5 or 10 degrees. The following year
I learned to sleep with no clothes on and no covers. That was worth
at least another 5, for a total of at least 15.
I live alone. If you have daughters, the naked part is probably not a
The roof fan turns on between 8 in the morning and noon, depending on
how hot and sunny it is. It turns off between 6 in the afternoon and
9, so it's always quiet when I go to sleep. It never lets the attic
get more than 5 or 10 degrees hotter than the outside temp, so the
wood that makes up the house never heats up more than that and the
second floor and the furniture and walls never get very hot. A whole
house fan doesn't work until it gets cool out, and there will some
days it will be 80 or more until you fall asleep, I believe. If it's
80 out, the fan won't make it cooler than 80 in.
I'm dubious about ceiling fans. My table fans I think blow far more
where I am than would a ceiling fan. Maybe in the kitchen where
someone moves around while cooking, but the fan on top of the tv has
some effect in the rest of the kitchen too.
My brother has one in my bedroom when I visit, in his airconditioned
Dallas home. I think a table fan might do more, but since the house
AC is sst to 72, I don't have a real comparison. Another friend here
who also has AC has 2 or 3 ceiling fans, one on a not that high but
cathedral ceiling. He doesn't have the money my brother has and he
gets a lot of heat in his living room window, and sets the AC to 75,
and the celing fan does something but i'm not sure how much. Should I
pay attention next time I'm there.
I hate having the windows closed. I hate the quiet.
I absolutely woudn't install more than one ceiling fan at first, and
compare the results with a table fan. Or two if you have more family.
I set up a "gable vent fan" in the attic access hole (exhausting) with
a X10 appliance module to turn if on and off. I open a window upstairs
and a window downstairs when I want to use the fan. It's sort of a
"poor mans" whole house fan. The fan had four mounting "arms" on it. I
sliped foam pipe insulation over each arm and place it in the access
hole to the attic. The foam de-couples the vibration to reduce noise.
It really makes a difference here in Seattle, It won;t do so well in
DC, but better than just a fan.
Even at 80 deg., a ceiling fan would make you feel cooler..won't cool
the house but would increase comfort level. Perhaps an exhaust fan in
the upstairs window that would draw cooler air into the house with a
window open downstairs/shady side. For windows on the sunny side, cheap
white or reflective window shades would keep a good deal of heat out.
Now that I finally think of it, re those hot ceilings, hot
via the attic, don't forget to consider the heat you yourself
receive via RADIATION from that ceiling.
And also window-glass that gets hot from the sun.
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